MSU LIBRARIES “ RETURNING MATERIALS: Place in book drop to remove this checkout from your record. FINES will be charged if book is returned after the date stamped below. RESIDUAL EFFECTS OF NITROGEN FERTIEIZERS AND LIME ON FRACTIONAL DISTRIBUTION OF ALUMINUM AND ASSOCIATED ELEMENTS By Darunee Tantiwiramanond A DISSERTATION Submitted to Michigan State University in partia] fquiTIment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Cr0p and Soi] Sciences 1981 6217.5“?7 ABSTRACT RESIDUAL EFFECTS OF NITROGEN FERTILIZERS AND LIME ON FRACTIONAL DISTRIBUTION OF ALUMINUM AND ASSOCIATED ELEMENTS by Darunee Tantiwiramanond Fourteen annual applications of N carriers (336 kg/ha yr) had profoundly altered the chemical pr0perties of a Hodunk sandy loam (Ochreptic Fragudalf). Seven years after termination of the experiment, soil (at 0-25 cm and 50-75 cm depth) were sampled from plots treated with (NH4)ZSO4 ("sulfate") or Ca(N03)2 ("nitrate"), with or without lime, to examine for forms of Al that may have contributed to retention and mobility of buffer acidity. Surface soils, amended with corn tissue without or with NH4+ (+0M or +OM+N), were incubated for 70 days. All soils were extracted with N NH OAc (pH 4.8) and with _h_l KCl before and 4 after a water-soxhlet extraction. "Free“ and "complexed" forms of extracted Al were differentiated by the 8-hydroxyquinoline method. Surface soils had a capacity for intercepting acidity and maintaining lime requirements without downward displacement of acidic complexes. When annual inputs of acidity from non-fertilizer sources Darunee Tantiwiramanond or from nitrification of NH4+ exceeded this capacity, net movement (M: buffer acidity into deeper soil layers occurred. Similar quantities of Al were removed by KCl and by NH OAc (pH 4 4.8) from unlimed-sulfate soil of pH(H20) 4.5. Recoveries in both extractants decreased with increasing pH and decreasing lime requirement. Lime requirements by buffer test were influenced mainly by Al exchangeable to KC], since both became negligible above pH(H20) 6.0. Substantnfl NH40Ac-extracted Al at this and higher pH were probably released from separate phase polymers and organic complexes by protonathnnand ligand exchange. Seventy percent or more of the Al extracted by NH40Ac was in “free" forms, whereas 60% or more of the Al in KC] was "complexed“. Water-soxhlet removed negligible Al and reduced subsequent extractabilities of Al by NH4OAc or KCl. But, percent complexation in NH4OAc extracts was increased. During incubation, increases in extractable Al occurred only when nitrification in +OM+N lowered the soil pH. In unlimed soils, the degree of complexation of Al in KCl was increased by +0M treatment. In the field, protonation of sesquioxides was mainly responsible for iruzreasing lime requirements, while chelation of charged Al species contributed to mobility and downward displacement of buffer acidity. DEDICATION To my parents, my brothers and my sisters... whose love and support are the main ingredients of my inner strength. To the farmers of Thailand... who give so much, but get so little in return. ii ACKNOWLEDGEMENT I would like to express my deep gratitude to Dr. A.R. Wolcott for his dedicating and rewarding guidance, and to Dr. 8.6. Ellis for his advice and understanding throughout the program of my study. I am thankful to the Department of Crop and Soil Sciences, Michigan State University for providing me necessary financial support. To my friends who helped me in field-soil sampling, and to Teresa Hughes for her laboratory assistance, I remain indebted. Special thanks are extended to Drs. B.D. Knezek, D.D. Warncke and T.J. Pinnavaia for their invaluable service as my guidance committee, and to Dr. C. Cress for his statistical consultation. Finally, to the Bhimji Family who provided me a "home away from home", and to S.R. Pandey for his friendship and encouragement, I deeply appreciated. TABLE OF CONTENTS INTRODUCTION 0 O O O O O O O O O O O I O O O O 0 LITERATURE REVIEW. . . . . . . . . . . . . . . . Effects of N Fertilizers on Soil Properties Availability of Plant Nutrients. . . . . Soil Acidity . . . . . . . . . . . . . . Roles of Organic Matter in Soils. . . . . . Nature of Soil Organic Matter. . . . . . Distribution in Relation to the Mineral Matrix Functional Groups in Relation to Soil Acidity, and Chelation . . . . . . . . . . . . . . . . Roles of Aluminum in Soils. . . . . . . . . . . . Forms of Aluminum in Relation to Soil Acidity. Variable Charge, Ion Exchange and Specific Adsorption. MATERIALS AND METHODS. . . . . . . . . Source. . . . . . . . . . . . . . Management History. . . . . . . . Cropping History in re Soil C and Soil Test History—T'TT. . . . . . Sampling of Soil. . . . . . . . . . . Laboratory Incubation of Surface Soil Extraction Procedures . . . . . . . . Analytical Procedures . . . . . . . . Statistical Analysis. . . . . . . . . RESULTS AND DISCUSSION . . . . . . . . . . . . Properties of Soils as Sampled in Soil pH. . . . . . . . . . . . Soil C and N . . . . . . . . . Forms of Extractable Al. . . . Effects of Incubation Treatments. Changes in Soil pH, C and N. . Changes in Distribution of Al. Relationships of Associated Elements and A June 1979 1' SUMMARY AND CONCLUSIONS. . . . . . . . . . . . . LITERATURE CITED. 0 O O O O O O O O O O O C 0 iv 000000000 Page 000000 5—: N 000000... a... 03 w \J N 00.00. m A Page APPENDIX 0 O O O O O O O O O O O O O 0 O 0 O O O O O O O O O I O A]. A1 Forms of phosphorus in surface soil (0-25 cm) and subsoil (50-75 cm): effects of incubation treatments and long- term residual effects of nitrogen carriers and lime . . . A1 A2 Forms of potassium in surface soil (0-25 cm) and subsoil (50-75 cm): effects of incubation treatments and long- tenn residual effects of nitrogen carriers and lime . . . A2 A3 Forms of calcium in surface soil (0-25 cm) and subsoil (50-75 cm): effects of incubation treatments and long- term residual effects of nitrogen carriers and lime . . . A3 A4 Forms of magnesium in surface soil (0-25 cm) and subsoil (50-75 cm): effects of incubation treatments and long- term residual effects of nitrogen carriers and lime . . . A4 A5 Forms of copper in surface soil (0-25 cm) and subsoil (50-75 cm): effects of incubation treatments and long- term residual effects of nitrogen carriers and lime . . . A5 A6 Forms of iron in surface soil (0-25 cm) and subsoil (50-75 cm): effects of incubation treatments and long- tenn residual effects of nitrogen carriers and lime . . . A6 A7 Forms of manganese in surface soil (0-25 cm) and subsoil (50-75 cm): effects of incubation treatments and long- term residual effects of nitrogen carriers and lime . . . A7 A8 Forms of zinc in surface soil (0-25 cm) and subsoil (50-75 cm): effects of incubation treatments and long- tenm residual effects of nitrogen carriers and lime . . . A8 A9 Forms of aluminum in surface soil (0-25 cm) and subsoil (50-75 cm): effects of incubation treatments and long- term residual effects of nitrogen carriers and lime . . . A9 A10 Forms of cadmium in surface soil (0-25 cm) and subsoil (50-75 cm): effects of incubation treatments and long- tenn residual effects of nitrogen carriers and lime . . . A10 All Forms of lead in surface soil (0-25 cm) and subsoil (50-75 cm): effects of incubation treatments and long- term residual effects of nitrogen carriers and lime . . . A11 A12 Forms of nickel in surface soil (0-25 cm) and subsoil (50-75 cm): effects of incubation treatments and long- tenn residual effects of nitrogen carriers and lime . . . A12 Bl HOdunk series 0 O O O O O O O O O O O O O O O O O O O O 0 A13 LIST OF TABLES Table Page 1 Percent nitrogen and relative residual acidity of eight nitrogen carriers used in the long-term field study . . . . 53 2 Crop and fertilizer history (1959-80) . . . . . . . . . . . 53 3 Partial history of crop yields (1961-76). . . . . . . . . . 56 4 Comparisons of soil C, N, and C:N between 1975 and 1979 in relation to residual effects of nitrigen carriers and lime. O O O O O O O O O O O O O O O O O O O O O O O O O O O 57 5 Soil pH in 1975: residual effects of nitrogen carriers and limEO O O I O I O O O O O O O O O O O O O O O I O O O O O O 58 6 Soil pH changes (1961-79) for treatments considered in present StUdy O O O O O O O O O O O O O O C O O C O O O O O 59 7 Lime requirement changes (1961-79) for treatments considered in present study . . . . . . . . . . . . . . . . 63 8 Changes in distribution of soil acidity after nitrogen treatments were discontinued (1972) . . . . . . . . . . . . 66 9 Soil pH, forms of carbon and nitrogen in surface (0-25 cm) and subsoil (50-75 cm): long-term residual effects of nitrogen carriers and lime . . . . . . . . . . . 77 10 Total aluminum (determined by 8-hydroxyquinoline) in surface soil (0-25 cm) and subsoil (50-75 cm): long-term residual effects of nitrogen carriers and lime. . . . . . . 85 11 Free and complexed aluminum in extracts of surface soil (0-25 cm) and subsoil (50-75 cm): long-term residual effects of nitrogen carriers and lime . . . . . . . . . . . 90 12 Free and complexed aluminum in fractional extracts of surface soil (0-25 cm) and subsoil (50-75 cm): long-term residual effects of nitrogen carriers and lime. . . . . . . 92 13 Percent complexed aluminum in extracts of surface soil (0-25 cm) and subsoil (50-75 cm): effects of incubation treatments and long-tenn residual effects of nitrogen carriers and lime . . . . . . . . . . . . . . . . . . . . . 94 vi Table 14 15 16 17 18 19 20 21 Soil pH, forms of carbon and nitrogen in surface soil: effects of incubation treatments and long-term residual effects of nitrogen carriers and lime . . . . . . . . . . Estimates of mineralization during incubation and net recovery of mineral nitrogen. . . . . . . . . . . . . . . Total aluminum (determined by 8-hydroxyquinoline) in surface soil: effects of incubation treatments and long-term residual effects of nitrogen carriers and lime. O O O O O I O C O O O O I O O O O O O O I O O O O 0 Free and complexed aluminum in extracts of surface soil: effects of incubation treatments and long-term residual effects of nitrogen carriers and lime . . . . . . . . . . Free and complexed aluminum in fractional extracts of surface soil: effects of incubation treatments and long-term residual effects of nitrogen carriers and lime. Organic carbon and C/Complexed Al Ratios in Fractional extracts of surface soil and subsoil. . . . . . . . . . . Forms of associated elements in.N NH 0Ac extracts determined by plasma emission photometry: effects of incubation treatments and long-term residual effects of nitrogen carriers and lime. . . . . . . . . . . . . . . . Forms of associated elements in N KCl extracts determined by plasma emission photometry: effects of incubation treatments and long-term residual effects of nitrogen carriers and lime. . . . . . . . . . . . . . . . . . . . vii Page 100 102 111 114 116 120 121 112 LIST OF FIGURE Figure Page 1 Relationships of soil pH (0.1M KCl) and distribution of Al in surface soils extracted by.N NH40Ac (pH 4.8), and by E KC] 0 O O O O O O O O O O I O O O O O O O O O C O O O 106 viii INTRODUCTION Civilization began when the human race started cultivating plants instead of wandering and hunting for food. The time has been dated back to about 2500 B.C. (Tisdale and Nelson, 1975). Through keen observation, early cultivators adepted the practice of adding animal and vegetable manures to the soil to restore soil fertility and increase crop yields. But, satisfactory answers to man's curiosity regarding the phenomena of plant growth came slowly. For centuries, man has tried to rationalize plant growth and nutrition. The pot experiment of Van Helmont (1577-1644) is credited as the turning point leading to a better understanding of plant nutrition. Despite his erroneous conclusion - that water was the sole nutrient for plants - his empirical and quantitative method stimulated a more scientific approach in later investigations. Discoveries during the late seventeenth and early eighteenth centuries were corner stones in the progress of scientific agriculture. The concept introduced by Liebig (1803-1873) - that growth of plants was pr0portional to the amount of mineral substances available in the soil or in added fertilizers - Opened the era of using chemical fertilizers in augmenting soil fertility. In the U.S.A., commercial chemical fertilizers have been used in increasing amounts since 1880 (Ibach and Mahan, 1968). The demand for fertilizers increased slowly until 1943. Following World War 11 there was rapid growth in the synthetic nitrogen industry and in potash production. It took only one decade for the use of chemical fertilizers l to increase 100%. By 1968, the demand reached 580% of that 25 years earlier. The main motive behind this tremendous increase in demand for fertilizers was increased productivity, supporting improved income and standard of living for the farmer. Despite the vast benefit of high analysis fertilizers on cr0p productivity, intensive management systems have been shown to create some undesireable effects in the soil and in associated environmental systems. Adverse effects of intensive cr0pping tend to increase in severity over time unless corrective management practices are employed. Accelerated cycling of carbon in cultivated soils leads quickly to greatly reduced levels of soil organic matter (SOM). Declining 80M and compaction by heavy equipment combine to degrade soil structure and aggravate problems of poor infiltration and percolation of water, soil crusting, restricted aeration, and impeded root development. Uncorrected, these physical changes interfere with the efficient use of water and nutrients by cr0ps. They can also lead to increased loss of soil and nutrients in runoff and increased pollution of surface waters that receive the runoff. Increasing public awareness of environmental pollution have aroused concern for the side effects of over-fertilization. Ground waters and streams were reported to be contaminated by excessive nitrate (N03') content in leachate or runoff from nearby farm land. Drinking water which contains more than 10 mg N/l as N03 can induce methemoglobinemia in humans and livestock. On occasion, the effect may be fatal for infants. Excessive phosphorus (P) from surface drainage of agricultural land is considered to be an important contributor to excessive growth of water plants, or eutr0phication, which impairs the to increase 100%. By 1968, the demand reached 580% of that 25 years earlier. The main motive behind this tremendous increase in demand for fertilizers was increased productivity, supporting improved income and standard of living for the farmer. Despite the vast benefit of high analysis fertilizers on cr0p productivity, intensive management systems have been shown to create some undesireable effects in the soil and in associated environmental systems. Adverse effects of intensive cr0pping tend to increase in severity over time unless corrective management practices are employed. Accelerated cycling of carbon in cultivated soils leads quickly to greatly reduced levels of soil organic matter (SOM). Declining SOM and compaction by heavy equipment combine to degrade soil structure and aggravate problems of poor infiltration and percolation of water, soil crusting, restricted aeration, and impeded root develOpment. Uncorrected, these physical changes interfere with the efficient use of water and nutrients by crOps. They can also lead to increased loss of soil and nutrients in runoff and increased pollution of surface waters that receive the runoff. Increasing public awareness of environmental pollution have aroused concern for the side effects of over-fertilization. Ground waters and streams were reported to be contaminated by excessive nitrate (N03') content in leachate or runoff from nearby farm land. Drinking water which contains more than 10 mg N/l as N03 can induce methemoglobinemia in humans and livestock. On occasion, the effect may be fatal for infants. Excessive phosphorus (P) from surface drainage of agricultural land is considered to be an important contributor to excessive growth of water plants, or eutr0phication, which impairs the quality of water for many uses (De Haan and Zwerman, 1978). For the U.S. as a whole, adverse environmental impacts of agricultural Operations appear to be much less than those due to domestic and industrial wastes (Viets, 1971). However, from the standpoint of the individual farmer, there remain compelling reasons for managing his resources in ways that are compatible with a sound environment. These include cost/price relationships that call for increasing efficiencies in use of fertilizers, water, fuel, chemicals and other production inputs, and concerns for maintaining or improving the productivity of his prime resource, which is the soil. Energy shortages and the expanding world population serve to intensify these self-interested concerns. The present study deals with adverse residual effects on soils of long-continued use of nitrogen (N) fertilizers. Soil materials used in the study were taken from a long-term field experiment in which residually acid and residually basic N sources were used, with and without lime to neutralize accumulating buffer acidity. The objectives of the study were: 1. To characterize forms of aluminum (Al) which may have been involved in retention of buffer acidity and its movement downward through the soil. 2. To observe effects of decomposing organic matter and nitrification of NH4+ during incubation on extractability and inferred mobility of Al. 3. To derive data that might be used in later studies to characterize distributions of P and 10 metallic cations in relation to distributions of Al. LITERATURE REV I EN EFFECTS OF N FERTILIZERS 0N SOIL PROPERTIES Nitrogen is a key element responsible for lush vegetative growth and dark green leaf color of plants. Nitrogen is demanded in greater quantity than other plant nutrients because it is an essential constituent of chlorOphyll and of amino acids which are required in protein synthesis (Pesek et al., 1971). Unlike other plant nutrients that are derived from minerals, the primary source of N is the atmosphere. Since N2 is a relatively inert gas, growing plants, except legumes, cannot use it directly, and N is often the first limiting element for Optimum crOp yields. For this reason, commercial N fertilizers are indispensable for economic production of crOps. Forms of N commonly supplied in fertilizers are NH4+, NH3, N03- and urea. Most fertilizer sources of N are water-soluble. Urea may be coated with sulfur or complexed with formaldehyde to reduce its solubility and rate of hydrolysis (Kilmer and Webb, 1968). Once they are incorporated in a well-drained soil, the reduced forms of N are rapidly converted to N03 . However, both NH4+ and N03' are readily taken up by plants. Availability of Plant Nutrients About half of the N applied in fertilizer is taken up and used for crOp growth. 0f the remainder of added N, a portion can be accounted for through microbial immobilization, fixation of NH4+ between the lattices of clay minerals, or losses through leaching, runoff or gaseous evolution. In most studies, a substantial prOportion (15 to 25%) of the added N cannot be accounted for and is assumed to have been lost through volatilization of NH or through denitrification (Kilmer and Webb, 3 1968). Where N is limiting, N fertilizer amendment leads to increased tOp growth and root proliferation. To keep physiological functions in balance, the plant increases its demand for other essential elements. The expansion of the root system increases its capacity to supply such demand by increasing the absorptive surface area and the volume of soil occupied by plant roots. In addition, the ion-exchange and nutrient uptake properties of roots, as well as transportation and utilization systems within the plant, may be altered by increases in protein content and changes in the nature and concentration of mobile metabolites. Increased protein content of roots enhances both their anion exchange and their cation exchange prOperties (Adams, 1980; Munson, 1968). In well-aerated soils, at favorable temperature and pH, nitrification is rapid, N03' quickly becomes the principal form of N available, and differences in crOp response to different forms of fertilizer N are not often observed. Nevertheless, the uptake and utilization of other nutrients is affected differently by NH);+ than by N03-. Seedling develOpment and early growth of plants may be influenced significantly in situations where the availability of some essential nutrient is marginally low or already high enough to be somewhat toxic. Some of these differences in direct response to ammoniacal sources If; nitrate salts can be explained by pH effects associated with hydrolysis of different N carriers at the time of application or with subsequent nitrification of NH4+ (see next section). Uptake mechanisms are also involved. Nhen NH4+ is taken up, it is NH3 that actually enters the plant, leaving the surface environment Of the root more acid (Kamprath and Fay, 1971). Since the Optimum pH for availability of many nutrients in mineral soils is about 6.0 to 6.8, the acidifying effects due to hydrolysis of NH»,+ salts or to uptake or nitrification of NH);r can be beneficial in soils of higher pH or detrimental in more acid soils (Giordano and Mortvedt, 1972; Lucas and Knezek, 1972). In alkaline soils, the availability of P is increased by NH + salts (Adams, 1980). Beneficial 4 increases in availability of Mn, Cu, Zn or Fe may be expected also where one or more of these nutrients are marginally deficient at pH's as low as 6.0 or 6.5 (Kamprath and Foy, 1971). At lower pH, the availability of Mo may be reduced and a native deficiency of this nutrient may be aggravated. In soils of pH 5.0 or less, additional acidity from NH4+ fertilizers can be expected to aggravate toxicities due to Al or Mn. Nhen NH4+ and orthOphosphate are applied together in mixed fertilizers, they may be c0precipitated with Al, Fe, Ca or Mg. Some of these metal-ammonium phosphates are relatively insoluble, depending on pH, and are only slowly available to plants (Adams, 1980; Lindsay, 1979). When nitrification is greatly delayed by an unfavorably low pH, NH4+ taken up by plants may not be utilized effectively in the plant (Kamprath and Foy, 1971). The metabolic interactions are complex, but it has been Observed that the nature and balance of anions and cations in the plant are altered greatly when NH4+ rather than N03' is taken up. The uptake of Ca is depressed relative to K, and the pH of the plant sap is lowered substantially. Apparently, a favorable balance in uptake and translocation of cations and anions depends on nitrate reductase systems and requires that at least a portion of the N be taken up as N03 (Munson, 1968). The interactions between N and other elements in soils and in plants occur simultaneously. The main factor governing such interactions in soils is the pH Of the ambient soil solution. Soil Acidity Most fertilizer salts have a direct acidic effect (”electrolyte effect") on the soil solution at the time of application. The added cation shifts exchange equilibria to favor dissociation of other basic cations, as well as protons, from the exchange complex: 2+ Ca + 2H(Soi]) i==========é 2H+ + Ca(soil) [11 The initial pH depression is greater with addition of the salt of a weak base and a strong acid, such as (NH4)2504. (hi the other hand, the introduction into soils of NH3, or of a carrier that dissociates or hydrolyses to release NH3, can raise the pH of the ambient soil solution by one or more pH units (Pesek et al., 1971): (NH4)2HPO4 ------------ > NH3 + NH4H2P04 [2] + 2- HzN-CO-NHZ + 2H20 ------------ > 2NH4 + C03 <==========\= 2NH3 + C02 + H20 [3] -..._.._.____.._.\. + - NH3 + 1120‘ ---------- NH4 + OH [4] The directly alkalizing effect of ammoniacal fertilizers is, normally, reversed quickly by nitrification: H o 20 + 2 + 2 + - NH =\=======\- H30 + NH3 ......... > H30 + N03 [5] Due to reactions [4] and [5], soil pH in the zone of placement of anhydrous NH or ammonia-releasing fertilizers, such as urea, can vary 3 seasonally over a range of several pH units (Kamprath and Foy, 1971). As nitrification proceeds, this localized pH may reach values one or more pH units lower than before the NH3 source was introduced. Such direct and seasonal effects are transient but can have significant effects on plant nutrition during critical periods of growth. ()ver a period of months, soil pH tends to return to the original level. The extent to which pH returns to earlier levels wdll depend on the nature and capacity of buffering systems in the soil. Long-term residual effects of N fertilizers may be acidic or basic, depending on the carrier and the extent to which anions or cations interact differentially with soil colloids, or are removed differentially by crOps or by leaching, or in the case of N itself by volatilization or denitrification (Kamprath and Foy, 1971; Pesek et al., 1971; Nolcott, 1964). Nitrification of 1 kg N as NH to N03" [Eq. 5] produces residual 3 acidity equivalent to the neutralizing value of 3.6 kg of CaCO3. The expected acidity from a given input of N will be less if NH3 is lost by volatilization. If some of the N is immobilized in soil organic matter (SOM), the corresponding increment of residual acidity will not appear until the N is mineralized again by decomposition of the SOM and then nitrified. Under conditions of restricted aeration, N03" is used by denitrifying bacteria as a terminal electron acceptor. To the extent that denitrification does occur, previously accumulated acidity will be neutralized (Chang and Broadbent, 1980; Hiltbold and Adams, 1960): 2H0 + 2NO- ---------- > N2 + 5[0] + 3H20 [6] Residual acidity may also be derived from the complementary anions in NH4+ salts. The theoretical "anion acidity" in NH4Cl or (NH4)ZSO4 is equivalent to the dissociable proton in NH4+ [Eq. 5]. In the case of orthOphosphate, the dominant species in soils of less than neutral pH is H2P04' (Lindsay, 1979). Thus, the theoretical anion acidity due to phosphate in mono-ammonium phosphate is equal to that expected from nitrification of the NH3 derived from NH4+, whereas in diammonium phosphate it would be half the NH + equivalent [Eq. 2]. 4 Nitrate salts of metal cations tend to leave a basic residue in soils due to differentially greater removals of the anion by crOps, leaching and denitrification. However, N03" taken up by plants is accompanied by an approximately equivalent quantity Of cations. In the plant, N03' is reduced to NH3 and assimilated as -NH2. This leaves an excess of mineral cations which are countered by organic acids produced 10 during N03“ reduction (Munson, 1968; Pierre et al., 1970). Plants vary uniquely in their capacity for taking up basic cations (K+, Na+, Ca2+, MgZ+) in excess of anions other than N03' (HZPO ', SD 2'). The ratio of 4 excess bases (EB) to total N in crOp tissues has been shown to be related directly to the quantity of residual acidity (lime requirement) that develOps (Pierre et al., 1970). Where buckwheat (EB/N = 1.81) was grown, the increase in lime requirement was 1.87 times greater than theoretical for nitrification of the NH4+ in NH4N03. Nith oats (EB/N = 0.70), the increase in lime requirement was 0.73 of theoretical. Numerous field and greenhouse studies have indicated that, on the average, the residual acidity attributable to N in fertilizers (regardless of the chemical form applied) is about one-half of the theoretical acidity due to nitrification of an equivalent quantity of NH3. In the fertilizer industry, CaC03 equivalents for individual elements have been derived from field and greenhouse studies (Faun Chemicals Handbook, 1977). These are used to estimate the residual acidity or basicity which may be expected on the average or in usual situations. Potential residual CaC03-equivalents (kg/kg of nutrient) assigned to fertilizer elements are as follows: Acid-forming nutrients: S (3.2), Cl (1.4), P (1.1), N (1.8) Base-forming nutrients: Ca (2.5), Mg (4.1), K (1.3), Na (2.2) If'free carbonates are present (native to the soil or added as lime), protons released by reaction [5] will be dissipated largely as water: 2+ - - CaCO3 + H20 ------ > Ca + HCD3 + 0H i - - +-=====\. ' HCO3 + 0H + 2H30 \ 4H20 + CD2 ii + ______\ 2+ CaC03 + 2:430 ‘ ------ Ca + 3H20 + CO2 [71 In the absence of free carbonates, protons tend to be retained by adsorption on soil colloids. Simplistically, the adsorptHNInmy be viewed as a reversal of Eq. [1]. The principal accumulators of protons in mineral soils are SOM and hydrated metal oxides and hydroxides, mainly of Al (Jackson, 1963; Kamprath and Foy, 1971). Residually acidic effects are cumulative. Potential acidity due to protonation of SOM and amorphous mineral colloids can build up quickly where nitrogen in excess of crop removal is applied (Kilmer and Webb, 1968). The increase in potential acidity is reflected in the equilibrium pH of the soil solution, which in turn influences solubilities and equilibria that determine the availability of numerous plant nutrients. Below pH 5.5, deficiencies and/or toxicities Of specific nutrients become increasingly problematic for most crOps. The lime requirement for correcting potential acidity is estimated as some arbitrary multiple (as 2x) of the CaC03-equivalent of H+ released when a soil sample is suspended in a standard buffer solution (McLean, 1980; Shoemaker et al., 1961). The appearance of buffer acidity in soils is greatly accelerated below pH 5.0 due to rapid depolymerization and release of highly charged Al polycations and monomers. Over a relatively short period of time, all of the surface charge previously accumulated by Al polymers that buffer at higher pH may be released. Thus, in field experiments from which soil materials for the present study were taken, the pH of a sandy loam soil declined 12 from 6.0 to values Of 4.0 to 5.1 after 5 annual applications (336 kg/ha yr) of 6 different acidic-N carriers. Increases in buffer acidity over the same period of time exceeded the input acid-equivalent for individual carriers by 1.5- to 6-fold. The phenomenon was referred to as "runaway acidity” (Wolcott et al.,1965). ROLES OF ORGANIC MATTER IN SOILS Nature of Soil Organic Matter The mass of organic C in soil constitutes the largest fraction of surface C reservoirs in the biosphere. 0f the total soil C, 60-70% occurs in humic materials (Schnitzer, 1978). Although soil humus may represent less than 5% of the dry weight of a productive agricultural soil (Stevenson, 1972), it makes unique contributions to soil fertility (Allison, 1973; Greenland and Hayes, 1978; Stevenson, 1972). The decomposition of SOM releases N, P, S and other nutrient elements for higher plants. Better soil structure in humus-rich soil is a result of soil aggregation which provides good drainage and aeration for plant growth, root expansion and microbial activities. Humified organic matter increases soil buffering capacity and ion exchange capacity, helping protect plant nutrients from leaching loss. The dark color of SOM absorbs solar radiation, hence it increases soil temperature. "Soil organic matter“ refers more specifically to the non-living components of a heterogeneous mixture of organisms and detritus. The mixture undergoes dynamic seasonal changes due to a combination of degradation and synthesis (Allison, 1973; Greenland and Hayes, 1978). 13 Soil microorganisms use part of the C in plant and animal residues for the building of their cells which may at times become a considerable portion of SOM. Both degradation and synthesis are simultaneously taking place, and any factor, such as soil temperature, that affects one is likely to affect the other. The net effect of changes in environmental parameters is to change the rate at which the balance between the two humification processes shifts toward lower energy levels. The more readily degraded energy sources include starchs, proteins, hemicelluloses, cellulose and aliphatic structures. As these are dissipated, the prOportion of aromatic structures in residual humus increases. Carbon content increases, but 0, N, P and S increase at a relatively more rapid rate. The ratio C:N:P:S provides a rough approximation index of degree of humification, approaching 100:10:1:1 in extensively humified SOM. Declining availability of energy substrates is reflected in declining numbers and activities of soil organisms. The actual energy level approached during humification in a given field situation depends upon the quantity and frequency of input of photosynthetic products, directly in the form of root exudates and plant debris or, indirectly, via animal food chains. In relation to humification processes, SOM is composed of two fractions: (1) unaltered organic materials and (2) transformed products, or "humus“ (Greenland and Hayes, 1978). The latter fraction serves as a reservoir of chemical elements that are essential for plant growth, such as N, P, S. Humus is further classified into non-humic substances (recognizable products of biological synthesis), and humic substances (amorphous polymers formed by condensation reactions that are l4 essentially non-enzymatic) (Greenland and Hayes, 1978; Schnitzer, 1978). Non-Humic Substances-- Non-humic substances are chiefly constituents of higher plants, animals and soil microorganisms (Lowe, 1978). Their physical and chemical structures are still recognizable and serve as a continuing energy-supply for soil microbes. The chemical components of non-humic materials are carbohydrates, proteins, lignin, fats, waxes, resins, tannins, pigments, other compounds with cyclic structures, organic bases, and small molecular organic acids (Allison, 1973; Schnitzer, 1978). Usually, these compounds are susceptible to microbial attack. Carbohydrates are the major non-humic constituent of SOM. They account for 5-20% of total SOM, and may occur as part of cell structure, or as exocellular or intracellular components and metabolic products (Lowe, 1978). Soil carbohydrates include cellulose, other polysaccharides and sometimes free monosaccharides. The composition of monosaccharide and polysaccharide materials varies from soil to soil, and depends on the rate of decomposition and the input of fresh organic matter. The most important class of polysaccharides in soils, quantitatively and qualitatively, are polyuronides of varying uronic acid content and molecular weight (Anderson et al., 1977; Greenland and Hayes, 1978; Mortenson, 1960; Swincer et al., 1968; Thomas et al., 1967). Polyuronides from different soils contain a similar suite of neutral sugars and amino sugars. The lower molecular weight polyuronides also contain a large prOportion of amino acids and have a rather high ash content, even after extensive purification. The amino 15 acids may be present as peptides bound to the polyuronides through complexed polyvalent ions (Fe, Al, Si, Ca, Mg). About 10% of SOM is in the form of polysaccharides. The spectrum of sugars present suggests that they are predominantly of microbial origin (Allison, 1973). Polysaccharides are not only an important structural component of SOM but also play an important role in soil structure, aggregation, exchange capacity, metal chelation, and in supplying C as energy for microorganisms. The N in constituent amino sugars and complexed amino acids appears to contribute importantly to the seasonal N supply for growing crOps. Smith and Stanford (1971) found that, when soils were autoclaved, NH4+ and sugars (anthrone method) were released in linearly proportional quantities. In a large group of soils from different soil groups, either NH4+ or the glucose equivalent correlated well with N mineralized when the soils were incubated. Humic Substances-- Humic substances are dark colored, amorphous polymers of relatively high molecular weight (Allison, 1973; Flaig et al., 1975; Kononova, 1966; Schnitzer, 1978; Schnitzer and Khan, 1972; Stevenson and Butler, 1969). The predominant structural units appear to be an aromatic rings, extensively cross-linked through linear or heterocyclic bridging structures involving C, 0, N or S. The three-dimensional network of organic building blocks is further stabilized by H-bonding and by salt bridging or ligand bonding through polyvalent cations. In soils taht have evolved in base-rich environments, the principal structure-stabilizing cation is Ca. In extensively weathered or I6 intensively leached soils, the principal stabilizing cations are mono- and dihydroxy Al and Fe. Humic substances can by partitioned into three fractions based on their different solubilities in acid or base. Humic acid (HA) is soluble in alkali but insoluble in acid, while fulvic acid (FA) is soluble in both. Humified organic matter that cannot be extracted with acid or alkali is referred to as humin. Portions of the humin fraction can be recovered by successive alternate extractions with acid and alkali. Humin components recovered in this way have essentially the same organic structural components, functional groups and prOperties as HA. Their resistance to extraction appears to be due to the fact that they are more extensively complexed with minerals. Fulvic and humic acids are polyelectrolytes. The molecular weight of FA is in the range of 5,000 to 300,000. The greater solubility of FA is due in part to smaller molecular size and in part to greater polarity due to a higher degree of oxidation and a higher concentration of acidic functional groups. fhmfic substances develOped in different geographical regions and under different pedological conditions are remarkably similar in chemical and physical prOperties, as well as in their essentially aromatic-based structural composition. The main differences encountered in different soils and different soil horizons are in the proportions of FA to HA and the degree of complexation with minerals. Numerous theories have been prOposed to account for the apparent dominance of aromatic structural units in humic substances and their resistance to enzymatic or chemical breakdown (Allison, 1973; Flaig et 17 al., 1975; Kononova, 1966; Stevenson and Butler, 1969; Schnitzer, 1978; Schnitzer and Khan, 1972). These theories differ mainly in the importance given to the sources of phenolic compounds (plants 12;. microorganisms) and the relative significance given to enzymatic 3‘s; non-enzymatic processes in the synthesis of resistant humic polymers. An early theory considered that plant lignins are biochemically recalcitrant and that the randomly condenses C6-C3 structures of lignin persist as the core structures in humus. These decompose slowly from the surface inward. Carboxyl groups are formed as C3-sidechains are degraded and phenolic-0H groups are exposed by demethoxylation. Nitrogen and S are incorporated as surface complexes with proteins or protein fragments. Later studies have shown that the content Of O-containing functional groups is much greater than can be accounted for by the lignin theory. Studies with model compounds have shown that humus-like substances are formed in mineral acid hydrolysates of sugars, during alkaline auto-oxidation or enzymatic oxidation of phenols and quinones, and in mixtures of amino acids and sugars of standing or rapidly at elevated temperatures. More recent studies have shown the formation of humic substances in laboratory cultures of soil organisms, using various substrates. Whether the humic polymers are formed within living organisms or by condensation after intra-cellular contents are released by autolysis is not clear. It appears that the biochemical recalcitrance of humus is due in part to the randomness of linkages formed during condensation of free radical intermediates in the oxidation of phenols and dienols. Resistance to degradation and difficulty of extraction is also due to 18 complexation with minerals and to the inaccessibility of humus that occurs in the interiors of soil aggregates. Distribution in Relation to the Mineral Matrix At any given time, organic materials may be found in soils, covering the full range from fresh detritus and living decay organicms ‘through transition products to extensively humified materials (Allison, 1973). These may appear initially as isolated masses in larger soil pores, associated with plant fragments or fecal pellets of soil animals. However, major microbial transformations take place in water films on the surfaces of pores large enough to permit colonization by microorganisms (Rovira and Greacen, 1957). Enzymes and soluble substrates and products can diffuse into micrOpores in the aggregated mineral matrix. However, the chemical environment in the interior of aggregates will be shielded from seasonal fluctuations associated biological cycles in extra-aggregated space (Spycher and Young, 1979). Aggregates form as parent materials weather during soil formation. Larger mineral grains (sand and silt size) are bonded together by amorphous mineral gels, secondary clay minerals and complexed or adsorbed organic materials, formed 111-3.131. or transported from sites of weathering and biological activity in upper soil horizons (Brewer, 1964; Jones and Uehara, 1973). The primary unit in the cementing complex, or ”plasma", may be viewed as a soil organo-nineral particle (SO-MP) comprised of a crystalline clay matrix and a coating of organic materials adsorbed to negatively charged clay surfaces through polyvalent cations or hydroxy metal polymers (Edwards and Bremner, 1967; T9 Hamblin and Greenland, 1977). The primary SO-MP particles are bound together through their gelatinous organo-mineral coatings to form microaggregates. These coalesece further to form aggregates of varying size. The larger aggregates are less firmly bonded and may undergo cycles of degradation and reaggregation in response to changes in the aqueous environment: wetting and drying, freezing and thawing, pH changes, and changes in the nature and concentration of mineral ions and organic species in the soil solution. The dynamics of aggregate formation and degradation are not well understood, particularly in cultivated soils where structural arrangements are disturbed periodically by tillage and by erosive movement of materials exposed at the land surface (Alberts and Moldenhauer, 1981; Spycher and Young, 1979). Organic materials at varying stages of decomposition or humification are not distributed uniformly among aggregated particles of varying size or density obtained when soils are dispersed in runoff or by shaking in the laboratory. Organic C, N and P are normally higher in clay sized microaggregates than in larger aggregates of silt or sand size, and the cm ratio is narrower. Clay sized aggregates vary widely in density (less than 1.6 to more than 2.0 g/cm). Here, the denser fractions are lower in C than the lighter microaggregates, C/N ratios are narrower, and a larger prOportion of the C can be extracted with alkali. Extractability in alkali and narrow C/N ratios (approaching 10) are characteristic for extensively humified materials. On the basis of such Observations, Spycher and Young (1979) prOpose that the distributions of C and N among water-stable aggregates of different densities in the clay-sized fraction reflect the position 20 of these aggregates within the soil fabric. The lighter microaggregates come from surfaces exposed to the soil biosphere; whereas, denser fractions come from intra-aggregate regions. The organic matter in lighter fractions is of recent origin, fluctuates in phase with seasonal cycles in the below ground living biomass, and is depleted of N because of removal by plants, leaching or diffusion into the aggregated interior matrix. Interiorized organic matter becomes enriched with N (narrow C/N ratios) and can only enter biological cycles again when occluding matrix structures are disaggregated. Methods commonly used for extracting organic matter from soils rely on alkali, acids, chelating agents or cation exchange resins to displace complexing polyvalent cations. Structural arrangements are therefore destroyed, and few inferences can be made regarding how the extracted fractions are distributed through the soil fabric. Various sequential extraction schemes have been use to isolate classes of organic compounds that can be interpreted in relation to biological systems or Spatial arrangements in the soil (Allison, 1973; Hamblin and Greenland, 1976; Schnitzer and Khan, 1972; Tan and McCreery, 1970). An early proximate extraction sequence involved floatation to remove detritus fragments, followed by nonpolar and weakly polar solvents to recover "free" lipids and resins, hot water to recover water-soluble polysaccharides, then mild acid hydrolysis (2% HCl) to remove 'hemicelluloses" (Stevenson, 1965). The water-soluble polysaccharides in soils have been studied extensively (Greenland and Hayes, 1978; Mortenson, 1960; Thomas et al., 1967). Those extracted by hot water are similar to those found, together with FA, in the supernatant after alkali extracts are acidified to precipitate HA. The 2] spectrum of component sugars indicate that they are of microbial origin. Thus, it may be inferred taht they originate at sites of microbial activity on surfaces of larger soil pores. The term ”hemicellulose" for polysaccharides released by mild acid hydrolysis was an early misconception, since they appear to be of microbial rather than plant origin. Their greater resistance to extraction may reflect a higher uronic acid content and more extensive complexation with minerals. However, recent studies suggest that this polysaccharide fraction may be bound by ester linkages to HA (Tan and McCreery, 1970). In any case, the inference that polysaccharides are associated with matrix structures at surfaces exposed to the biosphere is consistent with the wider C/N ratios found by Spycher and Young (1979) in less dense aggregates. Functional Groups in Relation to Soil Acidity, CEC, and Chelation The chemically significant roles of SOM can be attributed to its multi-functional groups. The weak-acid prOperty of most functional groups extends the soil buffering capacity over a wide range Of pH values. Electron donor groups in SOM form complexes with Fe3+, Zn2+, Mn2+ and other heavy metals. Water-soluble complexes with low molecular weight FA may increase plant uptake, while the more water-stable complexes with higher molecular weight HA can reduce uptake and prevent toxic pollutants from contaminanting the food chain. In a soil solution, there are three groups of reactive organic compounds (Geering and Hodgson, 1969). The first group is non dialyzable. This is the FA fraction and has a pKa that can only be 22 poorly defined as 3 to 4.7. The other two groups are dialyzable. with pK values of 4.5 and 9.5. These are attributed to aliphatic acids and amino acids, respectively. Titration curves for HA as initially recovered from soil may indicate pKa values as high as 6.0. After treatment to reduce ash content, apparent pKa values are in a range similar to FA (Coleman and Thomas, 1967). Fulvic and humic acids are hetero-polycondensed polymers. The basic structure is made up of aromatic rings that are held together by -O-, -CH -, -NH-, -N= and -S- linkages. The aromatic rings carry various functional groups (Greenland and Hayes, 1978; Stevenson, 1972; Stevenson and Butler, 1969; Zunino and Martin, 1977). llue substituted functional groups can be divided into three classes: 1. OH-containing groups: carboxyl-OH (-COOH), phenolic-OH, alcoholic—OH 2. N-containing groups: amine (-NH2, -NH-), azo (-N=). heterocyclic ring N 3. Miscellaneous groups: carbonyl (-C=0), methoxyl (-OCH3), sulfonic acid (-SOZ-OH), sulfhydryl (-SH) and phosphonic acid (-PO-(OH2)) The elemental composition of humic substances varies over a relatively narrow range in soils develOped under different vegetative and climatic conditions (Schnitzer and Khan, 1972; Zunino et al., 1975). Fulvic acids contain approximately equal prOportions of C and 0, each accounting for 40 to 50% of the dry weight. On average, HA contains 10% more C but 10% less 0 than the FA from the same soil. All Of the O in 23 FA can be accounted for in functional groups, whereas 20% or more of the O in HA appears to be tied up in condensed bridging structures within the molecule. Fulvic acids may contain more S than HA's (ranges of 0.1 to 3.6% 33:0.1 to 1.5%) and less N (ranges of 0.9 to 3.3% y_s_._ 0.8 to 5.5%). The principal acidic groups in FA and HA are carboxyl and phenolic hydroxyl. Other groups that may dissociate protons at apprOpriate pH's include alcoholic-0H and hydroxyls associated with dienols or with quinonoid structures (Schnitzer and Khan, 1972). Acidic functions have been ascribed also to N in heterocyclic structures where amide-imide tautomerism can occur (Coleman and Thomas, 1967). Total acidic groups in FA (600 to 1400 me/100 g) are generally higher than in HA (500 to 900 me/lOO g). A larger prOportion of the acidity in FA is due to carboxyl groups (average 80% 33;. 54% in HA) (Schnitzer and Khan, 1972). Important additional sources of acidity in SOM are uronic acid carboxyls in polysaccharide fractions which account for 5 to 20% of the organic C in soils (Lowe, 1978). The proton dissociating ability of different functional groups is governed by their own nature and the ambient solution acidity. Titration curves for FA and HA exhibit no sharp inflections. The changes in pH as base is consumed reflect the overlapping acid strengths of heterogeneous functional groups with dissociation constants that decrease in sequence as successive sites are deprotonated (Stevenson and Butler, 1969). 'The dissociability of oxygen-containing acidic groups varies with the kind, number and location of other nearby substituted groups. Thus, conductometric titrations indicate the presence in FA of at least three 24 types Of carboxyl groups with distinctly different acid strengths and capacities for complexing polyvalent cations (Schnitzer and Khan, 1972). Multiply substituted benzenecarboxylic acids may have pKa values as low as 4.0. Weaker carboxyls extend the range of acid buffering to pH 6.5 or 7.0. Substituted phenols and polyphenols may begin to dissociate in the upper part of this range. Above neutrality, more weakly acidic phenolic-0H groups are the principle organic sites of buffering to about pH 9.5, above which significant dissociation of alcoholic-OH groups can be expected (Coleman and Thomas, 1967; Jackson, 1963). As initially extracted, soil humic fractions frequently exhibit apparent pKa values around 6.0 -- substantially higher than after purification treatments to remove complexed cations. It appears that organic functional groups in acid soils may be tied up largely in complexes with hydrated and variously hydroxylated species of Al and/or Fe. In such cases, acid buffering ascribed to SOM may really be due to exchange hydrolysis of hydroxy-Al or -Fe (Coleman and Thomas, 1967). Dissociated acid sites are negatively charged. Individual sites dissociate at a characteristic and constant potential. Their number increases with pH, augmenting the permanent negative charges due to isomorphous substitution and broken edges in crystalline silicate clays. Thus, SOM contributes increasingly to soil cation exchange capacity (CEC) as pH increases (Coleman and Thomas, 1967). Cation exchange capacities reported for FA range from 800 to 1400 me/100 9. For HA the range is 300 to 500 (Bailey and White, 1964). These compare with permanent charge CEC for vermiculite (100-150), montmorillonite (80-150), hydrous mica (10-40), and kaolinite (3-15). Regression estimates (based on clay content, organic C, and CEC in whole 25 soils) indicate that the effective cation exchange capacity (ECEC) of SOM at pH 7.0 is substantially lower than would be expected from values reported for extracted FA and HA (Helling et al., 1964).. “This is due, in part, to the fact that phenolic-OH groups are largely undissociated at pH 7.0, so have no exchange activity. Also, many potential exchange sites are undoubtedly tied up in non—exchangeable complexes with polyvalent cations, especially Al3+ and Fe3+. Reported exchange affinities for HA increase with valence and decreasing ionic radius (Bloom, 1981 a): Na+ < K+ << Mg2+ < Caz+ < Ba2+ The order is similar for FA, except that Na+ is preferred over K+. Metal ions interact with acidic functional groups with bonding strengths that range from electrovalent to covalent. Complexes that involve only carboxyl groups (I and II) retain a greater degree of ionic character than do complexes where the metal is bonded simultaneously to a carboxyl and a nearby weaker acid group as in III or to oxygens in a conjugated double bond system as in IV (Schnitzer and Khan, 1972; Stevenson, 1972). o o o if \i II II 0“” @"jjy 0°» ,w\ ‘E‘ \° ll 0 I II III IV The affinity of important electron donor groups for metal ions 26 is reported to be (Schnitzer and Khan, 1972): O I \ - -c=c- > -MH2 > -N=N- > )N > -coo > -o- >-c=o enolate amine azo ring N carboxylate ether carbonyl In general, the stability of organo-metal complexes increases with increasing valence and decreasing ionic radius of the metal. Complex formation usually results in displacement of protons. The pH drOp when inorganic salts are added to aqueous solutions of FA or HA is often taken as a measure of the strength of bonding (Bunzl et al., 1976; Schnitzer and Khan, 1972). Using this criterion, the stability of FA and HA complexes increases in the following order: Ba2+ < Ca2+ < Mg2+ < Mn2+ < 002+ < Ni2+ < Fe2+ < Zn2+ 2+ 2+ 3+ 3+ << Pb < Cu < Al < Fe Reported relative affinities among metals fOr FA and HA vary with experimental conditions and with criteria used to characterize sites and strengths of bonding (Bloom, 1981; Schnitzer and Khan, 1972). Various Spectrosc0pic methods support the inference from chemical criteria that carboxyl groups are involved in most metal complexes with SOM. The more strongly bound cations bind also to phenolic-0H. Most divalent cations appear to form outer sphere complexes in which the metal ion is completely hydrated and held by forces that are essentially ionic. Direct inner sphere bonding sites do appear to be involved in complexes 2+ and trivalent cations. 3+ of Pb2+, Cu Trivalent Fe3+ and Al in solution are hydrated and hydrolyze to form mono- or di-hydroxy ions: 27 Ratios Of OH/Fe and OH/Al in complexes formed in the laboratory (Schnitzer and Khan, 1972) indicate that it is the hydroxylated products of hydrolysis that enter into complexes with FA and HA: 9 O /(&—on)n Alton); H. n‘f‘,°°‘)\ /(1-0—AI(OH)1 ow pH (Oi-l)m MPH)" ”film 0\Ai(on) Hijher PH (\H. 0\ / \oj [1o] “(OI-()1 At low pH and low ratios of Al or Fe to FA or HA, the more stable mono-hydroxy chelate is formed [Eq. 9]. The mononuclear, dihydroxy complex probably appears after electron donor groups spaced appropriately for chelation are saturated. Binuclear bridging complexes between random sites within the organic polymer or on different polymers probably form also as the concentration of Fe or Al species in solution is increased. Complex formation results in displacement Of protons and neutralization of negative charges associated with acidic functional groups. Solubility of the polymer complexes decreases as net negative 28 charge decreases and cross-linking increases, until a point is reached where flocculation occurs. If base is added to raise the pH, additional protons are dissociated from the aquo shell of Fe or Al, increasing the OH/metal ratio [Eq. 10]. As this ratio increases, the complexes acquire a net negative charge, and solubility increases again. In laboratory extractions or titrations, metal complexes with FA and HA begin to dissolve as pH rises above 7.0. Once peptized by base, the dissolved complexes remain in solution when acid is added until about pH 2.0, where HA's flocculate due to loss of charge by protonation of acidic groups. Flocculation at low pH is probably promoted also by inter- and intra-molecular H-bonding. Acidification tends to reverse the reactions that lead to complexation. However, FA and HA fractions separated at pH 2.0 retain substantial quantities of metals (5% ash or more) that cannot be removed by dialysis or electrOphoretic methods. Treatment with cation exchange resins can reduce ash contents to 1.5% or less. Calculated stability constants for soluble FA and HA complexes with metals are considerably lower than for complexes with synthetic chelating agents such as EDTA. Stabilities increase with increasing pH up to the point where more stable metal oxides start separating out as a separate phase. It appears that separate-phase hydroxy-Al polymers may start forming competitively with chelation processes at about pH 5.0. However, pre-formed Al complexes with FA remain stable to about pH 8 before breaking up to form amorphous Al(OH)3 polymers. The critical iHl for formation of separate-phase hydrous oxides of some other complexed 3+ 2+ 2+ metals is higher -- pH 8 for Fe and pH 10 for Cu . Apparently, Ni 29 remains complexed even at pH 10 (Schnitzer and Khan, 1972). Complexation reactions of a given metal are influenced by the nature and concentration of other ionic species present (Schnitzer and Khan, 1972; Zunino and Martin, 1977). The ionic strength of the soil solution is normally low, approaching zero. However, drying and freezing increase ionic concentrations in residual water films. When fertilizers are applied, electrolyte concentrations in the zone of placement may approach saturation and remain high for periods Of time, depending on method Of placement, moisture regimes, crOp removal and the capacity of soil systems to adsorb ionic species released by dissolution Of different fertilizer components. Increasing ionic strength increases the competition between different ions for reactive sites. An increasing prOportion of available organic functional groups are occupied by mononuclear complexes with relatively low stabilities. Stability constants decrease with increasing ionic strength. The decrease is more rapid for the more strongly bound cations (Fe3+. A13+Q 3+ and Cu2+, Ni2+). When ionic strength is increased to about 0.2 M, Fe Al3+ complexes with FA begin to flocculate (Schnitzer and Khan, 1972). Interactions between complexed cations and inorganic anions can influence the prOperties of complexes formed (Dhillon et al., 1975; Schnitzer and Khan, 1972). Thus, at low to medium metal/FA ratios, orthOphosphate bonds to complexed Fe or Al to form organo-metal phosphates in which the P is non-exchangeable and poorly available to plants. The P in similar complexes involving Mn is readily available. At higher metal/FA ratios, available sites for bonding with FA are weaker, and the metal phosphates precipitate as a separate phase. Flocculation (coagulation) through complexation with polyvalent 3O cations is undoubtedly an important mechanism producing stable; water-insoluble SOM in field situations. As recovered from soils (after peptization with base, precipitation with acid, and dialysis to remove excess salts), humic substances are hydrOphilic and water-soluble due to the mutual repulsion of negative charges at dissociated acid sites. The peptized FA and HA molecules may be viewed as linear polymers or as a three-dimensional cross-linked network. In either case, addition of salts leads to flocculation. Trivalent cations are most effective. Flocculation may be ascribed, in part, to reduced repulsion between negative charges due to the presence of added cations. Also bridging complexes are undoubtedly formed. These draw spatially separated structural components closer together. Linear molecules will tend to coil and three-dimensional networks will tend to contract. As water is expelled, coiled or contracted molecules become increasingly hydrOphobic (Schnitzer and Khan, 1972). When flocculated HA-metal complexes formed in the laboratory are air-dried, they are hydrOphobic (Greenland, 1971). In the field, dehydration of flocculated metal complexes due to ageing and cycles of drying or freezing may reasonably produce the very durable, non-extractable humin fractions of SOM. The residual ash constituents found in FA and HA fractions after extensive purification may also be held in such contracted, dehydrated molecular complexes. The organo-metal complexes in soils are further stabilized by adsorption on mineral colloids. In highly weathered trOpical soils and in soils of volcanic origin, the adsorbing mineral phase may be largely amorphous sesquioxides of Fe and Al. However, in soils of temperate regions, the primary unit of stable structure involves a central 3] crystalline clay matrix. This is the SO-MP prOposed by Edwards and Bremner (1967) and Spycher and Young (1977). Mechanisms Of interaction between SOM and crystalline clay minerals will vary with molecular size of organic molecules and the nature and steric distribution of functional groups, and with the nature and prOperties of the clay (its specific surface and the density, sign (+ or -) and location of sites of charge in the crystal lattice), and with the nature and prOperties of exchangeable cations (Greenland, 1971; Hamaker and Thompson, 1972; Mortland, 1970). Initially, adsorptive forces are mainly coulombic: anion exchange, ion-dipole interactions and H-bonding to “water bridges“ coordinated to exchangeable cations. Dominant factors affecting the energy of adsorption are the valence, electronegativity and hydration status of metal cations present in the system. With divalent cations, it appears that ion-dipole interactions and H-bonding to coordinated water shells are mainly involved. In the case of transition metals, such as Cu2+, a degree of covalent bonding may arise by charge transfer from unsaturated organic structures. The most energetic adsorption complexes with crystalline 3+ or Al3+ as the bridging cation. Ligand clays are formed with Fe exchange (inner sphere coordination) is probably involved. With aging and dehydration, weaker coulombic attractions and physical forces become additively more significant: H-bonding to surface oxygens or hydroxyls in the clay lattice, van der Waal's forces, and entrOpy effects due to displacement of adsorbed water at hydrOphobic interfaces. As in the case of dried metal-HA flocs, the resulting SO-MP would be expected to be hydrOphobic and water stable. Amorphous sesquioxides may have a greater capacity for adsorbing 32 FA and HA than do the crystalline clays (Greenland, 1971; Schnitzer and Khan, 1972; Spycher and Young, 1977). Adsorption probably involves anion exchange at sites of positive charge and ligand exchange for coordinated -OH or -OH2. Similar interactions probably occur at broken edges Of crystalline clays. However, such sites are sensitive to seasonal or long term changes in pH, ionic strength and hydration status. Surface organic complexes are subject to breakdown as separate-phase sesquioxide polymers or crystalline oxides form at higher pH, or with ageing and dehydration. Thus, SOM associated with amorphous clays is less durable than that associated with crystalline clays. The forces leading to stable organo-metal complexes derive mainly from the permanent negative charges in the lattices Of crystalline and semi-crystalline aluminosilicates. Over time, weaker forces acting at the clay surface are additive and become increasingly important. In soils develOped under similar conditions of climate and drainage, the organic C content is correlated closely with the quantity and kind of crystalline clay minerals present (Mortland, 1970). ROLES OF ALUMINUM IN SOILS Next to oxygen (0) and silicon (Si), Al is the most abundant element in igneous rocks and basalts from which soil parent materials are derived. Rocks and constituent minerals that are not at equilibrium under conditions of pressure, temperature and moisture at the interface between atmosphere and lithosphere are subject to physical disintegration and chemical alteration, leading toward a new equilibrium 33 state (Buol et al., 1973; Greenland and Hayes, 1978). These processes are exothermic and tend to transform the original materials to the lowest energy level stable at ambient conditions (Bohn et al., 1979; Foth, 1978). Initial weathering processes produce unconsolidated deposits of rock fragments that are the parent materials for soils. Further chemical alteration proceeds as primary mineral structures are exposed to the action of water, C02 and 02. Principle mechanisms are hydration and the displacement by protons of alkali and alkaline earth ions from interstices in an O-coordinated framework of Al (or Mg) and Si atoms. In these positions, the cations counter the residual negative charge of oxygen not satisfied by Al (or Mg) and Si (Pauling, 1960). Thus, they give stability to the crystalline structure. Initially, protons released by dissociation of water and by hydrolysis of CO2 are effective in displacing interstitial cations. Protization is accelerated as stronger acids (H2504, HNOB, organic acids) enter the system through activities in the biosphere (Jackson, 1963). The nature of secondary minerals formed is influenced in the beginning by the chemical composition and grain size of primary minerals present in the parent materials. Over time, however, the weathering environment becomes increasingly important. Dominant factors are pH and the nature and concentration of basic cations present. These, in turn, are governed by the rate at which different cations are removed by leaching (Buol et al., 1973; Foth, 1978; Grim, 1968; Jackson, 1963). In calcareous parent materials, weathering of primary minerals is 2+ retarded until Ca levels are reduced by removal Of carbonates. 34 Primary minerals may then be converted directly to secondary aluminosilicates by differential removal of basic cations. Or, removal of interstitial cations may destabilize the primary Al-O-Si or Mg-O-Si structures, releasing hydrated silica and alumina which then recombine 2+ and/or K+ to fOrm crystalline secondary aluminosilicates. 2+ with Mg Where rainfall and drainage conditions allow Mg and/or K+ to remain in the weathering zone, various 2:1 lattice aluminosilicates, such as montmorillonite, will appear. If KIr remains relatively more abundant than Mg2+ , a sequence of products may appear: hydrous mica (illite) ---> vermiculite ---> montmorillonite. Further removals of KI' and MgZ+ leads to the formation of 1:1 aluminosilicates (kaolinite). As basic cations are depleted further and pH declines, protonation and hydration of Al-O octahedra destabilize 1:1 lattice structures and 3+ release Al(H20)6 and SiO2 which, on hydration, yields silicic acid (SiOz.H20). These monomeric species may polymerize to form separate phases of amorphous alumina and silica. Or they may recombine to form amorphous aluminosilicates (allOphane) which, on ageing, may acquire a 2+ + degree of crystallinity. If, at this stage of weathering, M9 or K are introduced in surface drainage from other areas or in the form of lime or fertilizers, resynthesis of 2:1 layer silicates may occur. Potassium fixation, due to reversion of vermiculite to mica is a significant factor in availability of fertilizer K in some soils (Foth, 1978). The activities of Si and Al are pH dependent, therefore sensitive to the degree of base-saturation of exchange sites. The ratio of reactive species of Si to reactive Species of Al is an important factor determining the nature of secondary minerals that may appear in soils 35 (Grim, 1968). Acidity favors condensation and precipitation of hydrated SiO2 and promotes the removal of Al and Fe. Neutral to alkaline pH promotes the removal Of silicic acid and the retention Of polymeric hydroxides of Al and Fe. For example, feldspar may hydrolyze to 2:1 layer silicates (as montmorillonite) under moderate leaching, to 1:1 minerals (as halloysite or kaolinite) under more severe leaching, and to Al-hydroxides (bauxite) under severe leaching in the tropics (Jackson, 1963). Chelation is another factor affecting reactive Si/Al ratios (Buol et al., 1963; Grim, 1968). In temperate climates, decomposition of surface litter under forest vegetation is relatively slow, and a rather continuous supply of low molecular weight organic ligands is produced. Chelation reduces the susceptibility of Al to polymerize and facilitates its movement away from sites where it might recondense with silica. Soils formed under these conditions in coarse-textured parent materials have surface eluvial horizons enriched with silica and illuvial subsoils enriched with Al and Fe. In the trOpical regions, chelating substances appear only transiently because decomposition is so rapid that Al is readily stabilized in surface horizons by polymerization of its hydroxides. Polymerization is likely promoted also by the higher average annual temperatures and by extended dry periods in monsoon climates. Oxygen in 2:1 silicates retains or acquires a residual negative charge due to isomorphous substitution by cations of lower valence for 4+ in tetrahedral layers or for Al3+ in octahedral layers. These Si charged structures are stabilized by exchangeable basic cations and by organO-mineral coatings adsorbed on their surfaces by various 36 mechanisms, among which bridging through polyvalent cations is probably most important. Further protection is affOrded as gelatinous coatings coalesce to bind primary particles into aggregates (Jones and Uehara, 1973). As noted earlier, Spycher and Young (1979) have suggested that the shielded interiors of microaggregates may provide an environment where K+ and/or Mg2+ , diffusing inward from the soil solution and from sites of weathering near aggregate surfaces, can recombine with amorphous and semi-crystalline alumina and silica in cementing gels to form crystalline aluminosilicates. After laboratory treatments to break down aggregates and remove organO-mineral coatings, secondary crystalline aluminosilicates appear mainly in particle size fractions less than 20um in diameter (fine silt and clay). l>robable weathering sequences in a given soil can be inferred from X-ray diffraction patterns for these purified crystalline fractions, together with data on the cation exchange prOperties and Si/Al ratios of selectively dissolved amorphous and semi-crystalline fractions (Foth, 1978). Such data obtained by Schafer (1968) for soils from experimental plots sampled in the present study indicate the probable sequence: NYdrous mica (illite) ---> vermiculite ---> kaolinite ---> amorphous Al(OH)3 ---> allOphane-like, amorphous to weakly crystalline silicates. Montmorillonite was not found. Its absence in the sequence was consistent with the low levels of exchangeable Mg2+ encountered. In coarse-textured (sandy) parent materials, weathering products 37 formed in upper horizons move as colloidal su5pensions or soluble species in percolating water and then precipitate or recondense on entering an illuvial horizon where bases have accumulated, or at depths determined by seasonal cycles of wetting and drying (Buol et al., 1973; Foth, 1978; Grim, 1968). In less porous parent materials, when undisturbed, extensively aggregated, water-stable structures develOp. Tillage breaks up larger aggregates, exposing previously shielded surfaces to biological activity and to leaching and chemical weathering (Rovira and Greacen, 1957). Additions of fertilizer increase the concentratons of solutes and active sources of acidity in the weathering zone. These effects of cultivation serve to accelerate pedochemical changes leading to lower energy states and to soils of lower inherent productivity (Bohn et al., 1979; Buol et al., 1973; Coleman and Thomas, 1967; Greenland and Hayes, 1978). Forms of Al in Relation to Soil Acidity Jackson (1963) pointed out that Al and Si are the principal O-coordinators in soil minerals -- a role that is analogous to that of C in the acidic functional groups of organic compounds. In important primary minerals, both Al and Si appear in tetrahedral coordination. Due to protonation and hydration on weathering, Al becomes 6-coordinated in secondary minerals. The increase in O-coordination leads to an increase in acidic prOperties. Because of residual negative charges stabilized within their lattices, the crystalline secondary minerals behave as moderately strong acids. 38 The acid function of permanent negative charges in crystalline clay minerals is not expressed directly in equilibria involving H+ ions. Rather, the effect of crystalline clays on acid buffering in soils is expressed indirectly through the nature and prOperties of cations held exchangeably at their surfaces. It appears that this may be true also for the negative sites that arise as acidic functional groups in soil organic colloids are deprotonated; H+ equilibria associated with these sites are probably controlled by hydrolysis of Al or Fe complexed at these sites (Coleman and Thomas, 1967; Hargrove and Thomas, 1981; Mehlich, 1981). Monomeric Al released by weathering Of primary or secondary minerals appears as the hydrated cation, Al(H20)63+, which is itself an important weathering acid with pKa - 5 (Jackson, 1963). The buffering properties of acid soils (pH < 7.0) is influenced strongly or determined largely by the cation exchange and ligand exchange prOperties of Al, and its hydrolytic reaction products (Coleman and Thomas, 1967; Jackson, 1963; Kamprath and Foy, 1971; Schnitzer, 197B; Schnitzer and Khan, 1972). Acid buffering due to Al species over the pH range from 4.0 to 8.0 may be outlined as follows: H30+ H30+ (6-x) + nAl(H 0) 3+;sé:=é [Al(0H) .(6-x)H 0] "IT" I sé4:=> [Al(0H)3] 0[11] 2 6 pH 4_7 X 2 n an 7'8 n The course of hydrolytic reactions and the products formed depend on the pH of the system, the nature and prOperties of clay minerals present, the nature and concentration of other ions present, the rate of hydrolysis, temperature, and time (Bache and Sharp, 1976). At low pH, 39 the dominant Al species are monomers and low molecular weight polycations. As pH increases, deprotonation and dehydration lead to a series of polymers with increasing OH/Al ratios and decreasing charge per Al atom: 3+ 2+ + Al(H20)5 s ----- é Al(0H)(H20)5 + H30 [12] 3+ 2+—=====§ 4+ Al(H20)6 + Al(0H)(H20)5 ‘ Al2(OH)2(H20)8 + H3 4+ ______ s n+ + nAl2(0H)2(H20)8 . ------ [Al2n(0H)5n(H20)2n] + 3nH30 [14] 0+ + H20 [13] Due to its greater charge, the trivalent cation in Eq. [12] is adsorbed more strongly by crystalline clay minerals than the monohydroxy cation. Polycations, however, are adsorbed even more strongly because of their larger size, and increasingly with increasing degree of polymerization [Eq. 14]. Thus, the activities of Al in the soil solution are probably dominated by equilibria involving the monomers and the dimer in Eq. [13]. Their concentration is determined by their equilibria with exchangeable forms of Al in Eq. [12] and Eq. [14]. Above pH 5.0 or 5.4, the concentration of Al species in solution decreases abruptly due to their rapid removal into non-exchangeable interlayer precipitates and surface coatings with OH/Al ratios of about 2. Precipitates of higher OH/Al ratio will appear at higher pH. Also, supersaturation with respect to amorphous or crystalline Al(0H)3 may occur (Lindsay, 1979). With aging, hydrolysis and dehydration proceed slowly and spontaneously, leading to the formation of separate amorphous or crystalline phases of Al(OH)3 (Coleman and Thomas, 1967; Hsu, 1966). The reaction may be viewed as a dismutation in which portions of the polymer are deprotonated while Al at surfaces or crystal edges is protonated: 4O [Al(0H)2]n+ s=====s §pIAl(0H)3] + ‘gAl3+ [15] In Eq. [15], water of hydration has been ignored. Due to the dismutative nature of polymerization, residual surface charge on hydroxy-Al polymers is not fully neutralized until about pH 8. At higher pH, OH/Al ratios continue to increase, and hydroxy-Al surfaces become negatively charged. The relation of monomeric and polymeric forms of Al to pH ranges of significance in management of soil have been outlined by Coleman and Thomas (1967): Range Of pH in Reactant with added base acidity water I < 4.0 H3O+ II 4.0-5.6 A1(H20)63+ III 5.6-7.6 "Strong“ hydroxy-aluminum IV > 7.6 “Weak" hydroxy-aluminum A pH of 4.0 is the lower limit (the "ultimate pH") fOr soils, unless free mineral acids are present, such as H2304 (Jackson, 1963). Hydronium is the principle weathering acid in nature. When it is adsorbed by crystalline minerals, H30+ is unstable and dissociates. The protons migrate to sites Of negative charge within the lattice or at lattice edges. This leads to instability and accelerated breakdown of lattice structures. Free H30+ is then rapidly removed from solution by hydrolytic reactions with released Si, Al, and basic cations. Breakdown of primary and secondary minerals is the ultimate buffering mechanism in Range 1. Major agricultural soils of the world fall in Ranges II and III. 4] Three forms of acidity are of concern in their management: "active", “exchange", and “titratable” (or ”total") acidity (Coleman and Thomas, 1967; Kamprath and Fay, 1971). Active acidity(soil pH) is controlled by organic functional groups that dissociate and by Al species that hydrolyze in soil solution or when a soil sample is suspended in water. At salt concentrations normally present in soils, the concentration of Al in displaced soil solutions is the same as the solubility of Al in water at the same pH. It is generally considered that the principle species of Al in soil solution are those in Eq. [12] and [13]. However, during periods when root exudates or decomposing detritus are present, Al complexed with low molecular weight organic ligands [Eq. 9, 10] are undoubtedly present also (Hoyt and Turner, 1975; Kwong and Huang, 1979 a,b). Exchange acidity is estimated in the laboratory from the difference in sum of H)r plus Al3+ between suspensions in water and in unbuffered 1N salt solutions (KCl, NaCl, CaClz). A decrease in pH is attributable mainly to hydrolysis of exchangeable Al species displaced into solution. These include Al(H20)63+ as well as some of the lower molecular weight polymers in Eq. [14]. Polycations containing 10 or more Al nuclei and with OH/Al ratios approaching 2 have been reported (Bache and Sharp, 1976; Jackson, 1963). In the field, displacement of exchange acidity accounts for pH lowering and increased concentrations of Al and Mn -- "salt effect” -- in the vicinity of applied fertilizers (Coleman and Thomas, 1967; Kamprath and Fay, 1971). Titratable acidity, or total acidity, is the amount of acid neutralized by added base to a given pH. Theoretically, it is equal to 42 all of the active and exchange acidity associated with SOM and Al species with OH/Al ratios < 2, plus residual charge associated with non-exchangeable Al polymers up to the reference pH, usually pH 7.0. Base may be consumed also by exchange of hydroxyl for adsorbed sulfate or phosphate, if these are present (Mehlich, 1981). Nearly all of the titratable acidity in soils may be exchangeable at pH 4, where the soil CEC approaches saturation with Al(H20)63+. With increasing pH, the proportion of exchange acidity to total acidity decreases as acidic functional groups in SOM are dissociated and as Al is precipitated in non-exchangeable interlayer deposits and surface coatings on crystalline clay minerals. The decrease in exchange acidity above pH 5.0 or 5.5 is rapid. Exchange acidity becomes negligible above pH 6.0, when exchangeable Al accounts for only 10% or less of the exchangeable cations in soil. In many soils, considerable titratable acidity will remain above pH 6.0 in the form of non-exchangeable polymers with OH/Al ratios > 2. This will decrease with increasing pH, whereas acidity associated with organic functional groups will increase. In temperate climates, estimates of titratable acidity are used as the basis fOr determining how much lime should be applied to raise soil pH into a range favorable for the crOps being grown (frequently pH 6.5 to 7.0). In trOpical and sub-trOpical soils, liming criteria are based on controlling toxic concentrations of soluble Al at low pH's (Coleman and Thomas, 1967; Kamprath and Fay, 1971). Estimating lime requirements is complicated by the fact that non-exchangeable Al polymers with OH/Al ratios >2 react slowly with added base. Routine advisory estimates are derived from measurements of 43 exchange acidity or from the decrease in pH when soils are suspended in apprOpriately calibrated buffer solutions. These laboratory determinations are multiplied by some arbitrary factor (usually 2x) to allow for non-exchangeable charged Al that may react with lime over a period of months or years. Further complications arise in the field. The reaction rate of applied lime is affected by a number of factors, among them: particle size and purity of the liming material, the prOportion of dolomite (less soluble) to calcite (more soluble), and the degree of incorporation into the soil. In very acid soils containing large quantities of exchangeable Al, the addition of lime may result in the formation of Al gels in such volume that lime particles are occluded and their reaction retarded for long periods of time (Coleman and Thomas, 1967). Moschler et al. (1962) Observed that soil pH was still rising 5 years after applicatnniof 18 T/ha of dolomitic lime. In plots sampled for the present study, soil pH was still rising 14 years after 27 T/ha of dolomitic lime was applied on plots where soil pH had been lowered to 4.2 by previous applications of (NH4)ZSO4 (Burutolu, 1977). ‘Very weak acid buffering in Range IV (pH > 7.6) is derived mainly from phenolic-0H in SOM and from surface charges on non-exchangeable Al polymers. Bicarbonate equilibria become increasingly important also (Jackson, 1963). Even in acid soils, pH levels well up into this range may exist for periods of time in the placement zone after applications Of ammoniacal fertilizers. Violent seasonal fluctuations in pH, due to hydrolysis of NH3 and its subsequent nitrification, undoubtedly produce drastic changes in physical and chemical prOperties Of soil organO-mineral complexes in the affected soil volume. 44 Variable Chargg, Ion Exchange and Specific Adsorption Sites of negative or positive charge influence the internal soil environment in many important ways (Coleman and Thomas, 1967; Jackson, 1963; Mehlich, 1981). The negative charges stabilized in lattice structures of 3-layer phyllosilicates with Si:Al ratios of 2:1 or greater are not affected by pH and are the primary source of “permanent“ or "constant“ charge CEC (CECc). Sites of negative charge in SOM are pH-dependent and contribute to “variable charge“ CEC (CECv). Sites Of pH-dependent variable charge in 2-layer phyllosilicates with Si:Al ratios < 2:1, in sesquioxide coatings and interlayers, and in amorphous sesquioxide clays may contribute to CECv, to anion exchange capacity (AEC), and to anion sorption capacity (A50). The role played by a given site of variable charge at an interface is determined by atomic arrangements and bonding strengths within the core structure, and by the pH of the ambient solution and the nature and concentration (activity) of solute ions (Sposito, 1981). Effects Of pH on charges associated with a surface Al atom are illustrated in Eq. [16]: _ 1 _ _ \IL/ 004' r50 \AL/ T OH’ 1.50 T \ . 0.5- / | \f" om“. M <1), 1“ 0,3" ELL \I /OH w°\ / \ < “f /Al\ 0* [15] 35/ I \OH:'" HO OH ' OH \ I ’0” ; \A'./OH '- I o /AI‘\ ”I / |\ H L _ E _ L ._ 45 The central core structure depicted in Eq. [16] is that of crystallirue 3+ ions reside in octahedral coordination between two Al(OH)3 in which Al closely packed layers Of OH' ions, and the A13+ is distributed in multiples of hexameric rings (Hsu, 1968; Jackson, 1963). Coordinated aquo and hydroxo groups outside the core structure carry 1/2 unit charge. Net charge on the surface varies with degree of protonation (lower arrows) and with ligand exchange (upper arrows). Similar equations may be written for octahedrally coordinated Fe3+ (Mehlich, 1981; Parfitt and Smart, 1978). Anion Exchange and Anion Sorption-- Sites Of positive charge due to protonated Al3+ or Fe3+ (left side of Eq. [16]) may be countered by the constant structural charges in crystalline clay minerals or by anions held electrostatically or in weak (outer sphere) coordination (Mehlich, 1981; Mekaru and Uehara, 1972; Sposito, 1981). Anion sorption at these sites can be non-specific, in which case, these sorbed anions can be displaced exchangeably by 01', or ClO4-. The anion exchange capacity (AEC) is commonly estimated by saturation with NO monOprotic acids, as NO ' 3' followed by displacement with Cl'. Polyprotic mineral acids and organic acids may be held exchangeably but are subject also to specific adsorption by inner sphere coordination, or ligand exchange. This involves displacement of aquo or hydroxo ligands in Eq. [16] by an oxo ligand (coordinated -O') of the acid radicle. The specific interaction that occurs and its effect on surface properties will vary with pH and the nature and concentration Of counter ions present. Rajan (1978) has proposed the following sequence 46 of reactions between 5042' and hydrous alumina at pH 5.0: + 2- . - - . . : OH -. E/Ofla- O\ //0 . .1AI/ ’- 1'6) + 5 :\0 /° + H0 [17] ” :“cny_ o/'<\O ‘ ' i\‘ / z . /$\o 0 J __ O . .. . OH 1° 2 o ,o \ /° z/ \ / —_S . / j l s . - O ‘N ‘ ,.4\°H1 + / \ -.A-'/ o + 0H [18] ' O O ' :\0H ' l - L ' J : .. .. OH "'4' in/ I o ... _o 0’ 3“0 4' _______s, /'z '\. 47 - ' “ ° \\ ““‘——' \n’ /'SS 1 ‘OH‘‘ O J _-\ 0H, 3 OH 2‘ at pH 5.0 2.. Rajah (1978) Observed that initial additions Of SO4 displaced mainly aquO groups [Eq. 17]. With increasing SO4 concentration, an increasing proportion of hydroxo groups was displaced [Eq. 18]. The final surface after 5042' sorption carried close to zero charge. 'ha achieve a neutral surface, a further reaction was proposed in which a hydroxo or aquo group on an adjacent Al atom was displaced to form a bidentate complex [Eq. 19]. Infrared studies have shown that binuclear complexes are involved in the specific adsorption of both $042- and H2P04' by Fe oxides (Parfitt and Smart, 1978). Similar complexes are to be expected when either anion is adsorbed on hydrous Al oxides. Phosphate is adsorbed more strongly than sulfate, however, and in larger amounts at any one pH 47 value. Organic acids also form inner sphere complexes with Al and Fe oxides through ligand exchange for carboxyl oxygen. Benzoate forms a monodentate complex (Parfitt et al., 1976), oxalate forms a binuclear bridging complex (Parfitt and Smart, 1978), and citrate forms a tridentate chelate (Kwong and Huang, 1979 a,b). Organic and inorganic anions compete for adsorption sites. Anion sorption capacities (ASC) of soils high in organic matter are low (Mehlich, 1981). On the other hand, prior additions of organic matter to soils may have little effect on P sorption; whereas, additions of P to the same soils will reduce their capacity for adsorbing organic matter (Yuan, 1980). Thus, it appears that P is adsorbed more strongly than organics at sites for which they compete, but organics are sorbed also at sites and by mechanisms that are non-competitive with P. Sorption of organics increase with increasing Si:Al ratio Of mineral colloids, whereas sorption of P decreases. Specific adsorption of anions interferes with orderly polymerization and crystallization which normally occur as hydroxylation and dehydration proceed [Eq. 16]. Sesquioxides precipitated in the presence of specifically adsorbing anions have a large specific surface due to smaller size of stable polymers (Hsu, 1968; Rajan, 1978) and/or to stabilization of a porous, Open structure in the polymer (Kwong and Huang, 1979 b). Surface activities associated with sites of variable charge remain high. Important among these surface activities are anion exchange, specific anion sorption, cation exchange, and specific cation sorption. In general, specific adsorption of anions increases the net 48 negative charge of the surface and increases ECEC (Alvarez et al., 1976; Bloom, 1979; Hsu, 1968; Mekaru and Uehara, 1972; Sposito, 1981). In the case of multi-functional organic molecules, the increase in negative charge and ECEC due to sorption may be augmented by non-complexed acidic groups of the same molecule (Kwong and Huang, 1979 b). Cation Exchange and Cation Sorption-- The pH-dependent negative charges that arise by hydroxylation or ligand exchange [Eq. 16], together with the pH-dependent acid functional groups in SOM are mainly responsible for variable charge cation exchange capacity (CECv). The sum of CECv plus CECc is the ECEC. In the lab, ECEC is determined as the equivalent sum of cations 3+ 2+ + K+ + Na+) displaced by a neutral unbuffered (H+ + Al + Mg2+ + Ca salt solution (Coleman and Thomas, 1967; Mehlich, 1981). The ECEC is approximately equal to CECc in soils of pH 5.0 or less, where most Of the charged Al Species are exchangeable and where exchangeable HJr plus Al3+ account for 50% or more of the exchangeable cations (Kamprath and Foy, 1971; Mehlich, 1981). At about pH 4.0, the CECc approaches saturation with Al and can be estimated as the sum of A13+ plus H+ diSplaced by a neutral unbuffered salt solution (after first removing soluble salts). Above pH 5.0, hydrolytic reactions of Al affect cation exchange properties in two ways (Coleman and Thomas, 1967; Kamprath and Foy, 1971): 1. Above pH 5.0, ECEC quickly becomes greater than CECc due to decreasing positive charge and increasing negative charge in sesquioxide coatings and separate phases with OH/Al of 2 or more [Eq. 11, 14, 16]. 49 If the pH of a soil high in sesquioxides is increased.by liming from pH 5 to pH 6, there may be as much as a 50% increase in ECEC. 2.'Hmaexchangeability of Al decreases due to polymerization. Exchangeable Al, as percent Of ECEC, decreases from 50 to 60% in the range of pH 4.9 to 5.2, to 10% or less at pH 6, and approaches zero at. pH 73 'The equil Harium concentration of Al species in solution is very low when Al occupies less than 60% of the ECEC, then increases rapidly as non-exchangeable Al is mobilized by protonation below pH 5 or 5.4 and percent Al saturation of ECEC increases. In addition to non-specific cation exchange, polyvalent cations nwy be adsorbed specifically by interaction with oxygens on the sesquioxide surface. The strength and extent of adsorption depends less on the nature of the sesquioxide (Al or Fe) than on the nature and concentration of the adsorbed cation and the pH of the system (McBride, 1978). Structure V is based on the Observation that one proton appears in solution for each divalent cation adsorbed: AlOH + M2+ §======3 AlO-M+ + H+ [20] O“: l '3 I OH \ / \Ai/ __ >M/ \Cu/ o’n‘o’ I \OH L H: 1 Cl) ‘0’ ‘o O\AI/O\A'l /0H 2. O\Al /0\Al/O /| \o’l \ J / \0/ \ H v VI This mass action model ignores the fact that, as H+ in solution 50 increases, protons are also adsorbed. When the simultaneous adsorption of H+ is accounted for, it appears that two protons are displaced for each divalent cation adsorbed (Forbes et al., 1976). Thus, the adsorbed cation apparently attaches to two separated sites on the polymer surface or to sites on two different polymeric units. If the adsorbed cation has an ionic radius not too different than Al and has a similarly high electrochemical potential, it may enter into the hexagonal structure of the octahedrally coordinated Al or Fe polymer, as shown in V1 for Cu2+. When c0precipitated with alumina at near neutral pH, MgZ+, Cu2+ 3+ and Fe appear to enter into octahedral sites throughout the alumina structure (McBride, 1978). Isomorphous substitution Of the divalent 3+ cations for Al increases the number of negative charges on the alumina gel, as evidenced by change in electrOphoretic mobility and an increased capacity for adsorbing other cations. The larger ionic radius of Mg2+ (0.65 A) and Cu2+ (0.72 A), compared with 0.50 A for Al3+, distorts the alumina structure, makes it more porous (larger gel volume, increased specific surface), and reduces the rate at which the gel crystallizes on aging. 2+ is held much Despite their relatively equivalent ionic size, Cu more tenaciously than M92+. Much of the c0precipitated Mg2+ can be removed by leaching at low pH. That which remains is probably ”buried" mechanically in the alumina structure, since no significant permanent charge is retained. The much more stable Al-O-Cu bond is not broken by acid leaching, and surface charge is altered residually. Larger ions, such as Ca2+ (0.99 A) and Mn2+ (0.80 A) are adsorbed by surface mechanisms, but probably do not take up structural positions in alumina. They are likely retained at external or internal sites Of 5] excess negative charge. The relative affinity of alumina for alkaline 2+ > Ca2+ 2+ > earth metals has been found to follow the order: Mg > Sr Ba2+. The variation in affinity suggests that direct metal-surface oxygen interactions are involved (McBride, 1978). Forbes et al. (1976) found that the adsorption of a group of transition metals by goethite was consistent with an electrochemical model in which surface concentrations of M2+ and H+ were defined in terms of the difference in electrical potential between the plane of 2+ and the plane of adsorption of H. It was assumed adsorption of M that H+ was adsorbed in the plane of the oxide surface. Adsorption increased with increasing pH and decreasing surface charge (decreasing surface H+). COpper (II) and Pb2+ were adsorbed in appreciable quantities at pH 4.7 and approached maximum values at about pH 6.0. Zinc (II), Co2+ and Cd2+ were not adsorbed below pH 6.0 and approached maxima at pH 7.2 to 8.0. COpper appeared to be adsorbed in the surface oxide plane; others were diSplaced from the surface in direct relation to their intrinsic affinities. Apparently, the energy of adsorption was influenced by two Opposing factors: (1) intrinsic affinity (related to atomic weight) and (2) interaction between cations of the same species which increased with surface coverage and increasing ionic radius. Relative strength of adsorption increased in the order: Cation Cu2+ >» sz+ :> 2n2+ :> to2+ z» Cd2+ Atomic weight 64 207 65 59 112 Ionic radius 0.72 1.20 0.74 0.72 0.97 MATERIALS AND METHODS EXPERIMENTAL SOIL MATERIALS SOURCE Soil materials used in this investigation were taken from a field experiment initiated in 1959 on the Soil Science EXperimental Farm at Michigan State University, East Lansing. The soil has been classified as Hodunk sandy loam (Ochreptic Fragudalf). The soil series description is given in Appendix Bl. MANAGEMENT H I STORY The purpose of the field experiment was to compare N sources in terms of crap response and residual effects on soil acidity. The eight N carriers in Table 1 were applied annually on corn at the rate of 336 kg/ha. A uniform basal fertilizer (Table 2) was applied on all plots receiving the experimental sources of N and on one set of control plots. A second set of control plots received no basal fertilizer and no fertilizer N. The ten treatments were assigned to plots (4.3 x 7.6 m2) laid out according to a randomized complete block design with four replications. In the seventh year of the experiment (1965), liming treatments were 52 53 Table 1. Percent nitrogen and relative residual acidity of eight nitrogen carriers used in the long term field study . N carrier N Relative residual acidity* % (NH4)ZSO4 21 5.3 NH4Cl 26 5.3 NH4NO3 33 1.8 NH3 82.2 1.8 Urea 46.0 1.8 Ureaform 38.0 1.8 Ca(NO3)2 15.5 -1.3 NaNO3 16.0 -1.8 I Farm Chemicals Handbook, 1977. * Based on assumption that residual acidity is equal to one half of theoretical for complete nitrification (3.6 kg CaCO / kg N) plus acid equivalen§+of anions (SO4 , Cl') or minus bas c equivalent of cations (Ca , Na . Table 2. CrOp and fertilizer history (1959-80)+. Basal fertilizer Annual nutrient Year CrOp Annually N-RZOS-KZO N P K kg/ha kg/ha 1959-72 Corn 224 5-20-20 11 19 37 1973 Wheat 168 6-24-24 10 18 34 1974-75 Soybeans 392 12-12-12 47 20 39 1976 Soybeans 112 0-26-26 0 12 25 1977 Alfalfa-brome 224 0 2 1978-80 Alfalfa-brome* - * Burutolu, E.F.A. (1977). * Except for plots limed in 1965-66, alfalfa did not survive on plots where basal fertilizer for corn (1959-72) had been supplemented with acidifying N carriers (ammonium salts, NH3, urea or ureaform). 54 assigned to two of the four blocks to give a split block design with two replications. An initial application (4.5 T/ha) of dolomith: agricultural limestone was made in the spring of 1965, followed by a second application the following year. The sum for the two applications on each plot was equal to twice the lime requirement estimated by the SMP buffer test (Shoemaker et al., 1961) for that plot. The lime used was limestone No. 2 examined by Parfitt and Ellis (1966). Differential thermal analysis indicated a dolomite content of 55%, the balance being calcite and magnesite in prOportions giving a CaCO3 equivalent neutralizing value Of 100 to 105 for the bulk material. After 14 annual applications (1959-72), the experimental N treatments were discontinued. Wheat or soybeans were grown with uniform fertilization on all plots until 1977, when a mixture of alfalfa and bromegrass was seeded (Table 2). One cutting of hay was harvested in 1978. Soil samples for the present study were collected in June 1979. A in~inciple objective in the present investigation was to observe effects of decomposing organic matter on extractability of Al. For this reason, consideration will be given here to crOpping history as it may have affected the nature and quantity of soil organic matter (SOM), and to soil test histories showing changes leading to the soil pH levels and lime requirement status encountered in soil samples taken for this study in June 1979. CROPPING HISTORY _I_H__i3_i-:_ son. c AND N The two treatments selected for investigation included the residually acidic carrier, (NH4)ZSO4, and the residually basic carrier, 55 Ca(NO3)2. In Table 3, as in later tables, "sulfate“ refers to the ammonium carrier, and "nitrate" to the calcium salt. The historical data are taken from Burutolu (1977), who had summarized yield and soil test data through 1975. Moisture is frequently a limiting factor at the experiment site, and crOp yields for the selected years in Table 3 are low. The important point to note is that the sulfate plots without lime had been essentially barren since about 1966. When samples were taken in 1979, only a scattering of grass and weeds were present. It will be seen in the next section that the lack of vegetation on the sulfate plots was due to extreme soil acidity. Due to the negligible quantities of fresh organic matter being recycled through the soil in unlimed sulfate plots, the level of organic C in September 1975 was lower than in unlimed nitrate plots (Table 4). With both carriers, organic C in limed plots was lower by 40% or more than in unlimed plots. Similar reductions in SOM content due to liming have been ascribed to enhanced microbial activity and more rapid cycling of C (Leo et al., 1959). In the 1979 sampling, organic C had declined further in unlimed sulfate plots and had changed very little in unlimed nitrate plots. In limed plots of both carriers, however, organic C had increased by 70% or more. This dramatic increase in limed plots may be ascribed mainly to increased productivity and increased annual return of crOp residues (Table 3). Stands of alfalfa, seeded with bromegrass in 1977 were much better in all limed plots, and both grass and alfalfa were much more vigorous in 1978 (two cuttings harvested but yields not recorded). Unlimed plots with pH <5.0 (Tables 5,6) remained essentially barren. 56 Table 3. Partial history of crOp yields (1961-76)+. Corn* Soybean** N carrier 1961 1967 1971 1974 1975 1976 quintal/ha NO lime NO fert. 35.5 a# 16.4 a 18.4 a 12.9 b 10.6 b 3.9 Sulfate 33.0 a 0.6 a 0.0 a 1.4 a 4.6 a 2.5 Nitrate 54.4 b 53.7 b 52.3 b 13.8 c 8.0 b 2.9 Lime## No fert. - 24 4 a 34.1 13.7 b 10.2 6.2 Sulfate - 27.7 a 34.0 11.1 a 11.0 5.4 Nitrate - 52 8 b 38.2 15.1 b 10.5 5.1 L5005 13.9 19.7 1.6 2.9 ns I} Burutolu, E.F.A. (1977). * N carrier applied annually (336 kg N/ha) in addition to 224 kg/ha Of 5-20-20. ** All pl # ots homogeneously treated with 12-12-12 (392 kg/ha). Within lime, means followed by none or the same letter are not different at P(05). For comparisons between lime, the tabulated LSD and ni applies (Steel and Torrie, 1980). Lime applied in 1965-66. Total lime applied on no fert., sulfate, trate were 9, 27, and 9 T/ha, respectively. Table 4. 57 Comparisons of soil C, N and C:N ratios between 1975+ and 1979 in relation to residual effects of nitrogen carriers and lime. N Profile 0r anic C 0r anic N C:N carrier depth 1975 1979 1975 1979 I975 I979 ............ :,% ------------- No lime Sulfate Surface 0.87 0.79 0.080 0.055 11.9 14.5 Subsoil 0.12 0.14 0.010 0.017 8.8 8.2 Nitrate Surface 0.99 0.95 0.090 0.068 10.4 14.0 Subsoil 0.41 0.20 0.020 0.014 16.8 14.3 Lime* Sulfate Surface 0.52 0.91 0.080 0.061 6.5 14.8 Subsoil 0.12 0.40 0.020 0.021 10.0 19.1 Nitrate Surface 0.52 0.89 0.090 0.069 6.2 12.8 Subsoil 0.29 0.20 0.020 0.018 13.2 11.1 + Burutolu, E.F.A. (1977). * Lime applied in 1965-66. Total lime applied on no fert., sulfate, and nitrate were 9, 27, and 9 T/ha, respectively. 58 Table 5. Soil pH in 1975+: residual effect of nitrogen carriers and lime. Soil pH N carrier (1959-72) No lime Lime* (F25 223-50 SIT-75 U-Zb Zia-EU 50-75 cm cm cm cm cm cm No fertilizer 5.3 - - 6.7 - - Basal fertilizer 5.1 5.4 6.6 6.8 6.9 6.6 **(NH4)2504 4.2 4.2 4.4 6.2 5.4 4.8 NH4Cl 4.4 4.5 4.8 5.3 4.9 4.9 NH4NO3 4.2 - - 7.0 - - NH3 4.5 - - 6.2 - - Urea 4.4 - - 6.7 - - Ureaform 4.3 - - 5.9 - - **Ca(N03)2 5.3 5.7 5.7 6.8 6.9 7.2 NaN03 5.6 6.4 6.8 7.0 6.6 6.3 + Burutolu, E.F.A. (1977). * Lime was applied in 1965-66 on two of the four plots of each treatment. The total for two applications was twice the amount determined for each plot by the SMP method (Shoemaker et al., 1961). ** Treatments of concern in present investigation. 59 + Table 6. Soil pH changes (1961-79) fOr treatments considered in present study. Treatment Year N Profile Jul Aug Jul Sep Jul Aug May Jul Oct Sep Jun carrier depth 61 62 63 65 66 67- 70 71 71 74 75 79 No lime No fert Surface 5.9 5.7 5.7 5.2 6.0 5.8 5.8 6.1 5 3 5.5 5 3 - NO fert Subsoill 6.4 5.5 5.7 - 6.4 - - 6.3 - - - - Subsoil2 - - - - - - - 6.2 - - - Sulfate Surface 4.5 4.2 4.0 4.1 3.9 3.8 3.8 4.1 3 8 4.4 4.2 4.5 Sulfate Subsoill 5.3 4.5 4.6 - 4.5 - - 4.0 - - 4.2 - Subsoil2 - - - - - - - 5.0 - - 4.4 4.9 Nitrate Surface 5.7 5.8 5.8 5.6 5.7 5.4 5.8 5.8 5.3 5.5 5.3 5.4 Nitrate Subsoill 6.3 5.6 5.8 - 5.8 - - 6.0 - - 5.7 - SubsoilZ - - - - - - - 6.0 - - 5.7 6.2 Line* No fert Surface 5.8 6.5 6.7 7.2 7.0 6.7 6.7 6.7 - NO fert Subsoill - 6.2 - - 6.7 - - - - SubsoilZ - - - - 5.8 - - - - Sulfate Surface 4.2 4.4 5.1 5.4 5.8 5.5 6.0 6.2 6.6 Sulfate Subsoill - 4.7 - - 4.8 - - 5.4 - Subsoil2 - - - - 5.6 - - 4.8 7.2 Nitrate Surface 6.2 6.7 6.4 7.0 7.0 6 5 6.2 6.8 6.5 Nitrate Subsoill - 6.7 - - 6.8 - 6.9 - Subsoil2 - - - - 6.0 - - 7.2 7.3 Sampling depth (cm) Surface soil 0-15 0-20 o-13 0-13 0-13 0-15 0-15 0.25 0-25 0-25 o-2s 0-25 Subsoill 30-45 --25-43-- - 25-43 - - 25-50 - - 25-50 - Subsoil2 - - - - - - - 50-75 - - -- 50-75 -- I Data fOr 1961-1975 are taken from Burutolu, E.F.A. (1977). Values are fOurbplot means through 1963 and two-plot means after the first application Of lime in spring 1965. * Total lime fOr two applications (1965-66): 9, 27 and 9 T/ha on No fert., sulfate and nitrate plots, respectively. 60 Also contributing to increased C in limed plots in 1979 was the fact that the soil had not been disturbed by tillage Operations after the alfalfa-brome mixture was seeded in 1977. Carbon would have cycled more slowly than when soils were being plowed for cultivated crops every year (Rovira and Greacen, 1957; Allison, 1973). Organic N in September 1975 (Table 4) was essentially the same in all plots, reflecting the uniform inputs of fertilizer N on wheat and soybeans over a 3-year period (Table 2). By June 1979, organic N in surface soils had declined sharply on all plots. This is not surprising, since no fertilizer N had been applied since 1975. The largest decrease in organic N (840 kg/ha) occured in the surface soil of the unlimed sulfate plots. Removal of N in soybeans harvested in 1976 or in hay harvested in 1978 would have been negligible because of scattered stands and poor growth. Losses from the surface were apparently due to leaching of mineralized N (NH + and NO3') and 4 mobilized organic N. About a third of the loss from the surface 25 cm can be accounted for by the increase at 50-75 cm (235 kg/ha). *- 4 and organic N in the 25-50 cm layer (not sampled), and by leaching of Reasonably, the balance might be accounted for by interception of NH N0 - to depths below 75 cm. 3 For other treatments in Table 4, calculated losses from surface soil were 739, 706, and 638 kg N/ha, 25 cm depth, in unlimed-nitrate, limed-nitrate, and limed-sulfate, respectively. No more than half of these losses (250 kg/ha) might be accounted for by removal of 8.0 quintal/ha of soybeans (6% N) in 1975 (Table 3) plus an estimated 10 T/ha alfalfa-brome (2.2% N) harvested in 1978. These estimates do not take into account the biological fixation of N associated with alfalfa, 2 6] which could have introduced 100 kg/ha or more of N into these soil systems (Allison, 1973). Some leaching of mineralized N may have occurred. However, the fibrous root systems of grass species are efficient scavengers of available N. They also support a high level of denitrifying activity in the rhizosphere (Allison, 1973). It is possible that extensive denitrification occurred. Organic matter retained in surface soils of all plots in Table 4 had a wider C:N ratio in June 1979 than in September 1975. In limed plots, the ratio doubled from one sampling to the next. Wider C:N ratios may reflect an increase in the proportion of less extensively humified organic matter due to larger inputs of fresh organic matter and slower decomposition rates under sod. On the other hand, wider C:N ratios may reflect selective complexation of soil minerals with organic fractions low in N. The latter interpretation would be consistent with the large retention of C and the uniquely high C:N ratio in the subsoil of the limed-sulfate plot. As will be noted later (Table 8), substantial quantities of buffer acidity were displaced downward by lime applied on sulfate plots. The mobile buffering materials undoubtedly included chelated Al and Fe. These rapid changes in absolute and proportionate quantities Of organic C and N imply that the nature and quantity of active organic fractions available for interaction with mobile mineral species is determined largely by the nature and quantity of recent or current additions of fresh organic matter (root exudates, plant debris). For this reason, an organic amendment was included in one of the incubation treatments employed in the present investigation. 62 SOIL TEST HISTORY The soil in the experimental area was at about pH 6.0 at the beginning of the field experiment (1959). Changes in soil pH, exchangeable bases and lime requirement through 1963 were reported by Wolcott et al. (1965). Changes through 1975 were summarized by Burutolu (1977). In Table 5, data for the last year in which all plots were sampled (1975) are presented so that residual active acidity (soil pH) associated with the two treatments of concern in the present study may be compared with effects associated with the other treatments in the field experiment. Without lime, active acidity in surface soils of all plots had increased from the original pH 6.0. The uniquely acidifying effect of all ammoniacal N carriers is apparent. Differential effects on soil pH extended through the upper subsoil (25-50 cm) into the lower subsoil (50-75 cm). The time course of changes in active acidity (soil pH) and buffer acidity (lime requirement) for the three treatments considered here can be seen in Tables 6 and 7. Again, in these and later tables, "sulfate" refers to (NH4)ZSO4 and "nitrate" refers to Ca(N03)2. Some of the year-to-year variation in pH (Table 6) and lime requirement (Table 7) was associated with rainfall patterns and the season of the year when samples were taken (Schafer, 1967; Wolcott et al., 1965). Components of buffer acidity moved downward during periods of net percolation and upward during periods of net evapo-transpiration. Ammonium sulfate is among the most acidic fertilizer sources of N 63 Table 7. Lime requirement changes (1961-79)+ for treatments considered in present study. Treatment Year N Profile Jul Aug Jul Sep Aug May Jul Oct Sep Jun carrier depth 61 62 63 66 70 71 71 74 75 79 -“ T/hag 25 CT“ NO lime No fert Surface 8 10 15 7 22 3 12 14 9 - Subsoill 10 7 7 2 - O - - - - SubsoilZ - - - - - 1 - - - - Sulfate Surface 22 29 36 24 4O 32 38 19 19 7 Subsoill 12 11 18 8 - 40 - - 16 - SubsoilZ - - - - - 23 - - 10 4 Nitrate Surface 8 6 8 10 14 14 12 14 10 3 Subsoill 10 5 11 6 - 6 - - O - Subsoil2 - - - - - 6 - - 0 0 Lime* No fert Surface 2 O 1 0 O 0 Subsoill 5 - O - - - - Subsoil2 - - 1 - Sulfate Surface 19 16 19 12 7 7 0 Subsoill 8 - 13 - - 12 - SubsoilZ - - 6 - - 24 0 Nitrate Surface 2 0 0 5 O O Subsoill O - 1 - - O - Subsoil2 - - 0 - - O 0 Sampling_depth (cm) Surface soil 0-15 0-20 0-13 0-18 0-18 0-25 0-25 0-25 0-25 0-25 Subsoill 30-45 25-38 25-38 25-38 - 25-50 - - 25-50 - SubsoilZ - - - - - 50-75 - - 50-75 50-75 I Data for 1961-1975 are taken from Burutolu, E.F.A. (1977). * Total lime fOr two applications (1965-66): 9, 27 and 9 T/ha on no fert., sulfate and nitrate, respectively. 64 (Table 1). Changes in pH and lime requirement were rapid and extreme. By the time that samples for the present study were taken (June 1979). surface soils had been at pH 4.5 or less for a period Of 18 years. Soil in the lower subsoil (50-75 cm) did not enter this range until sometime after May 1971. By this time, however, a high level of lime requirement was distributed throughout the sampled profile. Theoretically, Ca(NO3)2 should have had a residually basic effect; (Table 1). However, there were no consistent differencs in pH or lime requirement between nitrate and no fertilizer in surface soil (Tables 6, 7). It appears that the expected alkalinity was counteracted by acidity from sources other than fertilizer. Likely sources would include acidity washed out of the atmosphere in precipitatnwi("acid rain“). organic products produced by microorganisms, proton released when metals are complexed by organic ligands, and protons from nitrification of NH3 released during annual cycles of orgnaic matter decomposition. Acidity from non-fertilizer sources maintained similar levels of lime requirement in surface soils of both unfertilized and nitrate-treated plots. Lime requirements declined quickly in subsoils of unfertilized plots, but substantial acid buffering was maintained in subsoils of nitrate plots until annual applications were discontinued in 1973 (cf. Tables 2, 7). Annual inputs of acidity from biological sources would have been greater with nitrate than in unfertilized plots because of much greater yields and residue return from corn (Table 3). These inputs apparently exceeded the capacity of surface soils to intercept and stabilize the acidity. Also, the mobility of charged sesquioxides would have been promoted by surplus N03", since the rate of application (336 kg N/ha.yr) greatly exceeded the rate of removal by 65 corn. Distribution of acidity through the profile continued to change after high annual rates of N application for corn were discontinued in 1973 (Tables 6, 7, 8). In the case of Ca(NO3)2, it appears that buffering acidity by accumulated sesquioxides was inactivated 12.31.1111 by aging and polymerization in unlimed plots and by neutralization in limed plots. By contrast, the gradual decline in soil acidity after annual applications of (NH4)ZSO4 were discontinued appears to have been due, in part at least, to downward displacement of charged sesquioxides. Changes in soil pH and lime requirement from 1971 to 1979, in both limed and unlimed plots, fit the picture of a wave of buffer acidity moving downward through the profile. Soil pH values associated with the wave as it passed through successive soil layers were in the range Of 4.0 to 5.0. In this range of acidity, buffering would have been due primarily to exchangeable, monomeric Al(H20)63+ or low molecular weight charged polymers of Al (Jackson, 1960, 1963; Schwertman and Jackson, 1964). Data in Table 8 support the view that exchange displacement by Ca2+ and Mg2+ may have accelerated downward movement of exchangeable Al in limed plots. For the highly charged Al monomers to move as indicated in percolating soil solution, a supply of complementary mineral anions or of organic ligands Of low molecular weight must be assumed. Monovalent anions, such as Cl' or N03" from commercial fertilizers, or N03" from mineralization and nitrification of organic N, would have served to maintain mobility of these Al Species. After fertilizer applications were discontinued, organic ligands of low molecular weight would have 66 Table 8. Changes in distribution 9f soil acidity after nitrogen treatments were discontinued (1972) . Treatment Soilng " Lime requirement N’ ‘PrOfile Why ‘Sep JUn Why *Sep ‘JUn carrier depth 71 75 79 71 75 79 an ‘T7ha,‘25’cm NO lime No fertz. 0-25 6.1 5.3 - 3 9 - 25-50 6.3 - - O - - 50-75 6.2 - - 1 - - (NH4)2SO4 0-25 4.1 4.2 4.5 32 19 7 25-50 4.0 4.2 - 4O 16 - 50-75 5.0 4.4 4.9 23 10 4 Ca(NO3)2 0-25 5.8 5.3 5.4 14 10 3 25-50 6.0 5.7 - 6 O - 50-75 6.0 5.7 6.2 6 O 0 Lime* No fertz. 0-25 7.0 6.7 - 1 O - 25-50 6.7 - - 0 - 50-75 5.8 - 1 - - (NH4)2S04 0-25 5.8 6.2 6.6 19 0 25-50 4.8 5.4 - 13 12 - 50-75 5.6 4.8 7.2 6 24 O Ca(NO3)2 0-25 7.0 6.8 6.5 O 0 0 25-50 6.8 6.9 - 1 O - 50-75 6.0 7.2 7.3 0 O O + Soil pH and lime requuirement for 1971 and 1975 were taken fran Burutolu, E.F.A. (1977). * Total lime fOr two applications (1965-66): 9, 27 and 9 T/ha on No fert., sulfate and nitrate plots, respectively. 67 been relatively more important. These would have come in the form of root exudates during periods of vegetative activity or as products of microbial metabolism in the rhizosphere or in association with decomposing plant residues. Lime applied in 1965 and 1966 reacted quickly with previously accumulated acidity in control and nitrate plots. In sulfate plots, however, the reaction was extremely slow. It was not until 1979 that the pH in surface soil had risen into the range (pH 6.5 to 7.0) that would have been expected with the quantity of lime (27 T/ha) applied in 1965-66. Apparently, lime particles were coated quickly by sesquioxide gels that retarded their reaction (Coleman and Thomas, 1967; Moschler et al., 1962). SAMPLING OF SOILS Plots that had received the two treatments of concern (Table 5) were sampled in June 1979. Three samples were taken to represent the surface soils (0-25 cm) in each plot, and three of the subsoil (50-75 cm). Each sample was a composite of 10 cores taken randomly over the same central area of each plot, so as to minimize contamination by soil materials carried over from adjacent plots by tillage Operations over the years. Also, the three samples per plot served to attenuate error variance in the calculation of mean squares. After passing the bulk samples through a 4-mesh (5 mm) screen and mixing thoroughly, experimental aliquots were placed in plastic bags and stored at 5 C. 68 LABORATORY INCUBATION OF SURFACE SOIL Incubation treatments for the present study were selected to provide a basis for differentiating quantitatively between chelation and protonation as mechanisms for mobilizing Al. Soils were amended with fresh corn tissue (0M) to serve as a source of organic ligands released during decomposition. A paralled series Of incubations received the corn tissue plus NH4+ to serve as a source of protons released by nitrification. Data obtained after incubation of amended samples (+0M and +OM+N) are compared with data for unamended samples (identified in tables as “None“ under incubation treatment). The unamended samples were analyzed directly as removed from storage at 5 C. Thus, they represent the status of soils at the beginning of incubation. As noted above, the samples had been stored in field moist conditions. Surface soils collected in June, 1979, were used. Duplicate subsamples (200 g oven dry basis) from each of the three composite samples taken per plot were thoroughly mixed with finely ground mature corn stalks (with leaves), added at the rate of 1500 ug/g (3360 kg/ha-furrow slice). One of these duplicate subsamples received also 150 ug N/g as NH4Cl (analytical reagent grade). The mixed soil was placed in a ZOO-ml plastic cup. Soil moisture was adjusted to 15% with distilled-deionized water (percent water holding capacity of the soils was 13-16%, estimated by pressure plate at 1/3 bar pressure). The cup was covered with a polyethylene sheet containing two pin holes, and then incubated for*70 days in a temperature controlled room at 27:3 C. The moisture content was 69 adjusted to 15 % by weight once every two weeks. EXTRACTION PROCEDURES Direct Extraction of Al from Whole Soils Total available Al and total exchangeable Al were extracted by shaking 10.0 g of field moist soil or ammended soil after incubatyion for 24 h with 50 ml of’N_NH4OAc (pH 4.8) or N_KCl, respectively (McLean, 1965). “The soils were filtered and washed twice with the corresponding extractant. The final volume was adjusted to 100.0 ml. Fractional Extractions About 190 g of moist soil was weighed into a 43 x 123 lllll Whatman cellulose extraction thimble for Soxhlet extraction with 150 nfl distilled-deionized water. After 24 h extraction, the extract was measured for its recovered volume, then centrifuged in a plastic tube at 4000 rpm at 4 C to separate soil residue. The clear supernatant was kept in a plastic bottle and stored in a freezer for further analysis. The wet soil residue left in the thimble after Soxhlet extraction was subsampled for water content and further extractions. Fifty g of wet soil was Shaken with 50 ml of N_NH OAC (pH 4.8) or 0.1 N KCl for 24 4 h. The soil suspensions were filtered through Whatman No. 2 filter paper into 100-ml volumetric flasks. Another 50 ml of the corresponding extracting solution was leached through the soil residue into the volumetric flask. These filtrates were made up to 100 ml with their 7O extraction solutions. Finally, the soil residue after 0.1 N KCl filtration (including the filter paper) was successively extracted by 50 ml Of N KCl and filtered as in 0.1 )1 KCl extraction. All the extracts were kept in plastic bottles and stored at 5 C until analysis. ANALYTICAL PROCEDURES Chemical Analyses of Whole Soils Soil pH was measured with an Orion 801 pH meter (glass and calomel electrodes), using 1:1 suspensions in water or 1:2 suspensions in 0.1 N_ KC]. 2 and N03 semimicrO-Kjeldahl method (Bremner, 1965). Ground, air-dry soil (0.5 g) Total N (excluding NO ') was determined by a was equilibrated with 2 ml distilled water for 30 min before digestion with conc. H2304 to which potassium sulfate-selenium mixture was added as a catalyst. The suSpensions were digested for 1 h until the solutions were clear. The solution was rendered alkaline with 10 _N_ NaOH to transform total N into NH3. The NH3 was immediately steam distilled into 2 % boric acid to which methyl purple was added as indicator. For NH4+-N, 20.0 g moist soil was extracted with 20.0 ml of 2 ! KCl for 30 min, then centrifuged at 30,000 rpm for 5 min. Ten ml of the supernatant was steam distilled in the presence of 0.1 N NaOH, and NH3 was collected in boric acid. Ammonia in boric acid was determined by titration against standardized H2504. The difference between total Kjeldahl N (TKN) and NH4+-N was taken as organic N. Nitrate plus nitrite were analyzed by a procedure prescribed for 7] the Technicon Auto-Analyzer (Technicon Industrial System, 1972). Moist soil was extracted with saturated CaSO4 (25 9 soil: 25 ml soluticni) for 30 min on a rotary shaker at 150-200 rpm. The N03- concentration in the filtrate was then determined with the Technicon Auto-Analyzer. Nitrate was first reduced to NOZ' in a Cu-Cd reactor column. Nitrous acid then reacted with sulfanilamide under acidic conditions to form a diazo-compound. The compound then coupled with N-1-naphthylethylenediamine dihydrochloride to form a reddish azo dye which was measured photometrically. Total carbon (TC) was determined by dry combustion (LECO, 1965), utilizing a LECO 70-second Carbon Analyzer. One tenth g of air-dry soil, ground to pass through a 80 mesh screen, was weighed into a ceramic crucible and LECO #501-76 tin metal was added as combustion accelerator. The crucible was put in a high frequency induction furnace where TC in the sample was converted to C02 in a stream of 02 at; temperatures higher than 1600 C. The hot combustion gases were passed through a series of traps: a dust trap to remove metal oxides, manganese dioxide to absorb sulfur gases, a heated COpper catalyst tube to convert C0 to C02, and Anhydrone to remove moisture. The purified mixture of 02 and CO2 was collected in a cylinder inside the analyzer system. The difference in thermal conductivity between the gas mixture and pure 02 was measured by a thermistor type thermal conductivity cell with digital read-out. Carbonate in soil samples as taken from the field was estimated, using an acid-neutralization method (Allison and Moodie, 1965). Fifteen g of air-dry soil was boiled gently with 20.0 ml of standardized 0.5 N HCl for'5 nfin. The cooled filtrate was then titrated with standardized 72 0.25 _N_ NaOH to pH 7 (neutralization monitered by pH meter). Carbonate content was calculated from the difference between amounts of HCl and NaOH added. Chemical Analyses of Soil Extracts The total organic carbon (TOC) in HZO-Soxhlet extracts and in 0.1 _N_ KCl extracts was determined by a DC-SO Total Organic Carbon Analyzer (Dohrmann Envirotec, 1972). A 30 ul acidified homogenized sample solution was injected into a sample boat containing MnO2 oxidizer at room temperature. The boat was advanced to the 90 C vaporization zone 2 where H20, CO2 (from dissolved CO CO3 ' and HCOB') and volatile 2. organic carbon (VOC) were swept into the bypass column where VOC was trapped at 60 C while other gases were vented to the atmosphere. The boat was then advanced to the pyrolysis zone where the residual organic carbon (ROC) reacted with MnO2 at 850 C to produce 002. bypass column was backflushed at 130 C thus sweeping the VOC through the Meanwhile, the pyrolysis zone. Both the VOC and CO2 (from ROC) were swept by helium (He) into a H -enriched nickel-catalyst reduction zone where all C was 2 converted to methane at 350 C. The methane was quantified by a flame ionization detector and read out as mg C/l. A plasma emission photometer (Spectraspan IIIA) was used to determine the concentration of P, K, Ca, Mg, Zn, Fe, Mn, Cu, Al, Pb and Ni in the "whole soil extracts" -- _N_ NH OAc (pH 4.8) and _N_ KCl -- and in 4 "fractional extracts" -- successive extracts in the following sequences: (1) HZO-Soxhlet, 0.1 11 KCl and y_ KCl, and (2) g NH4OAc (after HZO-Soxhlet). Lithium chloride was added to give 5 ppm of Li in 73 the extracts to suppress ionization of the samples. The liquid sample was pumped into the nebulizer, converted to aerosol form, and released into the excitation area (DC argon plasma). The temperature of this excitation zone was maintained between 6000 and 7000 K SO that direct analysis of non-metals, B, P, as well as refractory type metals, V, Ti was permitted. The intensity of Specific wavelengths emitted by the excited sample was prOportional to the concentration of each element. The radiation beam was separated into its component wavelengths by an Echelle grating and prism, then selectively passed to a photomultiplier tube where the incident radiation gave rise to a prOportional current. Finally, the signals from the photomultiplier were transformed electronically to concentrations for each element. The lower limit of detection is 1 ug/ml for Cu, Cr, Ni, Pb; 2 ug/ml for Cd; 10 ug/ml for P, Mg, Mn, Al, K; and 20 for Zn, Fe, Ca. Determination of Al by 8-Hydroxyquinoline Method The forms of Al in each extract were investigated further by using an 8-hydroxyquinoline (8-0Hq) complexation procedure (Bloom et al., 1978). The principle of this method is that 8-0Hq reacts instantaneously with uncomplexed, mononuclear Al but more slowly with Al that has been complexed by 0M (Turner and Sulaiman, 1971; Hoyt and Turner, 1976). The colored alumino-quinoline complex is soluble in butyl acetate (BuOAc) and can be assayed photometrically. The length of time that the test solution is in contact with 8-0Hq before extracting with BuOAc provides a basis for discriminating between free and complexed forms of Al. It appears that the exchange of 8-0Hq for 74 organic ligands in soil extracts is essentially complete after 30 min (Bloom et al., 1978). In the present study, 60 min was allowed for ligand exchange. Water was added to an extraction tube in sufficient amount that its volume plus the extract would equal 25 ml. Five ml of N NaOAc, containing 0.2% (w/v) of 1,10-phenanthroline, was added. The phenanthroline eliminates interference by Fe. A sample of extract containing 50 to 800 nmole of Al was then added and the tube shaken briefly. For the zero time of contact (Al-1), 5 ml of BuOAc*was added, then followed by 5 ml of mixed reagent (4:1 mixture of 1% (w/v) 8-0Hq and 20% (w/v) hydroxylamine hydrochloride). The tube was stoppered and shaken immediately for 15 sec. After standing for 30 sec for phase separation, the BuOAc phase was removed to a new extraction tube. For the 60 min time of contact (Al-2), the mixed reagent was added, shaken vigorously for 15 sec then allowed to stand for 60 min. The alumino-quinoline complex, in both cases, was determined in the BuOAc phase with a Beckman SpectrOphotometer at 395 nm, using borosilicate glass cuvettes and a BuOAc blank. The complexed forms of Al were taken as the difference between Al-2 and Al-l. The lower limit of detection for Al is 0.05 ug/ml. STATISTICAL ANALYSIS Analyses of variance were performed according to a split plot design (Cochran and Cox, 1950), using facilities of the Michigan State University Computer Center. Lime treatments were assigned to main plots and nitrogen 75 treatments to subplots. Sampling depths were treated as sub-subplots in analyzing data for samples as taken from the field (before incubation). Only surface soils were incubated, and incubation treatments were assigned to sub-subplots in analysing effects of these treatments. In both cases, variation associated with the three subsamples collected from each plot was excluded as sampling error. The number of subsamples was included in the divisors used for calculating standard errors . RESULTS AND DISCUSSION PROPERTIES OF SOILS AS SAMPLED IN JUNE 1979 The chemical status of soil materials collected in June 1979, and used in this study reflect management practices over a 20-year period (1959-79). Part of the data in Table 9 are taken from Tables in the Materials and Methods section (p. 52-75), where their relation to management history and earlier analytical data are discussed. Important relationships will be reviewed briefly here. Discussion on the present results will be concentrated on means Of the eight treatment-combinations which are located in Part e of each Table (Second order interactions). As elsewhere in the thesis, “sulfate" refers to (NH4)ZSO and “nitrate" refers to Ca(N03)2. 4 Soil pH At the time the soil samples were taken, the differential N treatments had been discontinued for six years (since 1973), and the soil had been left undisturbed by cultivation since alfalfa-brome was seeded in 1977 (Table 2). The pH values in Table 9 do not reflect the fact that the surface soil of unlimed sulfate plots had been buffered by mineral breakdown (Jackson, 1963) at about pH 4.0 for about 10 years before the annual applications of (NH‘,()ZSO4 were discontinued in 1973 76 77 Table 9. Soil pH, farms of carbon and nitrogen in surface soil (0-25 cm) and subsoil (50-75 cm): long-term residual effects of nitrogen carriers and lime. ‘Treatment 2% _ N carrier Profile 2 ApH 00‘3- Org.- Org. Org.- NH4+- N03"- in field depth C C C/N N N N ug/g sOill a. Main effects Sulfate Mean 5.8 5.0 - 0.06 0.56 14.5 385 1.27 4.5 Nitrate Mean 6.3 5.4 - 0.04 0.56 13.2 425 1.01 9.1 Mean Surface 5.7 a+5.0 - 0.03 0.88 b 13.9 635 b 1.64 b 11.8 b Mean Subsoil 6.4 b 5.5 - 0.08 0.23 a 13.1 175 a 0.63 a 1.8 a Mean fOr no Time 5.2 a 4.3 a - 0.03 0.52 a 13.5 385 1.36 5.3 Mean fOr Time 6.9 b 6.1 b - 0.07 0.60 b 14.1 425 0.92 8.3 b. Interactions of N carriers with lime No Time Sulfate Mean 4.7 a 3.9 - 0.02 0.47 a 13.1 360 1.51 3.9 Nitrate Mean 5.8 b 4.7 - 0.03 0.57 b 13.9 410 1.21 6.7 Lime Sulfate Mean 6.9 6.1 - 0.10 0.65 b 15.8 410 1.03 5.0 Nitrate Mean 6.9 6.1 - 0.05 0.54 a 12.4 435 0.80 11.6 LSD05 0.97 1.0 - - 0.07 - ns* ns 6.7 c. Interactions of profile depth with lime No Time Mean Surface 4.9 4.1 0.02 0.87 b 14.1 615 b 2.1 b 9.2 b Mean Subsoil 5.6 4.6 0.04 0.17 a 11.0 155 a 0.62 a 1.4 a Lime Mean Surface 6.6 5.8 - 0.03 0.90 b 13.7 655 b 1.2 14.5 b Mean Subsoil 7.2 6.4 0.11 0.30 a 15.4 195 a 0.64 2.1 a LSD 1.5 1.7 - - 0.51 - 317 ns 11.9 05 78 Table 9 (cont'd.). ‘Treatment’ ' p? - N carrier Profile 2 ApH CO} Org.- Org. Org.- NH +- NO '- in field depth ' ' C ' C' ' C/N N N N ug/g soiT’ d. Interactions of_profile depth with N carriers Mean of no lime + lime Sulfate Surface 5.5 4.8 - 0.02 0.85 b 14.6 580 b 1.96 b 7.5 Sulfate Subsoil 6.0 5.3 - 0.10 0.27 a 14.2 190 a 0.58 a 1.5 Nitrate Surface 5.9 5.2 - 0.03 0.92 b 13.3 690 b 1.3 16.2 b Nitrate Subsoil 6.7 5.7 - 0.05 0.20 a 12.5 160 a 0.68 2.1 a LSD05 ns ns - - 0.51 - 317 ns 11.9 e. Second order interactions No Time Sulfate Surface 4.5 3.7 -0.8 0.02 0.79 b 14.5 545 b 2.5 b 6.9 Sulfate Subsoil 4.9 4.1 -O.8 0.04 0.14 a 8.2 170 a 0.52 a 1.0 Nitrate Surface 5.4 4.5 -0.9 0.02 0.95 b 14.0 680 b 1.7 11.5 Nitrate Subsoil 6.2 5.0 -1.2 0.03 0.20 a 14.3 140 a 0.71 1.8 Lime Sulfate Surface 6.6 5.8 -0.8 0.03 0.91 14.8 615 b 1.4 8.2 Sulfate Subsoil 7.2 6.4 -O.8 0.16 0.40 19.1 209 a 0.64 1.9 Nitrate Surface 6.5 5.9 -0.6 0.03 0.89 b 12.8 695 b 0.96 20.8 b Nitrate Subsoil 7.3 6.3 -1.0 0.06 0.20 a 11.1 180 a 0.65 2.4 a LSD05 2.2 2.4 - - 0.72 - 449 ns 16.8 +a,b = Duncan's equivalent ranges. Within columns and subsets of two means, means fOllowed by none or the same letter are not different at P(05). For other comparisons in a given category of interactions, the tabulated LSD05 applies (Steel and Torrie, 1980). ns = not significant at P(OS) * 79 (Table 6). Over the following six years, the pH increased again to 4.5. Subsoil pH had also increased from a low of 4.0 reached in 1971. The pH 4.9 found in 1979 may reflect also the fact that a deeper subsoil layer (50-75 cm) was sampled than in earlier years. The rapid develOpment of acidity with sulfate can be ascribed to retention of protons released by hydrolysis and nitrification of (NH ) SO 4 2 4 protonated sesquioxides of Al and Fe. The latter were mobilized in [Eq. 5], and to displacement of exchangeable bases by large quantities, as evidenced by the SMP-buffer test for lime requirement (Table 7). Calcium nitrate is nominally a basic carrier (Table 1). However, differential removal Of bases by corn (Table 2) and by leaching resulted in a gradual decline in pH of the plow layer (Table 6). The rate Of decline was similar to that in unfertilized control plots. By 1971, pH in both control and nitrate plots had reached the range of 5.3 to 5.5 where strong buffering due to mobilization of charged colloidal sesquioxides had begun to develOp much earlier (1961-63) with acidifying carriers in the same field eXperiment (Shafer, 1968; Wolcott et al., 1965). Illuviation of bases undoubtedly helped to maintain a rather stable pH in the subsoil of these plots. Lime requirements (Table 7) indicate that leaching of bases in control and nitrate plots was due to exchange displacement by charged sesquioxides, just as in sulfate plots. Levels of buffer acidity were lower than in sulfate plots, but continuing inputs of acidity frmn sources other than fertilizer are indicated. Likely sources include nitrification of NH3 released from decomposing organic matter (0M), and the production of organic acids by metabolic processes in plant roots 80 and microbes. The rate Of production of acidity from these sources would have been a function of the rate of cycling of C and N through crops and detritus systems. In unlimed-sulfate and nitrate soils, lime requirements decreased between 1971 and 1975, reflecting the greatly reduced level of N applied annually (Table 2). A further Sharp decrease occurred between 1975 and 1979. Annual tillage Operations were discontinued and a sod crOp established in 1977. It would be expected that the rate of cycling of C and N would be reduced in the absence of tillage. )A large increase in organic C was observed in limed plots (Table 4). However, there was a 20 to 30% reduction in organic N in surface soils of all plots. Apparently, a substantial portion Of annually recycling N was lost by denitrification. Denitrifying activity is high in the rhizosphere of sod crOps (Allison, 1973). To the extent that N03" was denitrified, the acidity produced by nitrification Of mineralized N would have been neutralized [Eq. 6]. Beneficial effects of lime (9 T/ha) applied on nitrate plots in 1965-66 were generated quickly throughout the profile (Table 6). This would suggest that accumulated acidity was neutralized lg git! by hydrolysis of carbonates leaching downward through the profile [Eq. 7]. Subsoil pH continued to increase, even after pH in surface soil had declined again from a maximum of 7.0 reached in 1970-71. By contrast, in sulfate plots, it took 10 years for a 27 T/ha application of lime to raise the pH of the plow layer from 4.1 in 1965 to 6.2 in 1975 (Table 6). Over the same period of time, subsoil pH increased very Slowly also, from about 4.5 to 5.4. It appeared that Time particles were coated quickly by Al- and Fe-hydrous oxides polymers 8] formed by adsorption Of OH' released in the hydrolysis of carbonate ([Eq. 7], p. 11). Further hydrolysis of carbonate and neutralization of charge on sesquioxides were greatly retarded. A large prOportion of the decline in buffer acidity since 1971 appears to have been due to leaching of exchangeable Al displaced by Ca2+ and Mg?'+ released as the dolomitic lime continued slowly to dissolve (Table 8). In Table 9, the differences between pH in water and pH in _N_ KCl provide a clue to changes in stable colloidal fractions in these soils. Mekaru and Uehara (1972) prOposed the use oprH (pH KCl - pH H20) as a parameter for estimating the net surface charge on soil colloids and their relative anion adsorption capacity. Many trOpical soils contain negligible quantities of permanent-charge clay minerals. At.;Hi lower than 5.0, such soils adsorb anions strongly and may exhibit a positive ApH. Where these soils are suSpended in a neutral, indifferent electrolyte such as KCl, both ions are adsorbed non-specifically, but the anion is adsorbed more extensively than the cation, leaving an excess of cations in solution. There is a corresponding shift in distribution of OH' and H+ between the interfacial environment and the ambient solution (Sposito, 1981). Most soils of temperate regions contain sufficient quantities of permanent-charge crystalline clays so that they exhibit a negative 21 pH. This was true of all the soils as shown in Part e. of Table 9. The ApH for soils from sulfate plots was the same (-0.8) at both depths and with or without lime. It would appear that the surface charge properties oi'stable colloidal fractions were determined by the nature of layer silicate-sesquioxide complexes formed during periods when soils at various depths in these profiles were under similar 82 conditions of high acidity (Tables 6, 7, 8) and low base saturation (Burutolu, 1977). The fact thatApH was unaffected by Time indicates that a characteristic prOportion of permanent negative charge was countered by sesquioxides retained by mechanisms that were irreversible with increasing pH and increasing base status. By contrast, the ApH in surface soil of the nitrate plots was reduced by lime from -0.9 to -0.6. As noted earlier, data in Tables 7 and 8 suggest that accumulated buffer acidity in limed-nitrate plots was neutralized infl. The decrease in ApH indicated that neutralization was incomplete and that considerable positive charge was retained in sesquioxides immobilized in precipitates or complexes not readily accessible for interaction with the SMP-buffer test but, nontheless, effective in adsorbing Cl' and influencing pH in KCl. A larger net negative charge was retained on colloids in the subsoils of nitrate plots (apH = -1.2 in unlimed plots, ApH = -1.0 in limed plots). Several mechanisms may have been involved. Annual inputs of Ca2+ would have tended to inhibit mobilization of charged sesquioxides in the surface soil, thereby reducing their concentration in percolating soil solution (Grim, 1968). Higher levels of exchangeable Ca2+ were maintained throughout the profile in nitrate 2+ would have been plots (Burutolu, 1977). Exchange sites occupied by Ca less available for incorporation of charged sesquioxides into stable complexes with crystalline clays and OM. Soil C and N With only two field replications, statistically significant 83 effects of N carriers or lime on soil C and N could not be shown (Tables 4, 9). Nevertheless, certain relationships to management and cropping history appear to be real and relevant to the present study. Of particular interest are the changes that occurred between 1975 and 1979 (Table 4). Unlimed sulfate plots had been essentially barren at the time that lime was applied in 1965-66 and had remained so through 1979. Depletion of SOM in the surface of unlimed-sulfate soil was accompanied by leaching of extensively humified fractions (narrow C:N ratio) into the subsoil. Materials retained in the surface soil had a much wider C:N ratio (Tables 4, 9). For the other three treatments, changes in C and N between 1975 and 1979 appear to have been influenced mainly by effects of the sod crOp established in 1977 and by the fact that the soil was no longer disturbed by annual tillage Operations. The discontinuation of N fertilizer application since 1976 (Table 2) caused a decline in N accumulation.. Organic N in surface soil decreased by 23%, but there was no evidence that leaching of organic or mineral N occurred (Table 4). Decreases were two-fold greater than estimated removals by soybeans in 1976 and alfalfa-brome in 1978. Also unaccounted for was N fixed by alfalfa. Thus, extensive losses by denitrification might have occurred. Large increases in organic C occurred between 1975 and 1979 in surface soils with both limed treatments and in the subsoil of the limed sulfate plots where C:N ratios were doubled (Table 4). The wider ratios would be characteristic for fresh OM introduced as root exudates and debris from the sod crOp, particularly where humification proceeds at a 84 more “natural“ rate, that is, in the absence of tillage. However, as was suggested in earlier discussions of Table 4 (p. 55-60), wider C:N ratios may also reflect preferential stabilization of organic ions low in N through complexation with soil minerals. For example, the large increase in subsoil C in limed sulfate plots is consistent with movement of carbonaceous ligands complexed with sesquioxide components of buffer acidity that were displaced from 2+ exchange sites in upper soil layers by Ca and (492+ released by the slowly dissolving dolomitic limestone. It Should be noted in Table 9 that 0032' was higher in all subsoils than in surface soils, but notably in sulfate plots. 'This would indicate that complexed 0032' is also involved in the cheluviation of potentially acidic sesquioxides (Fenn et al., 1981; Pleysier and Juo, 1981). Bicarbonate (HCOB') and N0 ' in solution would have contributed 3 to mobility of both basic and acidic cations. Significant quantities of N03" were present in all surface soils in the June 1979 sampling and was present at substantially lower concentrations at 50-75 cm (Table 9). Forms Of Extractable Al .A primary objective of this study was to characterize forms of Al that may have contributed to retention of buffer acidity in soil profiles of the field study. A first approximation was based on differences in quantity of Al that could be extracted directly in N NH4OAc (pH 4.8) or in unbuffered N KCl (Table 10). Differences here were expected to distinguish between Al exchangeable at the pH of the KCl suspension (Table 9) and Al held in 85 Table 10. Total aluminum (detenmined by 8-hydroxyquinoline) in surface soil (0-25 cm) and subsoil (50-75 cm): longbterm residual effects of nitrogen carriers and lime. Treatment *Direct eXtraction ‘FractiOnaT’extraCtiOn N'carrier ‘Profile TNHAOAC 1 N ‘HéO 0.1 N” I NFNHAOAC in field depth KCT' KCT" KCT' —— —— ug/g sOiTF--- a. Main effects + Sulfate Mean 123 b 78 0.132 b 23.8 33 - Nitrate Mean 56.8 a 10.9 0.037 a 0.83 3.5 - Mean Surface 78.6 a 47 0.142 b 14.3 22.2 26.5 Mean Subsoil 101 b 41 0.027 a 10.3 14.1 - Mean fOr no lime 132 b 85 b 0.157 b 23.9 35 - Mean fOr lime 48.1 a 3.0 a 0.012 a 0.81 1.01 - b. Interactions Of‘N carriers with lime No lime Sulfate Mean 191 b 148 b 0.061 a 46 64 - Nitrate Mean 72 a 21.6 a 0.253 b 1.64 6.9 - Lime Sulfate Mean 55 5.8 0.013 1.60 2.00 - Nitrate Mean 41 0.17 0.012 0.016 0.018 - LSD05 41 105 0.127 ns* 59 - c. Interactions of profile depth with lime No lime Mean Surface 120 94 0.265 b 28.7 44 41 Mean Subsoil 143 75 0.049 a 19.1 26.1 - Lime Mean Surface 37 0.042 0.019 0.021 0.003 12.5 Mean Subsoil 59 6.0 0.006 1.60 2.01 - LSD 62 ns 0.160 ns ns - 05 86 Table 10 (cont'd.). ‘Treatment DireCt eXtraction ‘Fractional eitraction NcarrTer Profile WHOM 1 N H20 0.1 N l N NHOAc in field depth 4 KCT KCl" KCT 4 ug/g soiT d. Interactions of profile depth with N carriers Mean of no lime + lime Sulfate Surface 97 a 76 0.230 b 27.4 38 31 Sulfate Subsoil 149 b 78 0.034 a 20.2 27 - Nitrate Surface 60 18.0 0.053 1.23 5.9 22.1 Nitrate Subsoil 54 3.8 0.021 0.43 1.10 - LSD05 62 ns 0.16 ns ns - e. Second order interactions No Time Sulfate Surface 154 a 152 0.440 b 55 77 49 Sulfate Subsoil 228 b 144 0.065 a 37 50 - Nitrate Surface 86 36 0.090 b 2.45 11.7 32 Nitrate Subsoil 59 7.4 0.033 a 0.84 2.16 - Lime Sulfate Surface 40 0.024 0.021 0.026 0.002 12.9 Sulfate Subsoil 70 11.6 0.003 3.2 4.0 - Nitrate Surface 34 0.061 0.017 0.015 0.004 12.0 Nitrate Subsoil 49 0.279 0.009 0.017 0.031 - LSD05 88 ns 0.227 ns ns - +a,b = Duncan's equivalent ranges. Within columns and subsets of two means, means fOllowed by none or the same letter are not different at P(05). For other comparisons in a given category of interactions, the tabulated L5005 applies (Steel and Torrie, 1980). ns = not significant at P(OS) * 87 forms that would equilibrate at a pH just below the point where polymerization-depolymerization equilibria are particularly sensitive to pH changes (Coleman and Thomas, 1967; Jackson, 1963; Kamprath and Foy, 1971). A second approach was based on the premise that removal of hot-water extractable 0M would alter the subsequent extractability of Al in KCl or NH OAc. Two sequences of extraction were employed: 1) H O 4 2 soxhlet ---> 0.1N KCl ---> N KCl, and 2) H20 soxhlet ---> N NH OAc (pH 4 4.8). These are identified as " Fractional extractions“ in Table 10 and later tables. Each Of the direct and fractional extracts was then subjected to the 8-hydroxyquinoline (8-0Hq) procedure (Bloom et al., 1978) for differentiating between mononuclear Al3+ and polynuclear or complexed Al (Tables 11 and 12). Direct Extractions-- Recoveries by direct extraction in Table 10 (e.) are the result of equilibria attained after shaking field moist soil for 24 h in NH4OAc or KCl. These equilibria would have included the initial exchange displacement of Al by NH4+ or K+, as well as further equilibria involving H3O+ released by hydrolysis of the displaced Al species ([Eq. 12, 13, 14] p. 39). The direction taken by these equilibria in KCl may be inferred from the difference between pH(H20) and pH(KCl) in Table 9. It is likely that the initial exchange was determined mainly by the degree of base saturation. There was a generally negative relationship between Al 2+ 2+ (Table 10) and Ca and Mg recovered in KCl (Appendices A3, A4, A9). 88 However, the final distribution of charged Al species between the solution and soil colloidal surfaces would have been determined primarily by the pH at equilibrium. Except for surface soil from the unlimed-sulfate soil, recoveries in NH4OAc were much greater than in KCl. Bache and Sharp (1976) had a Similar result with acid Scottish soils. They pointed out that protons released by dissociation of HOAc in the buffered extract would favor protonation and depolymerization of hydroxy-Al structures too large to be directly exchangeable at pH greater than the pKa = 4.7 of acetic acid. They speculated that protonated Al appearing in their N NH OAc 4 extracts may have originated in interlayer precipitates, separate phase alumina gels or organic-Al complexes. In support of this suggestion, it should be noted that all NH40Ac-extractable forms of Al in the surface of unlimed-sulfate soil was also exchangeable to KCl. Since the net surface charge of the subsoil was the same (Table 9), the much larger recovery in NH40Ac must have come from sources not associated with exchange surfaces. The same observation can be made for the surface and subsoils of the limed sulfate plots which produced the same pH. As a further speculation, it should be noted that dissociation of HOAc would Oppose the hydrolysis of Al species in solution, thereby inhibiting the reprecipitation of charged Al displaced by the initial exchange for NH4+. In any case the quantities Of Al in NH40Ac (Table 10) declined Sharply as pH(H20) increased from 4.9 in unlimed-sulfate subsoil to 5.4 in unlimed-nitrate surface soil. Further decreases with increasing pH(H20) were gradual, as might be expected if Al removal were, in fact, controlled by the buffering capacity of a constant initial concentration of HOAc. 89 Bache and Sharp (1976) observed that differential recoveries of Al with NH40Ac were unusually high in soils high in Fe. Since Fe3+ forms more stable complexes with humic substances (Schnitzer and Khan, 1972) and with phosphates (Lindsay, 1979), its presence may have facilitated diSplacement of Al3+ by NH4+ through mechanisms other than cation exchange. For example, NH4-Al-PO4 (taranakite) is much more soluble than AlP04. 2H20 (variscite), which in turn, is more soluble than FePO4. 2H20 (strengite). In the present study, phosphate was recovered in larger quantities in NH OAc than KCl (Appendix A1); Fe was also, but in 4 much lower concentrations than Al (Table 10, Appendices A6, A9). These results would be consistent with the proposition that NH +, as distinct from K+, may enter into complex reactions in addition to cation exchange. The amination of phenolic compounds and humic substances by NH4+ can be catalyzed, at acid pH, by extracellular phenoloxidases as well as by alumina or silica (Mortland and Wolcott, 1965; Sulfita and Bollag, 1981). This can be a mechanism for breaking coordinate-covalent bonds between A1 and phenolic ligands. Data in Table 11 provide evidence that, in addition to + in NH electrovalent cation exchange, the NH OAc displaced Al from 4 4 sites of stronger binding also. Equivalent quantities of Al were removed by NH4OAc and by KCl from the surface of the unlimed-sulfate soil (Table 10). However, the complexed Al in Table 11 represents only 15% of the total in NH40Ac , whereas 68% of the Al in KCl was complexed (Table 13; "None" under incubation treatment). The same pattern Of percent complexation is found in the subsoil, even though a much larger total quantity was recovered in NH4OAc (Table 10). Hr .nr Erwin Ht ?‘ OI (AE3: hm NW 90 Table 11. Free and complexed aluminum in extracts of surface soil (0-25 cm) and subsoil (50-75 cm): long-term residual effects of nitrogen carriers and lime. ‘Treatment Direct extractiOn N carrier ‘Profile NHAOAc ' 1 N KCl in field depth ‘Free Al 'COmpTexed“Al' ‘Free AT ‘COmplexed”Al ugfg soil a. Main effects Sulfate Mean 103 b+ 19.9 b 25.7 b 51 Nitrate Mean 45 a 11.4 a 4.8 a 6.1 Mean Surface 63 a 16.0 16.0 31 Mean Subsoil 86 b 15.2 14.5 26.2 Mean fOr no Time 111 b 20.6 b 29.2 b 55 Mean fOr Time 37 a 10.6 a 1.29 a 1.71 b. Interactions of N carriers with lime No lime Sulfate Mean 163 b 27.9 b 49 b 99 b Nitrate Mean 59 a 13.4 a 9.5 a 12.1 a Lime Sulfate Mean 43 11.9 b 2.46 3.4 Nitrate Mean 32 9.3 a 0.118 0.053 LSDO5 40 2.02 26.5 78.4 c. Interactions of profile depth with lime NO lime Mean Surface 99 21.6 32 62 Mean Subsoil 124 19.6 26.5 49 Lime Mean Surface 27 10.3 0.013 0.029 Mean Subsoil 48 10.9 2.56 3.4 LSD 60 5.7 ns* ns 05 9] Table 11 (cont'd.). Treatment Direct extractiOn N’carrier Profile NH’OAc l N KCl in field depth Free AI |Complexeil Al Free Al Complexe'cl AI ug/g sOiT' — d. Interactions of profile depth with N carriers Mean of lime + no lime Sulfate Surface 80 a 17.1 a 24.5 51 Sulfate Subsoil 126 b 22.6 b 27.0 51 Nitrate Surface 45 14.9 b 7.5 10.5 Nitrate Subsoil 46 7.8 a 2.12 1.70 LSD05 60 5.7 ns ns e. Second order interactions No Time Sulfate Surface 131 a 23.2 a 49 103 Sulfate Subsoil 196 b 33 b 49 95 Nitrate Surface 66 20.1 b 14.9 20.9 Nitrate Subsoil 52 6.7 a 4.0 3.3 Lime Sulfate Surface 29.3 11.0 0.001 0.022 Sulfate Subsoil 57 12.7 4.9 6.7 Nitrate Surface 23.8 9.6 0.026 0.036 Nitrate Subsoil 40 9.0 0.210 0.070 LSD05 85 8.1 ns ns +a,b = Duncan's equivalent ranges. Within columns and subsets of two * [15 means, means fOllowed by none or the same letter are not different at P(05). For other comparisons in a given category of interactions, the tabulated LSD05 applies (Steel and Torrie, 1980). = not significant at P(05) 92 Table 12. Free and complexed aluminum in fractional extracts of surface soil (0-25 cm) and subsoil (50-75 cm): long-term residual effects of nitrogen carriers and lime. FraCtiOnaT'extraCtiOn Treatment TTH’O 0.1'N KCl 1 N KCl" NHADAC N Profile Free 2(3me Free Compx Free 001an Free ' Canpx carrier depth Al Al Al Al Al Al Al Al —- — ug/g soiT ‘ a. Main effects Sulfate Mean 0.029b*0.103b 9.2 14.6 12.6 20.2 - Nitrate Mean 0.014a 0.023a 0.277 0.55 1.40 2.07 - - Mean Surface 0.039b 0.103b 4 0 10 13 0 17.1 9.4 Mean Subsoil 0.004a 0.023a 5 5 4 9 2 Mean fOr no lime 0.036b 0.121b 9.1 14. Mean fOr lime 0.007a 0.005a 0.38 0 b. Interactions of N carriers with lime NO lime Sulfate Mean 0.050b 0.202b 17.6 28.5 b 24.9 b 39 - - Nitrate Mean 0.022a 0.0396 0.55 1.09a 2.80a 4.1 - - Lime Sulfate Mean 0.008 0.004 0.76 0.84 0.256 1.74 - - Nitrate Mean 0.006 0.007 0.0002 0.015 0.012 0.006 - - LSD05 0.016 0.113 ns** 22.4 21.5 37 - - c. Interactions of profile depth with lime No lime Mean Surface 0.065b 0.200b 7.9 20.7 b 18 3 26.1 33 13.7 Mean Subsoil 0.007a 0.042a 10.2 8.9 a 9.4 16.7 - - Lime Mean Surface 0.013 0.006 0.001 0.019 0.0002 0.003 9.2 5.1 Mean Subsoil 0.001 0.005 0.76 0.84 0.268 1.75 - - LSOO5 0.030 0.140 ns ns ns ns - - 93 Table 12 (cont'd.). ‘FractTOnaT’eXtraCtiOn Treatment 7190 0.1 N KCl TN KClw NH 0Ac N Profile Free ' (1an Free 00me Free 00me Free I Compx carrier depth Al Al Al Al Al Al Al Al ugfg soiT d. Interactions of_profile depth with N carriers Mean of lime + no lime Sulfate Surface 0.057b 0.173b 7.6 19.8b 15.8 22.7 25.6 10.0 Sulfate Subsoil 0.001a 0.033a 10.7 9.5a 9.4 17.7 - - Nitrate Surface 0.020 0.033b 0.35 0.89 2.48 3.4 16.3 8.8 Nitrate Subsoil 0.008 0.013a 0.207 0.22 0.33 0.77 - - LSD05 0.030 0.14 ns ns ns ns - - e. Second order interactions No line Sulfate Surface 0.099b 0.34 b 15.2 40 b 32 b 45 34 15.0 Sulfate Subsoil 0.001a 0.064a 20.0 17.3a 18.2a 32 - - Nitrate Surface 0.031 0.059b 0.69 1.76 5.0 6.7 19.7 12.5 Nitrate Subsoil 0.014 0.020a 0.41 0.42 0.63 1.54 - - Lime Sulfate Surface 0.016 0.005 0.003 0.023 0.000 0.00 7.8 5.1 Sulfate Subsoil 0.0005 0.003 1.52 1.66 0.51 3.5 - - Nitrate Surface 0.010 0.007 0.0002 0.015 0.000 0.004 7.0 5.1 Nitrate Subsoil 0.001 0.007 0.0002 0.016 0.024 0.00 - - LSD05 0.043 0.198 ns ns ns ns - - + Compx = Complexed Al * a,b = Duncan's equivalent ranges. Within columns and subsets of two means, means fOllowed by none or the same letter are not different at P(05). For other comparisons in a given category of interactions, the tabulated LSD05 applies (Steel and Torrie, 1980). ** ns = not significant at P(05) 94 Table 13. Percent complexed aluminum in extracts of surface soil (0-25 cm) and subsoil (SO-75 cm): effects of incubation treatments and long-term residual effects of nitrogen carriers and lime. Treatment Direct eXtraCtiOn FraCtiOnaTFeXtractiOn N* ‘IncubatiOn ‘NHAOAc 1N* ‘HéO ‘021N 1N ‘NHAOAC carrier treatment KCl ' KCT' KCl No Time Surface Su ate None 15 68 77 72 58 31 +OM 11.7 69 82 49 78 21 +0M+N 9.9 73 63 49 78 18.6 Subsoil u ate None 14 66 98 46 64 - Surface Nitrate None 23 58 66 72 57 39 +OM 22 62 73 31 76 24 +0M+N 20 68 77 53 82 22 Subsoil Nitrate None 11.4 45 59 51 71 - Lime Surface Sulfate None 27 96 24 88 91 39 +0M 20 50 49 32 91 23 +OM+N 18.1 53 78 46 69 21 Subsoil Sulfate None 18.2 58 86 52 87 - Surface 'NTtFEtE' None 29 58 41 99 95 42 +OM 29 69 61 29 64 27 +OM+N 22 42 60 18.6 43 26 Subsoil Nitrate None 18.4 25 87 99 22 - 95 With the other three treatments, the prOportion complexed in NH40Ac increased with pH in the surface soil to 29% in the limed plots (Table 13). The range in subsoils was lower (rising with pH from 11% to 18%). For the same three treatments, the percentage complexed in KCl was 2 to 3-fold greater than in NH OAc and was also higher in surface 4 soils than in subsoils. (This comparison for three of the limed samples may be questioned because of the very low quantities of Al recovered in KCl (Table 10).) It should be emphasized that the 8-0Hq procedure does not distinguish between complexed and polymeric Al (Bloom et al., 1978). This was recognized by Bache and Sharp (1976), whose results paralleled those of the present study to the extent that the prOportion of polymeric or complexed Al in NH40Ac and KCl extracts increased with pH. However, in their investigation, the prOportion was not consistently greater in KCl, but varied from soil to soil and with the ionic strength of the NH4OAc or KCl. Their soils contained 3 to 5% organic C, which is higher than the less than 1% in all soils of this study (Table 9). The degree of complexation or polymerization in both extractants (Tables 11, 13) was greater in surface soils, which were also much higher in organic C than subsoils (cf. Table 9). The prOportion complexed or polymerized in NH4OAc increased with increase in pH and was associated with increased C content in both surface soils and subsoils. The prOportion complexed in KCl was 2 to 4 fold greater than in NH4OAc for all soils. Total recoveries from subsoils and limed-surface soils were very low, and a consistent relationship between percent complexation and pH did not appear. This suggests that organically complexed Al tended to remain in solution in both extracts, but the 96 proportion was reduced in NH40Ac by displacement of some of the Al from its complexes by NH4+. Possibly NH4+ may also interfere with polymerization of Al by specific mechanisms which had not been investigated in literature reviewed for this thesis. Fractional Extractions-- Effects of prior hot-water treatment provide further evidence that organic complexes contributed to extractability of Al. An important result in Table 10 is that Al recovered in NH40Ac after the HZO-soxhlet extraction was less by 60% than in direct NH40Ac extracts. Very little Al was removed in the hot water. Reduced extractability after this treatment may have resulted for a number of reasons. The chemical potential of water would have been increased substantially at 100 C, and processes Of polymerization and crystallization that occur naturally with aging would have been accelerated (Bloom et al., 1981; Coleman and Thomas, 1967; Hsu, 1966). Aluminum removed into separate amorphous or crystalline phases, would not have been exchangeable, nor easily extracted. Displacement of complexing Al and Fe would have released water-soluble organic ligands into the hot water extracted. Their removal prior to extraction with NH40Ac would have further reduced the solubility of Al. The data suggest that non-complexed and weakly complexed forms of Al were precipitated selectively, since a larger prOportion of the Al in NH40Ac after hot-water treatment was polymerized or complexed (Table 12) than in the direct extraction (Tables 11, 13). The comparison can be made only for surface soils, where the percentage polymerized or complexed in NH4OAc after the HZO-soxhlet extraction ranged from 31% in 97 unlimed-sulfate to 42% in limed-nitrate plots. The corresponding range in the direct NH4OAc extracts was 15% to 29%. In the K01 sequence after hot-water treatment (Table 10), smaller quantities of Al were recovered in 0.1N KCl than in the IN KCl extraction that followed. This is the eXpected mass action effect. The summed recoveries (0.lN_KCl plus lN_KCl) were less than in the direct N_ KCl extract, reflecting the removals of Al by precipitation in hot water. For soils in Table 12 where reliable estimates could be made, the prOportion complexed in extracts from surface soils was greater in 0.1N_ KCl (>70%) than in N_KCl (<60%) (Table 13), whereas the reverse was true in subsoils (50% complexed in 0.1N KCl _v__s__._ 64 to 87% in N KCl, increasing with pH). Estimates of organic C could not be made in N KCl because of nor'hiNH OAc because of the C in the acetate. ’ 4 Organic C was determined in the hot-water extract and in 0.1N_KCT. The interference from CT- data may be seen in Table 14 where the analyses that relate to Table 12 are identified by "None" under “Incubation treatment”. The larger total recoveries of Al from unlimed surface soils (Table 10) and the larger quantity complexed in H20 and in 0.1N_KCl (Table 12) were associated with much greater quantities of C extracted from surface soils than from subsoils (Table 19). This apparent correlation between extractable C and degree of complexation or polymerization did not appear in limed plots, where quantities of extractable C were similar to those in unlimed plots, but very little Al was extracted. These observations are consistent with the accepted view that Al 98 moves in soils mainly in association with organic ligands introduced by biological systems in upper soil horizons (Buol et al., 1963; Foth, 1978; Grim, 1968; Jackson, 1963). Complexes of lower molecular weight are stable and exchangeable at low pH but are flocculated at higher pH (Schnitzer and Khan, 1972), or are broken down as Al separates into amorphous or crystalline phases on dehydration and aging (Greenland, 1971; Spycher and Young, 1977). EFFECTS OF INCUBATION TREATMENTS A second Objective in this research was to Observe effects of 0M decomposition and nitrification during incubation on extractability and inferred mobility of Al. For this purpose, 200 9 samples of field moist surface soil from limed and unlimed-sulfate and nitrate plots were amended with finely ground corn tissue (1500 ug/g = 3.36 T/ha, 25 cm) with and without 150 ug/g N supplied as NH4CT. The amended soils were incubated at near water holding capacity and 27 C for 70 days. They were then subjected to the same extractions and analyses as the unincubated field moist samples. In tables that follow, data presented earlier for unincubated surface soils are entered again for comparison with the data for soils amended with corn tissue (+0M) or with corn tissue plus NH4CT (+OM+N). 99 Changes in Soil pH, C and N Incubation of the soil samples with corn tissue alone did not alter soil pH (Table 14). With the OM+N treatment, pH changes were related directly to the extent to which added NH);+ was nitrified. In limed soils, all of the added N (150 ug/g) was nitrified, and pH(KCl) was reduced by 1.4 pH units. In unlimed-nitrate soils, approximately one-third of the added NH4+ was nitrified, and the corresponding decrease in pH(KCl) was only 0.4 pH units. In unlimed-sulfated soils, net nitrification of added NH4+ did not occur (no increase in N03' over the OM treatment), and there was no change in pH(KCl). Calculated data in Table 15 indicate that only 67% (100 ug/g) of the NH +-N added could be accounted for in mineral forms, 4 after allowing for mineral N released from soil and from added corn tissues. Of the 50 ug/g not accounted for, more than half will be accounted for by the increase in organic N from 570 ug/g in the 0M treatment to 605 ug/g in the OM+N treatment. The balance can be reasonably ascribed to denitrification of N03" initially present or of N02' of N03' produced during incubation. If denitrification had greatly exceeded nitrification, an increase in pH would have been expected (Chang and Broadbent, 1980; Hiltbold and Adams, 1960). However, the difference in N03' between 0M and OM+N treatments is negligible and there is no difference in pH. The calculated releases of mineral N due to added 0M (Table 15) indicate that heterotrOphic activities were not inhibited, even in the most acid sulfate soils. The N content of the added corn tissue was not determined but would have been of the order of 1.5%, or 22 ug/1500 ug lOO Table 14. Soil pH, farms of carbon and nitrogen in surface soil: effects of incubation treatments and long-term residual effects of nitrogen carriers and lime. Treatment pH Tot. TKN Org. MineraT N N Incubation (KCl) C N C/N “NH; NO3 Tot. carrier treatment % ug/g soil ------ ugfg soil ----- a. Main effects Sulfate Mean 4.5a+ 0.90 640 620 14.5 24.0 40 63 Nitrate Mean 4.8b 1.00 710 690 14.5 18.6 53 74 Mean None 5.0b 0.91 635a 635a 14.3 1.64a 11.8a 12.5a Mean +OM 4.9b 0.93 655a 650a 14.3 5.4 a 24.4b 31 b Mean +0M+N 4.1a 1.01 735b 680b 14.8 57 b 103 c 162 c Mean fOr no lime 3.9a 0.94 665 630 14.9 39 b 20.2a 62 Mean fOr lime 5.4b 0.97 680 680 14.3 3.3 a 73 b 75 b. Interactions of N carriers with Time No lime Sulfate Mean 3.6a 0.86 615 570 15.1 44 11.1 57 Nitrate Mean 4.2b 1.01 715 680 14.8 34 29.4 68 Lime Sulfate Mean 5.4 0.94 665 660 14.2 3.5 68 70 Nitrate Mean 5.4 0.99 700 700 14.1 3.1 77 79 LS005 0.2 ns* ns ns - 26.2 24.9 ns c. Interactions of incubation treatments with Time No Time Mean None 4.1b 0.89 615 615 14.5 2.10a 9.2a 10.0a Mean +OM 4.0b 0.92 630 620 14.8 7.7 a 18.6a 28.3b Mean +0M+N 3.7a 1.00 755 650 15.4 108 b 33 b 149 c Lime Mean None 5.8b 0.93 655 655 14.2 1.19 14.5a 15.1a Mean +OM 5.8b 0.95 680 680 14.0 3.0 30 b 34 b Mean +0M+N 4.5a 1.02 710 705 14.5 5.6 173 c 174 c LSD 0.3 ns ns ns - 38 33 32 05 Table 14 (cont'd.). )0] Tot. TKN Urg . lreafifint ' Mineral N N Incubation (KCl) C N C/N “NH4 N03 Tot. carrier treatment "* “ ‘ ' * % ug/g sOil ------ ug/g sOil ---- d. Interactions of incubation treatments with N carriers Sulfate None 4.8b 0.87 Sulfate +OM 4.7b 0.88 Sulfate +0M+N 4.0a 0.94 Nitrate None 5.2b 0.95 Nitrate +0M 5.1b 0.98 Nitrate +0M+N 4.2a 1.07 L3005 0.3 ns e. Second order interactions Sulfate None 3.7 0.81 Sulfate +04 3.6 0.86 Sulfate +0M+N 3.6 0.90 Nitrate None 4.5b 0.97 Nitrate +OM 4.3b 0.97 Nitrate +0M+N 3.9a 1.10 Sulfate None 5.8b 0.94 Sulfate +0M 5.8b 0.91 Sulfate +0M+N 4.4a 0.99 Nitrate None 5.9b 0.92 Nitrate +0M 5.9b 0.99 Nitrate +0M+N 4.5a 1.05 LSD05 0.4 ns Means of lime + no lime 580a 620a 725b 580 610 665 690 690 740 690 690 690 ns - 550a 585a 720b 550 570 605 685a 675a 790b 685 675 690 615a 615 655ab 655 725b 720 695 705 695 695 700 690 ns '- 15.0 14.4 14.1 13.8 14.2 15.5 No lime 14.7 15.1 14.9 14.2 14.4 15.9 Lime 15.3 13.9 13.7 13.2 14.4 15.2 1.96a 7.5 a 7.5a 20.4b 91 c 16.2a 28.5b 115 c 33 8.7a 30 b 151 c 16.3a 32 b 172 c 32 8.1a 31 b 132 c 11.8a 25.7a 166 b 9.3a 29.8b 171 c 20.8a 39 b 178 c 45 +a,b,c = Duncan's equivalent ranges. Within columns and subsets of two or three menas, means fOllowed by none or the same letter are not different at P(05). For other comparisons in a given category of interactions, the tabulated LSDO5 applies (Steel and Torrie, 1980). *ns = not significant at P(05) l02 Table 15. Estimates of nitrogen mineralization during incubation and net recovery of mineral nitrogen. Treatment ‘Mineral”N7 Net mineralized"Net recovery' N7 ‘IhcdbatiOfi NHg- N03- Tot. Soil Due to % of carrier treatment alone added OM added N ----------- ug/g soil’ 7%? No Time Surface soil u a one 2.50 a* 6.9 8.1 a 8.1 - Sulfate +0M 12.6 a 16.2 31 b - 22.9 - Sulfate +0M+N 118 b 10.1 132 c - - 67 Subsoil SulfatE None 0.52 1.03 1.55 1.55 - - Surface soil Nitrate ’NOne 1.71 a 11.5 a 11.8 a 11.8 - - Nitrate +OM 2.90 a 21.0 a 25.7 a - 13.9 - Nitrate +0M+N 98 b 56 b 166 b -. - 93 Subsoil ra None 0.71 1.82 2.54 2.54 - - Lime Surface soil ‘SUTfate ‘NOne 1.42 8.2 a 9.3 a 9.3 - - Sulfate +0M 2.44 24.5 b 29.8 b - 20.5 - Sulfate +0M+N 6.7 172 c 171 c - - 94 Subsoil Sulfate None 0.64 1.92 2.56 2.56 - - Surface soil ’Nitrate 'NOne 0.96 20.8 a 20.8 a 20 8 - - Nitrate +0M 3.6 36 b 39 b - 18.2 - Nitrate +0M+N 4.6 175 c 178 c - - 92 Subsoil Nitrate None 0.65 2.4 3.0 3.0 - - LSD05 1.44 10.6 10.5 - - - TNet recovery = Mineral N in excess of that fOund where only organic matter was added. * a,b,c = Duncan's equialent ranges. Within column and subsets of three means, means fOllowed by none or the same letter are not diferent at P(05). For comparison between surface soil (no incubation) and subsoil in a given category of interaction the tabulated LSD05 applies (Steel and Torrie, 1980). l03 tissue. Calculated releases ranged from 14 to 23 ug/g. The indicated release is less in nitrate soils than in sulfate soils. Also, the data in Table 14 shows a net loss of organic N in unlimed-nitrate soil when 0M was added and that very little of the added organic N was retained in the limed-nitrate soils. These results suggest that the addition of fresh tissue to nitrate soils enhanced the release of mineral N from native SOM ("priming action“, cf. Allison, 1973), and that a substantial prOportion of the mineralized N was lost by denitrification. Residual SOM in both limed- and unlimed-sulfate soils was more resistant to decomposition as evidenced by lower levels of mineral N in the non-amended soils (Table 15). Surface prOperties in the sulfate soils apparently favored retention of N in organic forms both from added tissue and from added NH4+ (Table 14). At the low pH levels of the unlimed-sulfate and nitrate soils, nitrification may have been carried out by acid-adapted strains of chemoautotrOphic bacteria, or by acid-tolerant heterotrOphic nitrifiers (Alexander, 1977; Ishaque and Cornfield, 1972; Weber and Gainey, 1962). At a minimum, organisms capable of oxidizing NH4Jr to NO ' would need to 2 be present. Nitrite can be converted to N03“ non-enzymatically. The conversion can be catalyzed by transition metals such as Mn (Bartlett, 1980). Such reactions are favored by acid pH. Also at this acid pH, N03' can be formed spontaneously by dismutation of undissociated nitrous acid (Allison, 1973). Losses Of N may have occurred in several systems in this study (Table 15). Biological denitrification requires anaerobic conditions and pH near neutral or higher (Alexander, 1977). On the other hand, chemical denitrification can occur in well-aerated soils and over a wide l04 pH range (Clark, 1962; Hiltbold and Adams, 1960). Chemodenitrification is favored in acid media since it involves undissociated nitrous acid and various nitrosating species to be found in equilibrium with it (Bremner and Fuhr, 1966; Mortland and Wolcott, 1965; Stevenson et al., 1970): HNO ==E N O fizz} NO + N02 §==E N204 §==§ NO+ + NO '- 2" 23 3 Reactions of undissociated HNO2 and its equilibrium products at acid pH with lignin, lignin derivatives, and humic substances lead to the formation of a number of gaseous products, which are mainly N2 and N20. In the initial reactions, N is incorporated into organic combinations, some of which are stable to alkaline or acid hydrolysis. Additional N enters into labile combinations which break down to release NH3 in alkali or NH4+ in acid -- evidence that the nitroso or nitro groups formed initially are readily reduced to amines or amides. Further nitrosation leads to unstable N-nitroso groups which decompose on dehydration to release N2 or N20. From the above, it is apparent that reductive reactions of HNO2 and its equilibrium products with organic fractions in acid soils can give rise to chemo-immobilization, chemo-nitrification and chemo-denitrification. Ammonium can also be immobilized at acid pH by oxidative coupling in the presence of extra-cellular phenoloxidases, AlOH, Fe3+, or $102 (Flaig et al., 1975; Mortland and Wolcott, 1955). However, for denitrification to occur, NH4+ must first be nitrified at least to NO '. This first step in nitrification apparently requires the 2 mediation of enzymes in living organisms. 105 Changes in Distributions of Al It was anticipated that incubation of soils amended with fresh tissue only, and with fresh tissue plus N (NH4Cl) would permit some judgment to be made regarding the relative importance of chelation .‘_'.§_°.. acidification in mobilizing Al in the first place. It was hOped that the distribution of "free" and “complexed“ Al (8-0Hq method) in direct and fractional extractions would provide evidence regarding the role of chelation in promoting the movement of mobilized Al downward through the profile. Direct Extractions-- The incubation treatments affected soil pH (Table 14) which in turn influenced the levels of Al extractable in N NH OAc (pH 4.8) and 4 exchangeable in N KCl (Fig. 1; Table 16). Above pH 5.0, exchangeable Al became negligible. In the soils of this experiment, pH(KCl) “ 5.0 corresponds to pH(HZO) “ 6.0 (Table 9). It is usually found that exchangeable Al occupies less than 10% of the ECEC at pH 6.0 (Coleman and Thomas, 1967; Kamprath and Fay, 1971). Above pH(KCl) 5 3.7, larger amounts of Al were extracted in NH OAc 4 than in KCl. As noted earlier, the extra Al in NH4OAc must come from sources not associated with exchange surfaces but nonetheless susceptible to mobilization by protons released as HOAc in the buffered NH40Ac dissociates with increasing pH(HZO) of the soil. Since the lines drawn visually in Fig. 1 cross at pH(KCl) ’3 3.7, it leads to the expectation that exchangeable Al in soils with pH(HZO) less than pH 4.8 of the buffered NH OAc (or less than pKa = 4.7 of HOAc) 4 lat) I6£> I44) l2!) 5 " ICHD ID - c! 3 so 2 GI) 40 2!) C) Figure l. 106 \)N <3 )Alin NHN‘OUAO - (J Iklln KK3I \e '9 P, pH(O-IN KCl) Relationships Of soil pH (0.1N KCl) and distribution of A1 in surface soils extracted by N NH40Ac (pH 4.8), and by N KCl. 107 Table 16. Total aluminum (detenmined by 8-hydroxyquinoline) in surface soil: effects of incubation treatments and long-term residual effects Of nitrogen carriers and lime. A 4 Treatment Direct extraction Fractional extraction N carrier InEfibation NH40Ac 1 N H20 0.1 N_ FTIPN NH4OAc in field treatment KCl KCl KCl -------------------- ug/g soil ------ - a. Main effects Sulfate Mean 98 83 b+ 0.75 28.5 b 31 b 36 b Nitrate Mean 70 29 a 0.155 3.4 a 7.8 a 24.88 Mean None 79 47 a 0.142 a 14.3 a 22.2 b 26.58 Mean +OM 80 51 b 0.130 a 15.2 ab 15.4 a 29.0b Mean +0M+N 93 70 c 1.08 b 18.2 b 20.2 ab 35 c Mean fOr no Time 120 b 107 b 0.81 31 b 38 b 46 b Mean for Time 47 a 5.0 a 0.094 0.30a 0.68a 14.68 b. Interactions of N carriers with Time No lime Sulfate Mean 144 b 159 b 1.34 56 b 60 b 56 b Nitrate Mean 97 a 54 a 0.281 6.5 a 15.3 a 36 a Lime Sulfate Mean 52 6.4 0.159 0.42 1.01 15.4 Nitrate Mean 42 3.7 0.029 0.18 0.35 13.8 LSD05 39 40 1.22 20.0 20.3 12.0 c. Interactions of incubation treatments with lime No Time Mean None 120 94 a 0.2658 28.6 a 44 41 a Mean +0M 115 101 a 0.2258 30 a 31 45 b Mean +0M+N 126 125 b 1.93 b 36 b 38 51 c Lime Mean None 37 a 0.04 a 0.019 0.021 0.003 12.58 Mean +0M 44 ab 0.1188 0.035 0.044 0.019 12.6a Mean +0M+N 60 b 14.8 b 0.229 0.85 2.03 18.8b LSD 54 51 ns* 25.1 26.7 15.3 108 Table 16 (cont'd.). g 4. Treatment Direct extraction Fractional extraction N carrier Incubation NH40Ac 1 N H20 0.1 N 1 N NH40Ac in field treatment KCl KCl KCl -------------------- ug/g soil - d. Interactions of incubation treatments with N carriers Mean of no lime + lime Sulfate None 97 76 a 0.230 a 27.4 38 b 31 a Sulfate +0M 93 82 ab 0.200 a 29.0 26.3 a 36 b Sulfate +0M+N 103 91 b 1.81 b 29.1 27.2 a 40 c Nitrate None 60 a 18.0 a 0.053 1.23 a 5.9 ab 22.18 Nitrate +OM 66 a 20.0 a 0.061 1.49 a 4.5 a 22.48 Nitrate +0M+N 83 b 49 b 0.35 7.3 b 13.1 b 30 b LSD05 ns 51 ns 25 27 15.3 e. Second order interactions No lime Sulfate None 154 152 0.44 a 55 77 b 49 a Sulfate +OM 137 163 0.36 a 58 53 a 58 b Sulfate +0M+N 140 163 3.21 b 57 51 a 60 b Nitrate None 86 36 a 0.089 2.45 a 11.7 a 32 a Nitrate +OM 93 40 a 0.090 2.92 a 9.1 a 33 a Nitrate +0M+N 112 87 b 0.66 14.2 b 25.2 b 42 b Lime Sulfate None 40 0.0248 0.021 0.026 0.002 12.98 Sulfate +0M 49 0.1268 0.040 0.028 0.024 13.38 Sulfate +0M+N 66 18.8 b 0.42 1.22 3.01 20.0b Nitrate None 34 0.061 0.017 0.015 0.004 12.08 Nitrate +0M 40 0.110 0.031 0.059 0.014 12.08 Nitrate +0M+N 54 10.9 0.040 0.48 1.04 17.5b LSD05 77 72 3.1 35 38 22 + a,b,c Duncan's equivalent ranges. Within columns and subsets of two means, means fOllowed by none or the same letter are not different at P(05). For other comparisons in a given category of interactions, the tabulated LSD05 applies (Steel and Torrie, 1980). ns = not Significant at P(05) 109 will tend to polymerize during equilibration, and therefore recoveries in NH4OAc will be less than in KCl. For the soils of this experiment, pH(KCl) = 3.7 corresponds to pH(H20) 3 4.5 (Table 9). Equivalent quantity of Al were recovered by direct extraction in NH4OAc and KCl from amended sulfate soil of pH(KCl) ‘3 3.7 (Table 16). At pH(KCl) = 3.6 of amended soils, recoveries of Al in NH4OAc were reduced while Al exchangeable to KCl was increased. The change in pH is small but consistent with generation of protons in complexation reactions of Al and other metals with organic ligands (Schnitzer and Khan, 1972). The pH reduction in unlimed-sulfate soils was the same for OM and OM+N treatments, as might be eXpected Since net nitrification of added NH4+ did not occur (Table 14, 15), and recoveries of exchangeable Al were the same (Table 16). It is likely that recoveries in both extractants were determined primarily by the pH of the suspensions at equilibrium. Most of the Al in unlimed-sulfate soil appears to have been associated with exchange surfaces, since differences in extractability in NH4OAc or KCl and effects of added 0M and N were small. These soils had been at pH < 4.5 for 18 years or longer (Table 6). Aluminum in separate amorphous or crystalline phases had been largely mobilized and displaced downward in the profile (Tables 7, 8). By contrast, data in Table 16 indicate that in unlimed-nitrate and in limed soil of both carriers, substantial quantities of Al were retained in non-exchangeable sesquioxides susceptible to mobilization by chelation or by protonation. It appears that protonation was much more effective than chelation, since increased recoveries in both direct llO extractions were much greater when both 0M and N were added than when 0M was added alone. It is likely that some of the mobilized Al came from charged coatings associated with exchange surfaces, where they would have contributed to the variations in ApH given in Table 9.. However, the much larger recoveries in NH OAc indicate that separate phase alumina 4 was a principal source in this extractant. Although protonation appears to have been a main mechanism for mobilizing Al from non-exchangeable sources, chelation appears to have been important in stabilizing displaced Al in solution (Table 17). Complexed Al in KCl represented 42 to 96% of the total extracted (cf. Tables 17 and 13). In NH40Ac, the percentage complexed was much lower (10 to 29%). As suggested earlier, the lower degree of complexation in NH4OAc might be due to displacement of Al by NH4+ from phenolic ligands. This suggestion is supported by the greater reduction in percent complexed in the NH40Ac extract from unlimed-sulfate soil (Table 13), where nitrification was strongly inhibited by low pH and substantial levels of NH4+ were maintained through the incubation period ‘in both 0M and OM+N treatments (Tables 14, 15). In limed-sulfate soil, displacement of complexed Al by NH + during incubation might have contributed to lower 4 percent complexation in both NH40Ac and KCl extracts from amended soils. In limed-nitrate soil complexation was reduced in both extracts only when N was added (Table 13). It must be recognized that the 8-0Hq method does not differentiate between complexed and polymeric forms of Al that are slow to react. Also, there is evidence that forms that react instantaneously with 8-0Hq 111 Table 17. Free and complexed aluminum in extracts of surface soil: effects of incubation treatments and long-term residual effects of nitrogen carriers and lime. Treatment Direct extraction N carrier Incubation NH40Ac 1 N KCl in field treatment Free Al Complexed Al Free Al Complexed Al ---------------------- ug/g soil - (a. Main effects Sulfate Mean 84 5* 14.3 25.4 b 57 b Nitrate Mean 54 a 15.9 10.7 a 18.3 a Mean None 63 16.0 b 16.0 a 31 a Mean +OM 66 14.1 a 16.6 a 34 a Mean +0M+N 78 15.2 ab 21.7 b 48 b Mean fOr no Time 101 b 19.4 b 34 b 73 b Mean for lime 36 a 10.8 a 2.54 a 2.468 b. Interactions of N carriers with lime No lime Sulfate Mean 126 b 17.7 48 b 111 b Nitrate Mean 76 a 21.2 19.3 a 35 8 Line Sulfate Mean 41 11.0 2.97 3.3 Nitrate Mean 32 10.5 2.12 1.57 LSD05 32 8.4 12.4 27.5 c. Interactions of incubation treatments with lime No Time Mean None 99 21.6 b 32 62 a Mean +0M 97 18.2 a 33 68 a Mean +0M+N 108 18.4 a 36 89 6 Lime Mean None 26.6 10.3 0.0138 0.029 Mean +0M 34 10.0 0.0488 0.070 Mean +0M+N 48 11.9 7.6 b 7.3 L8005 49 3.6 17.9 36 112 Table 17 (cont'd.). Treatment Direct extraction N carrier Incubation NHAOAc 1 N KCl _ in field treatment Free Al ”Complexed Al Free Al Complexed Al — - — —— ug/g soil - d. Interactions of incubation treatments with N carriers Mean of lime + no lime Sulfate None 80 17.1 b 24.5 51 a Sulfate +0M 80 13.0 a 25.6 56 ab Sulfate +0M+N 90 12.9 a 26.3 65 b Nitrate None 45 14.9 7.5 a 10.5 a Nitrate +OM 51 15.2 7.5 a 12.5 a Nitrate +0M+N 66 17.4 17.1 b 32 b LSD05 ns* 3.6 17.9 36 e. Second order interactions No lime Sulfate None 131 23.2 b 49 103 a Sulfate +0M 121 16.0 a 51 112 ab Sulfate +0M+N 126 13.9 a 44 119 b Nitrate None 66 20.1 14.9 a 20.9 a Nitrate +OM 72 20.4 15.1 a 24.8 a Nitrate +0M+N 89 22.9 27.8 b 59 b Lime Sulfate None 29.3 11.0 0.001 0.022 Sulfate +OM 39 10.0 0.062 0.063 Sulfate +0M+N 54 11.9 8.8 9.9 Nitrate None 23.8 9.6 0.026 0.036 Nitrate +OM 29.7 10.1 0.034 0.076 Nitrate +0M+N 42 11.9 6.3 4.6 LSD05 70 5.1 25.3 52 +a,b,c = Duncan's equivalent ranges. Within columns and subsets of two NS = or three means, means fOllowed by none or the same letter are not different at P(05). For other comparisons in a given category of interactions, the tabulated LSD05 applies (Steel and Torrie, 1980). not significant at P(05) 113 ("free" Al) may include complexed forms, as well as monomers. For example, 80% of a complex of Al with salicylic acid prepared in the laboratory reacted with 8-0Hq at zero time (Hoyt and Turner, 1975). Thus, increases in "free" Al in both extractants (Table 17) when fresh 0M was added might have included low molecular weight organic chelates of Al. Fractional Extractions-- Hot water removed negligible Al, except from the OM+N amended unlimed-sulfate soil (Table 16). Organic C in the water extract for this treatment was unusually high (Table 19), but the increased C was not associated with an increase in degree of complexation of Al (Table 13). Except for this treatment, the degree Of complexation of water-soluble Al was greater for 0M and OM+N treatments than in unamended soil. The atomic ratios of C to complexed Al in Table 19 indicate that adequate quantities of water-soluble organic matter were present to allow for extensive complexing. The ratios were sharply lower in OM+N treated soils, which suggests that organic ligands of lower average molecular weight were involved. At the same time, residual C in whole soil increased which would indicate that Al complexes of higher average molecular weight were precipitated out. As noted earlier, precipitation of Al during hot-water extraction reduced recoveries in the following KCl sequence and in NH40Ac (Table 16). The pattern of recovery in relation to incubation treatments was the same as in the direct extraction, i.e. greater quantities of Al were found in soils which had received OM+N. 114 Table 18. Free and complexed aluminum in fractional extracts Of surface soil : effects of incubation treatments and long-tenn residual effects of nitrogen carriers and lime. Fractional extraction Treatment H90 + 0.1 N KCl 1 N KCl ' NHAOAc ' N Incubation Free “Compx Free Compx Free Compx FreeT"Compx carrier treatment Al Al ' Al Al Al Al Al' ' Al ----------------------- ug/g soil ----------------------- a. Main effects Sulfate Mean 0.245 0.506 12.4 b*16.1 b 9.3 b 21.1 b 27.1b 8.5 Nitrate Mean 0.040 0.1158 1.638 1.728 2.068 5.8 a 17.78 7.1 Mean None 0.039 0.1038 4. 0 a 10. 4 b 9.1 b 13.0 ab 17.18 9.4b Mean +OM 0.030 0.1008 7. 9 b 7.3 8 3.5 a 11.9 a 22.4b 6.68 Mean +0M+N 0.36 0.72 b 9. 2 c 9.0 ab 4.4 a 15.8 b 27.7c 7.48 Mean for no lime 0.258 0.55 b 13.9 b 17.7 b 11.1 b 26.8 b 34 b 11.4b Mean for lime 0.028 0.0678 0.198 0.128 0.258 0.438 10.48 4.28 b. Interactions of N carriers with lime No lime Sulfate Mean 0.45 0.89 b 24.6 b 32 b 18.2 b 42.1 b 43 b 12.8 Nitrate Mean 0.068 0.2128 3.1 a 3.4 a 3.9 a 11.4 a 25.88 9.9 Lime Sulfate Mean 0.042 0.117 0.226 0.198 0.31 0.70 11.3 4.1 Nitrate Mean 0.013 0.017 0.145 0.040 0.200 0.153 9.6 4.3 LSD05 ns** 0.39 12.8 7.1 4.7 15.6 ns ns c. Interactions of incubation treatments with lime No lime Mean None 0.0658 0.2008 7.9 a 20.7 b 18.3 b 26.1 b 26.88 13.7b Mean +OM 0.0458 0.1818 15.8 b 14.6 a 7.0 a 23.9 a 35.3b 10.18 Mean +0M+N 0.66 b 1.27 b 17.9 c 17.7 ab 8.0 a 30 c 41 c 10.38 Line Mean None 0.013 0.006 0.001 0.019 0.0002 0.003 7.48 5.1 Mean +OM 0.016 0.019 0.031 0.013 0.004 0.015 9.58 3.1 Mean +0M+N 0.054 0.175 0.52 0.32 0.76 1.27 14.4b 4.4 LSD ns 1.17 15.9 9.9 3.6 ‘19.6 ns 5.7 05 115 Table 18 (cont'd.). *Fractional extraction f Treatment HZQL 0.1 N KC1 1,N KC1 ' NHAQAC N Incubation Free Compx Free Compx Free Compx Free T Compx carrier treatment Al Al Al Al Al 'Al Al fiAl --— ug/g soil —— d. Interactions of incubation treatmentstith N carriers Mean of lime + no lime Sulfate Sulfate Sulfate Nitrate Nitrate Nitrate LSD None +0M +0M+N None +0M +0M+N 05 0.0578 0.173 0.0438 0.158 0.64 b 1.18 0.020 0.033 0.018 0.042 0.082 0.269 [IS ns e. Second order interactions Sulfate Sulfate Sulfate Nitrate Nitrate Nitrate Sulfate Sulfate Sulfate Nitrate Nitrate Nitrate , LSD None +0M +0M+N None +0M+N None +0M +0M+N None +04 +0M+N 05 0.016 0.020 0.091 0.010 0.012 0.016 "S 0.005 0.019 0.33 0.007 0.019 0.024 1.65 22.5. 0.003 0.023 0.019 0.009 0.66 0.0002 0.015 0.042 0.017 0.39 0.089 [14.0 7.6 a 19.8 b 15.8 b 22.7 14. 8 b 14.1 a 5.9 a 20.4 14.9 b 14.2 a 6.1 a 21.1 0.358 0.89 2.48 3.4 a 1.028 0.47 1.09 3.4 a 3.5 b 3.8 2.61 10.5 b ns 9.9 3.6 ns No lime . 32 b 45 3 11.8 8 41 . .8 11.3 a 40 0.698 1.768 5.0 6.78 2.008 0.928 2.17 6.98 6.7 b 7.5 b 4 6 20.6b 0.0002 0.002 0.002 0.021 0.92 2.09 0.0002 0.004 0.005 0.009 0.59 0.45 5.1 27.7 20.88 28.0b 32 c 13.38 16.88 22.9b 34 a 46 b 49 b 19.78 25.08 32.8b 10.0b 7.68 7.88 8.8b 5.68 6.98b 11S 15.0b 12.38b 11.28 12.5b 7.98 9.48 :Come = Complexed A1 *8 ,b, c = Duncan's equivalent ranges. ** different at P(05). interactions, the tabulated LSD ns = not significant at P(05) 05 Within columns and subsets of two or three means, means followed by none or the sane letter are not For other comparisons in a given category of applies (Steel and Torrie, 1980). 116 Table 19. Organic carbon and C/Complexed Al Ratios in fractional extracts of surface soil and subsoil. Whole Treatment soil Hot-water extract 0.1N KCl+ N7 *Incubation org. Org. Ratiof‘ Org. Ratio carrier treatment C C C/Compx. Al C C/Compx. Al % % % No lime Surface Sulfate None 0.79 0.0096 635‘ 0.0023 1.3 +OM 0.84 0.0085 640 0.0030 2.4 +0M+N 0.88 0.0230 255 0.0023 1.9 Subsoil Sulfate None 0.14 0.0029 1,020 0.0004 0.50 Surface Nitrate None 0.95 0.0120 4,600 0.0034 43 +OM 0.95 0.0122 4,200 0.0018 20 +0M+N 1.08 0.0123 540 0.0024 7.2 Subsoil Nitrate None 0.20 0.0049 5,500 0.0003 16 Lime Surface Sulfate None 0.91 0.0104 47,000 0.0030 2,900 +0M 0.88 0.0097 12,000 0.0022 5,500 +OMWN 0.96 0.0072 490 0.0020 80 Subsoil Sulfate None 0.40 0.0053 40,000 0.0002 2.1 Surface Nitrate None 0.89 0.0093 30,000 0.0030 5,100 +OM 0.96 0.0097 11,000 0.0016 2,100 +0M+N 1.02 0.0069 6,500 0.0020 506 Subsoil Nitrate None 0.20 0.0055 18,000 0.0001 140 + = Preceeded by hotrwater extraction * = Atomic ratios of organic C/complexed farms of Al 117 The degree of complexation (Table 13) in NH40Ac was greater after hot-water extraction in unamended soils, but was reduced by additions of OM more sharply than in the direct extract and to similar values in limed soils and in unlimed-nitrate soil. Again this would suggest that some complexed Al as well as separate phase Al were precipitated out by the hot-water treatment, leaving extractable complexes Of lower molecular weight. Percent complexation in 0.1N KCl was also reduced by both organic amendments (Table 13). Atomic ratios of C to complexed Al (Table 19) indicate that there was not enough C in unlimed-sulfate soil to allow for extensive complexation with organic ligands. Therefore, much of the "complexed" Al must have been polymeric rather than complexed. In unlimed-nitrate soil, 8 larger prOportion of Al reacting slowly with 8-0Hq could have been complexed, although the prOportion may have been reduced substantially in the OM+N treatment. In limed soils, the quantities of organic C in all 0.1N KCl extracts were sufficiently high to allow for extensive complexation. However, sharp reductions in the atomic ratios of C to complexed Al in soils receiving OM+N indicate that organic ligands were of much lower average molecular weight. After extraction with hot water and with 0.1N KCl, substantial quantities of Al exchangeable in N KCl remained (Table 16). In amended soils, the prOportion complexed (Table 13) was much greater in N KCl than in the preceding 0.1N KCl. Organic C could not be determined in the N KCl extracts because of interference from Cl'. It is likely that some of the polymeric and complexed Al precipitated in hot water was displaced by the more concentrated KCl. As in the case of 0.1N KCl , 118 more of the Al exchanged at low pH may have been polymeric. With increasing pH, chelation would have been increasingly necessary to keep Al in solution in the extract. The atomic ratios of organic C to complexed Al in subsoils (Table 19) indicate that much of the water-soluble Al could have been complexed with organic matter. After these were removed, much of the Al exchangeable to 0.1N KCl may have been in polymeric forms (cf. Tables 13, 19). These results with fractional extracts support the earlier suggestion that protonation is the principle mechanism for mobilizing non-exchangeable Al in soils, whereas chelation is mainly responsible for stabilizing Al in solution and facilitaing its movement downward in the profile. RELATIONSHIPS BETWEEN ASSOCIATED ELEMENTS AND AL DISTRIBUTION Plasma emission analyses for Al and eleven other elements were performed on all extracts, except the fractional N KCl extract after water-soxhlet extraction. Interference by CT' was unacceptably high in this extract. These data for soils as sampled are recorded in Appendix A. It is beyond the sc0pe of this thesis to evaluate these data in detail. Recoveries from field moist soils and incubated soils in direct extractions with NH OAc and in N KCl are given in Tables 20 and 21 and 4 will be discussed briefly here. 119 Macronutrients Substantial quantities of P were found in NH OAc (Table 20) but 4 not in KCl (Table 21). In the unlimed soils, it is likely that extractable P was derived mainly from complexes with sesquioxides of Fe and Al. In limed soils, calcium phosphate would have been the more likely source. Increased recoveries of NH40Ac extractable P, K, Ca and Mg in amended soils reflect that they were mainly released from the added corn tissue. Potassium levels were higher in nitrate soils, with or without lime, than in sulfate soils. This may be related to the higher levels of exchangeable Ca which would have tended to preserve net negative charge on exchange surfaces. Recoveries of both Ca and Mg were greater in NH4OAc than KCl. Thus, some portion of both cations was present as salts or complexes not associated with exchange sites. In the subsoils of limed plots, very large increases in NH40Ac over KCl extractable Ca and Mg could be due to dissolution of free carbonates by protons dissociated from HOAc (cf. data for co 2’ in Table 9). 3 Micronutrients and Heary Metals Larger quantities of micronutrients and heavy metals were extracted in NH40Ac than in KCl. It may be inferred that the increased quantities in NH40Ac came from salts or complexes not closely associated with exchange surfaces. The prOportion derived from nonexchangeable sources in sulfate soils decreased in the order: 120 Table 20. Fonns of associated elements in NH OAc extracts determined by plasma emission photometry: effect of incubation treatments and long-term residual effects of nitrogen carriers and lime. ‘Treatment N7 *IncubatTOn P K Ca Mg Cu Fe Mn Zn Al Cd Pb Ni carrier treatment ‘ " ‘ ‘ "' ug/g soiT — No Time Surface soil Sulfate None 20.9 59 121 36 0.020 43 4.0 0.45 168 0.020 1.09 0.30 Sulfate +0M 21.7 82 107 88 0.065 50 7.3 0.56 177 0.011 1.23 0.82 Sulfate +0M+N 21.7 80 103 25 0.048 50 8.4 0.60 177 0.015 1.23 0.61 Subsoil Sulfate None 2.58 81 703 200 0.189 12.8 24.6 0.38 289 0.083 1.55 0.56 Surface soil ‘Nitrate ‘NOne 13.0 86 475 36 0.028 6.1 8.0 1.04 115 0.046 1.54 0.51 Nitrate +OM 16.0 111 520 43 0.052 5.9 7.9 1.30 124 0.037 1.75 0.62 Nitrate +0M+N 16.2 112 535 45 0.076 11.2 75 1.65 149 0.046 2.02 0.74 Subsoil Nitrate None 2.80 46 660 71 0.076 16.8 2.6 0.32 76 0.055 1.28 0.24 Lime Surface soil u a one 14.3 60 840 246 0.016 4.6 6.6 0.40 56 0.056 1.76 0.36 Sulfate +OM 20.3 85 900 275 0.067 5.5 8.5 2.19 68 0.052 1.97 0.43 Sulfate +0M+N 16.0 84 900 280 0.029 8.0 23.4 2.41 89 0.061 1.89 0.42 Subsoil SulfatE None 3.2 47 6900 3255 0.249 10.6 27.9 0.77 88 0.126 7.9 0.96 Surface soil ‘Nitrate *NOne 18.8 87 960 187 0.062 2.5 11.0 0.61 47 0.043 1.97 1.34 Nitrate +0M 22.2 108 1010 204 0.027 2.9 14.5 0.68 54 0.077 1.84 0.35 Nitrate +0M+N 16.2 109 1030 206 0.029 3.9 58 0.90 72 0.064 1.97 0.41 Subsoil 'NTtFEtE' None 1.94 56 2380 790 0.161 4.9 7.3 0.24 62 0.058 3.9 0.57 121 Table 21. Forms of associated elements in N KCl extracts determined by plasma emission photometry: effects of incubation treatments and long-term residual effects of nitrogen carriers and lime. ‘Treatment N *InCUbatiOn P K Ca Mg Cu Fe Mn Zn Al Cd Pb Ni carrier treatment * ‘ ' " ‘ ug/g soil No Time Surface soil Salfate None 2.4 - 91 17 0.045 0. 25 4.2 0.32 182 0.011 0.74 0.23 Sulfate +0M 2.5 - 97 16 O. 056 O. 48 5.0 0.31 184 0.011 0.86 0.33 Sulfate +0M+N 1.9 - 101 17 0.038 O. 42 5.7 0.35 189 0.015 0.83 0.21 Subsoil Sulfate None 0.70 - 610 171 O 040 0 O9 19 1 0 26 166 0 011 1.48 0 36 Surface soil ’Nitrate ‘NOne 0.64 - 475 31 0.032 0.12 4.7 0.71 46 0.030 1.11 0.25 Nitrate +0M 1.25 - 466 31 0.011 0.16 4.3 0.76 46 0.029 0.65 0.09 Nitrate +0M+N 1.28 - 517 31 0.019 0.14 53 1.05 107 0.023 0.76 0.14 Subsoil 'NTEFEtE' None 1.00 - 580 42 0.023 0.02 1.5 0.10 12 0.011 1.18 0.23 Lime Surface soil *Sulfate ‘NOne 0.57 - 665 172 0.010 0.09 0. 91 O. 03 0.7 0.026 1.18 0.09 Sulfate +0M 1.13 - 680 163 0.011 0.24 0. 86 0.02 1.6 0.076 0.38 0.03 Sulfate +0M+N 0.87 - 770 176 0.015 0.25 15 0.46 23 0.050 0.77 0.05 Subsoil Sulfate None 0.85 - 1360 243 0.020 0.01 1.2 0.07 18 0.023 2.6 0.20 Surface soil ‘Nitrate ‘NOne 1.0 - 805 139 0.016 0.01 0. 60 0.02 0.7 0.035 1.5 0.09 Nitrate +0M 0.14 - 835 130 0.035 0.30 O. 80 O. 03 1.1 0.012 0.96 0.09 Nitrate +0M+N 0.36 - 925 149 0.025 0. 09 37 0.31 15 0.011 1.25 0.10 Subsoil Nitrate None 0.86 - 1535 202 0.013 0.06 0.31 0.03 3.1 0.015 2.9 0.18 122 Fe >> Cd > Cu > Zn > Mn = Ni > Pb > Al In nitrated soils the order was different: Fe >> Al > Mn > Cu = Ni > Zn > Cd > Pb Very little Fe was exchangeable. Iron is often found together with Al in soil colloidal fractions but tends to be mobilized at lower pH, as is indicated by the data in Table 20. Oxides of Mn serve as extracellular electron acceptors in facultative metabolism (Alexander, 1977). Except in the unlimed-sulfate soils, substantial qunatities of Mn were released in soils to which OM+N were added. The release of Mn with exchangeable Al was less with the same treatment. Thus, the combined acidity from nitrification during incubation and from HOAc during extraction were necessary to mobilize Mn from its sesquioxides. More Mn was released from nitrate surface soils than from the sulfate surface. The reverse was true in subsoils. Since MnSO4 is very soluble (Lindsay, 1979), it appears that sulfate surface soils had lost Mn into the subsoil during the 14 years of annual application of (NH4)2SO4. Zinc occurred in the same treatments where the Mn increases were observed in NH4OAc. There was evidence also that Zn had been lost from surface soils to subsoils in unlimed sulfate plots. Of the remaining metals, Pb was found in larger quantities in both extractants than Cu, Cd or Ni. There was evidence that Pb may have been displaced from surface soils to subsoils in limed plots. The behavior of all elements studied is undoubtedly influenced by the behavior of Al as affected by pH. At acid pH, exchange reactions of monomeric Al and low molecular weight polymers and complexes are likely important. Above pH 5.0, polymerization leads to the formation of 123 amorphous gels. Cations of apprOpriate radius and affinity may be incorporated or occluded in the develOping amorphous structure. Their subsequent behavior will depend increasingly on the fate of the amorphous alumina. Thus, the larger recovery of all elements in buffered NH4OAc than in KCl may result, at least in part, because occluding sesquioxides are removed by the action of the buffering HOAc. SUMMARY AND CONCLUSIONS Soil samples for this study were taken from a field experiment (begun in 1959), in which (NH4)ZSO4 ("sulfate") and Ca(N03)2 ( were applied annually at high rates (336 kg N/ha) for continuous corn. "nitrate") Dolomitic lime had been applied to half the plots after six annual applications of the N carriers. After 14 annual applications, the high-rate N treatments were discontinued. The experimental area was then under uniform management for six years (the last two without tillage). Surface soils (0-25 cm) and lower subsoils (50-75 cm) were sampled for the present study in the Spring of the 21st year (1979). Data collected by others over the 20-year period were summarized. Dramatic changes in chemical properties of the sandy loam profile had occurred. With the sulfate carrier, buffer acidity (lime requirement) accumulated quickly (within 2 or 3 years), first in surface soils and then in subsoils. According to the literature reviewed, residual buffer acidity attributable to (NH4)ZSO4 would have been due mainly to retention by sesquioxides of protons released by hydrolysis and nitrification of the ammonium salt. Increasing charge on sesquioxides was accompanied by displacement of exchangeable bases and rapid declines in pH. Before annual applications were discontinued (1973), high lime requirements were distributed rather uniformly through the profile: 32, 41 and 23 T/ha, which corresponded with soil pH 4.1, 4.0 and 5.0, at the depths of 0-25, 25-50 and 50-75 cm, respectively. 124 125 With the nitrate carrier, 8 residually basic effect would have been expected due to differential retention of Ca“. However, substantial levels of lime requirement (10-14 T/ha) were maintained, both in surface soils and subsoils. The pH in surface soils declined slowly from 6.0 at the beginning of the experiment to 5.3 in 1971. Illuviation of bases helped to maintain a relatively stable pH of 5.6 to 6.0 in subsoils. Inputs of acidity from non-fertilizer sources were indicated. Probable sources would have included acid rain, root exudates, microbial metabolic products, protons released in complexing reactions between cations and organic ligands, and protons generated by nitrification of NH3 released during annual cycles Of organic matter decomposition. Profile distributions of acidity continued to change after annual applications of the N carriers were discontinued in 1973. By 1979, lime requirement in sulfate surface soil had decreased to 7 T/ha, and pH had increased to 4.5. The corresponding values for nitrate surface soil were 3 T/ha and pH 5.4. In the lower subsoil (50-75 cm) of sulfate plots, lime requirement was 4 T/ha, and pH had increased to 4.9 from a low Of 4.4 in 1975. Buffer acidity at this depth in nitrate plots had disappeared quickly, and the pH in 1979 had risen to 6.2 from 5.7 in 1975. When lime (9 T/ha) was applied to nitrate plots in 1965-66, buffer acidity disappeared quickly from the profile, but pH continued to rise to 8 maximum of 7.0 in 1970-71. Subsoil pH increased more slowly. A 3-fold greater application of Time (27 T/ha) on sulfate plots reacted very slowly. It appeared that buffer acidity was not neutralized .111 Situ to any great extent. Rather, its Slow disappearance over a period 126 of 10 or more years was due mainly to downward displacement and leaching out Of the profile. After 13 years, buffer acidity had disappeared from the profile, but soil pH was still rising. In this last sampling (1979), the pH in limed sulfate and nitrate profiles was the same--pH 6.5 in surface soils and pH 7.2 at 50-75 cm. It is apparent that acidity introduced from fertilizer and other sources was retained by buffer systems which, in this sandy loam soil, were mobile and capable of extending acidity rapidly downward in the profile. Crystalline colloids may have had 8 limited capacity for retaining positively charged sesquioxides in complexes that were not exchangeable to KCl and that appeared to have altered surface charge. Surface charge was estimated by A pH (the difference between pH(HZO) and pH(KCl)). In surface soils and subsoils of both limed and unlimed-sulfate plots, this value was the same (ApH = -0.8). In surface soils of nitrate plots, pH was lowered by the lime treatment from -0.9 to -0.6. This is evident that sesquioxides with incompletely neutralized positive charge were complexed at exchange surfaces. Higher net negative charge (ApH = -1.0 to -1.2) in subsoils of limed and unlimed nitrate plots appeared to reflect the much reduced downward movement of charged sesquioxides due to lower inputs of acidity and to higher pH and higher base status throughout the profile. Because historical data were available to document changes that had occurred over a 20-year period in the field, it appeared useful to examine the behavior of Al in the soil samples taken in 1979 with 8 view to identifying forms that may have contributed to mobile buffer acidity. At the pH of surface soils in unlimed sulfate plots (pH(HZO) = 127 4.5, pH(KCl) = 3.7) similar quantities of Al were extracted by unbuffered N KCl and by _N NH4OAc (pH 4.8). Extractability in both extractants decreased with increasing soil pH, i.e. unlimed sulfate > unlimed nitrate > limed sulfate > limed nitrate. Above soil pH(HZO) = 6.0 (pH(KCl) = 5.0), A1 exchangeable by KCl became negligible and lime requirement by the SMP buffer test was zero. Thus, it appears that the buffer test was influenced mainly by exchangeable Al. Except for the unlimed sulfate surface soil, larger quantities Of Al were removed by the buffered NH OAC than by unbuffered KCl. Greater 4 recoveries in buffered NH40Ac have been observed by others and ascribed to protonation and polymerization of hydroxy-Al structures too large to be directly exchangeable. This explanation is consistent with the finding that only 15 to 29% of the Al in NH 0Ac was in complexed or 4 polymeric forms (slow to react with 8-hydroxyquinoline (8-0Hq)), as compared with 68 to 96% in KCl. This result would be consistent also with the proposition that NH +, as distinct from K, may have displaced some Al by ligand exchange in addition to cation exchange. Negligible amounts of Al were removed by water-soxhlet. This pretreatment reduced recoveries in subsequent extractions with NH40Ac or KCl. The percent complexed Al in NH40Ac was increased. These results indicate that dehydration and polymerization (aging processes that occur slowly in the field) were greatly accelerated at the temperature of boiling water. Incubation of surface soils with added corn tissue (+014) for 70 days did not significantly affect soil pH or the extractability of Al. In incubates to which NH4+ had also been added (+0M+N), soil pH was sharply reduced, except in the very acid unlimed sulfate soil where 128 nitrification was severely restricted and some loss of N03' by Chemodenitrification may have occurred. In soils where pH reductions did occur, recoveries of Al in both NH40Ac and KCl were sharply increased. Increased recoveries in NH40Ac were due mainly to increases in "free" Al. Reliable estimates of "free" and "complexed" Al, using the 8-0Hq procedure, could not be made in KCl extracts from limed soils because of the very small quantities of Al that were extracted. In unlimed soils, there was 8 tendency in direct extracts with N KCl toward increased complexation where 0M alone was added and additional complexation where OM+N were added. After hot-water extraction, the percentage complexed-Al in dilute KCl (0.lN) was lower for both treatments (50% or less) than in unamended soil (70%). In the succeeding extraction with .N KCl, the degree of complexation was greater in the amended soils (80% x§;,60%). Organic C was not determined in NH40Ac orlN KCl because of interference from acetate or Cl". In hot water and 0.1N KCl, the atomic ratio of organic C/complexed Al decreased with soil pH and was sharply reduced by the addition of N (+0M+N). Thus, the probability for chelation of Al decreased with pH, while the probability for protonation as 8 major mechanism for release of Al from non-exchangeable polymers or organic complexes increased. The C/complexed-Al atomic ratio in 0.1N KCl was about 2, so much of the readily exchangeable Al that reacted slowly with 8-0Hq must have been polymeric rather than complexed with OH. 'The probability for exchangeable chelates would have been somewhat greater in unlimed nitrate soils where the atomic ratio of C/Al ranged from 40 in unamended soil to 7 where OM+N were added. 129 The behavior of Al in KCl extracts is consistent with the view that protonation was primarily responsible for liberating Al from non-exchangeable sources in these soils. Chelation was undoubtedly important in stabilizing Al in solution and facilitating its movement downward through the profile. The very slow disappearance of buffer acidity from the profile of unlimed sulfate plots may have been due, at least in part, to the fact that the plots were essentially barren and negligible quantities Of soluble organic ligands were produced. Data were reported for P and 10 metallic cations associated with Al in KCl and NH4OAc extracts. All were recovered in larger quantities in the buffered NH40Ac. Aluminum was the dominant-exchangeable cation in unlimed sulfate surface soil, along with lesser quantities of Ca, K, and Mg, in that order. In unlimed nitrate soils, Ca was the dominant exchangeable cation. Magnesium had been extensively depleted in unlimed soils of both carriers. In limed soils, Ca and Mg were the dominant cations. Extractable Mn and Zn had been displaced from the acid surface soils of sulfate plots and had accumulated in subsoils. Substantial quantities of Mn and Zn that had been retained in less acid surface soils became extractable in NH40Ac when the pH was lowered during incubation by nitrification of added NH4+. The Mn and Zn were released in forms that were not exchangeable to N KCl. Extractable Fe was also largely non-exchangeable, but was encountered in substantial amounts in the acid-sulfate-surface soil. Extractable K was higher in nitrate-surface soils than in sulfate. This was true also for Mn, Zn and several metals found at very low levels (Cu, Pb and Ni). As soil pH increased, recoveries of Mn and Ni 130 in NH40Ac increased, whereas KCl-extractable Mn, Zn and Ni decreased. Copper and Cd were removed in less quantities than other elements. Both extractants removed less Cu but more Cd as soil pH increased. LITERATURE CITED LITERATURE CITED Adams, F. 1980. Interactions of phosphorus with other elements in soils and in plants. p. 655-674. In F.E. Khasawaeh, E.C. Sample and E.J. Kamprath (ed.) The role of phosphorus in agriculture. ASA., CSSA.,SSSA., Madison, Wis. Alberts, E.E., and W.C. Moldenhauer. 1981. Nitrogen and phosphorus transported by eroded soil aggregates. SSSA. 45:391-395. Alexabder, M. 1977. Introduction to soil microbiology. John Wiley and Sons, New York. Allison, F.A. 1973. Soil organic matter and its role in crop production. DeveTOpments in soil science 3. Elsevier Scientific Publishing Co., New York. Allison,luE., and 0.0. 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Measurement of metal-complexing ability of polyfunctional macromolecules: A discussion of the relationship between the metal-complexing properties of extracted soil organic matter and soil genesis and plant nutrition. Soil Sci. 119:210-215. APPENDIX A1 Appendix A1. Fonns of phosphorus in surface soil (0-25 cm) and subsoil (50-75 cm effects of incubation treatments and long-term residual effects Of nitrogen carriers and lime. Treatment Extractable ' P Fractional ‘ extraction N carrier Incubation WON: 1 N 1120 0.1 N 1 N NH40Ac in field treatment KCT"* * ' KClT' KCT' —— ug/g soiT’ No Time Surface soil + ‘SUTfate None 20.9 2.44 0.96 0.46 - 4.1 a +0M 21.7 2.49 0.75 0.68 5.6 b +0M+N 21.7 1.89 2.14 0.59 - 5.8 b Subsoil Sulfate None 2.58 0.70 0.059 0.176 - - Surface soil Nitrate None 13.0 0 .64 0.83 0.118 - 2.1 a +0M 16.0 1. 25 0.66 0.227 - 3.1 b +0M+N 16.2 1. 28 0.65 0.165 - 3.4 b Subsoil Nitrate None 2.80 1.00 0.215 0.34 - - Lime Surface soil 'SUTfate None 14.3 a 0.57 1.19 0.093 a - 2.7 a +0M 20.3 b 1.13 1.06 0.316 ab - 4.4 c +0M+N 16.0 ab 0.87 0.61 0.48 b - 3.7 b Subsoil . Sulfate None 3.2 0.85 0.03 0.43 - - Surface soil ‘Nitrate None 18.8 ab 1.00 0.77 0.176 a - 3.3 a +0M 22.2 b 0.140 0.69 0.69 b 5.0 b +0M+N 16.2 a 0.36 0.59 0.67 b - 3.8 a Subsoil Nitrate None 1.94 0.86 0.017 0.30 - - LSDOS * Within N carrier 12.6 ns 0.91 ns - - Between lime 15.0 ns ns ns - - +a,b,c = Duncan's equivalent ranges. Within columns and subsets of three means, means fOllowed by none or the same letter are not different at P(05). For comparisons between surface soil (None) and subsoil in a given category of interactions, the tabulated L5005 applies (Steel and Torrie, 1980). ns = not significant at P(05) A2 Appendix A2. Forms of potassium in surface soil (0-25 cm) and subsoil (50-75 cm): effects of incubation treatments and long-term residual effects of nitrogen carriers and lime. Treatment Fxtractabl e R FractionaT extraction N carriEr Incubation NH4OAc 1 N H20 0.T N 1 N NPQOAC in field treatment KCT' ' KCl” KCT' ug/g soiTl No Time Surface soil sulfate None 59 a'* 13.1 a - - 69 b +OM 82 b - 18.2 a - - 46 a +0M+N 80 b - 70 b - - 33 a Subsoil Sulfate None 81 - 14.4 - - - Surface soil ‘Nitrate None 86 a 10.4 - 113 c +OM 111 b - 16.4 - - 65 b +0M+N 112 b - 33 - - 44 a Subsoil N'i'tr-a‘t'e' None 46 - 9.1 - - - Lime Surface soil ’SUTfate None ' 60 a - 9.2 - - 81 b +0M 85 b - 17.3 - - 53 a +0M+N 84 b - 23.7 - - 43 a Subsoil Sulfate None 47 - 6.7 - - - Surface soil ‘Nitrate None 87 a - 11.6 - - 102 b +OM 108 b - 18.0 - - 67 a +CM+N 109 b - 23.5 - - 55 a Subsoil Nitrate None 56 - 5.3 - - - LSD05 * Within N carrier 24.4 - ns - - - Between Time 31 - ns - - - +a,b,c = Duncan's equivalent ranges. Within columns and subsets of three means, means fOllowed by none or the same letter are not different at.P(05). For comparisons between surface soil (None) and subsoil in a given category of interactions, the tabulated LSD05 applies (Steel and Torrie, 1980). ns = not significant at P(05) A3 Forms of calcium in surface soil (0-25 cm) and subsoil (50-75 cm): effects Of incubation treatments and long-term residual effects of nitrogen carriers and lime. Appendix A3. ‘Treatment ‘EXtractabTe ca ‘FraCtTOnal extraCtiOn Ncarrier Incubation NH OAc 1 N N 0 0.TN 1 N NHOAc in field treatment 4 KCT A 2 kci" KCT 4 - ug/g sOiTF No Time Surface soil 4 'Sulfate None 121 91 4.4 a 52 - 87 +OM 107 97 5.9 a 50 - 72 +0M+N 103 101 50 b 31 - 50 Subsoil Sulfate None 703 610 18.3 330 - - Surface soil Nitrate None 475 477 6.3 a 224 b - 375 b +0M 520 466 10.9 a 249 c 405 b +0M+N 535 517 65 b 174 a 245 a Subsoil Nitrate None 660 580 14.8 360 - - Lime Surface soil . sulfate None 840 665 a 10.3 a 223 a 600 b +OM 900 680 a 17.7 a 268 b 665 b +0M+N 900 770 b 86 b 240 a - 460 a Subsoil Sulfate None 6900 1360 40 600 - - Surface soil ‘Nitrate None 960 805 a 13.2 a 262 a 630 a +0M 1010 835 a 15.8 a 315 b - 790 b +0M+N 1030 925 b 90 b 280 a - 550 a Subsoil tra None 2380 1535 30 700 - - LSD05 * Within N carrier ns ns 25 394 - - Between lime ns ns ns 640 - +a,b,c = Duncan's equivalent ranges. Within columns and subsets of three means, means fOllowed by none or the same letter are not different at P(05). For'comparisons between surface soil (None) and subsoil in a given category of interactions, the tabulated LSD05 applies (Steel and Torrie, 1980). ns = not significant at P(05) Appendix A4. A4 Forms of magnesium in surface soil (0-25 cm) and subsoil (SO-75 cm): effects of incubation treatments and long-term residual effects of nitrogen carriers and lime. v v Treatment Extractable M0, .__Eraction81_extraction______._ N carrier Incubation NH4OAc 1 N H20 0.l N 1 N NH40Ac Mm KCl KCl KCl 4 ug/g soil No Time Surface soil Sulfate None 36.2 16.8 0.90 8* 14.8 15.8 +0M 88 15.5 2.30 a 15.5 20.5 +0M+N 25.0 16.9 20.2 b 8.3 - 12.0 Subsoil Sulfate None 200 171 5.2 91 - - Surface soil Nitrate None 36 31.3 0.74 a 26.5 b 24.3 +OM 43 31.4 1.55 a 24.2 ab - 33 +0M+N 45 31.4 8.7 b 13.8 a - 18.6 Subsoil Nitrate None 71 42 3.1 27.6 - - Lime Surface soil Sulfate None 246 172 3.8 a 111 b - 154 b +OM 275 163 8.8 a 109 b 201 c +0M+N 280 176 37 b 77 a - 128 a Subsoil Sulfate None 3255 243 1.85 109 - - Surface soil Nitrate None 187 139 3.1 a 91 b 115 a +OM 204 130 4.6 a 89 b 167 b +0M+N 206 149 29.0 b 63 a - 106 a Subsoil Nitrate None 790 202 2.05 96 - - LSD05 Within N carrier ns* ns ns nS - Between lime ns ns ns ns +a,b,c = Duncan's equivalent ranges. Within columns and subsets of three mean?,5Neans fOllowed by none or the same letter are not different at P 0 . ns = not significant at.P(05) 05 For comparisons between surface soil (None) and subsoil in a given category of interactions, the tabulated LSD applies (Steel and Torrie, 1980). A5 Appendix A5. Forms of COpper in surface soil (0-25 cm) and subsoil (50-75 cm): effects of incubation treatments and long-term residual effects of nitrogen carriers and lime. Treatment Extractable Cu Fractional extraction N carrier Incubation NH4OAc 1 N H 0 O.l N 1 N NH40Ac in field treatment KCl 2 KCl KCl ug/g soil - No Time Surface soil + Sulfate None 0.020 0.045 0.012 a 0.055 b - 0.052 +OM 0.065 0.056 0.014 a 0.017 a - 0.020 +0M+N 0.048 0.038 0.040 b 0.017 a - 0.024 Subsoil Sulfate None 0.189 0.040 0.014 0.022 - - Surface soil Nitrate None 0.028 0.032 0.007 0.030 b - 0.045 +0M 0.052 0.011 0.009 0.008 a - 0.012 +0M+N 0.076 0.019 0.012 0.017 a - 0.008 Subsoil Nitrate None 0.076 0.023 0.010 0.008 - - Lime Surface soil Sulfate None 0.016 0.010 0.009 0.021 b - 0.250 b +0M 0.067 0.011 0.009 0.009 a 0.004 a +0M+N 0.029 0.015 0.012 0.007 a - 0.006 a Subsoil Sulfate None 0.249 0.020 0.004 0.007 - - Surface soil Nitrate None 0.062 0.016 0.007 0.019 b 0.044 +0M 0.027 0.035 0.008 0.009 a - 0.007 +0M+N 0.029 0.025 0.010 0.011 ab - 0.008 Subsoil Nitrate None 0.161 0.013 0.006 0.007 - - LSD05 * Within N carrier 0.124 ns ns ns - - Between lime ns ns ns ns - - a,b = Duncan's equivalent ranges. Within columns and subsets of three neans, means fOllowed by none or the same letter are not different at P(05). For comparisons between surface soil (None) and subsoil in a given category of interactions, the tabulated LSD05 applies (Steel and Torrie, 1980). * ns = not significant at P(05) Appendix A6. A6 Forms Of iron in surface soil (0-25 cm) and subsoil (50—75 cm): effects of incubation treatments and long-term residual effects Of nitrogen carriers and lime. Treatnent W ’ N carrier Incubation NH4OAc 1 N H20 0.l N, 1 N NH4OAc in field treatment KCl KCl KCl ug/g soil - No Time Surface soil Sulfate None 43 0.250 0.38 a 0.47 a - 3.1 a +0M 50 0.48 0.36 a 1.35 b 9.1 b +0M+N 50 0.42 1.35 b 1.89 b 9.9 b Subsoil Sulfate None 12.8 0.088 0.35 0.143 - - Surface soil Nitrate None 6 1 0.115 0.100 0.086 1.93 a +OM 5 9 0.162 0.096 0.147 2.17 a +0M+N 11 2 0.139 0.143 0.263 - 4.06 b Subsoil Nitrate None 16.8 0.023 0.241 0.133 - - Lime Surface soil Sulfate None 4 6 0.090 0.072 0.063 - 1.17 +OM 5 5 0.242 0.064 0.176 - 1.41 +0M+N 8 0 0.249 0.245 0.156 - 2.16 Subsoil Sulfate None 10.6 0.011 0.009 0.140 - - Surface soil Nitrate None 2 53 0.011 0.056 0.060 - 0.88 +OM 2 86 0.30 0.025 0.155 - 0.85 +0M+N 3 9 0.093 0.044 0.149 - 1.26 Subsoil Nitrate None 4 9 0.058 0.008 0.122 - - LSDOS * Within N carrier 20 ns ns 0.175 - - Between lime 34 ns ns 0.31 - +a,b,c = Duncan's equivalent ranges. Within columns and subsets of three "S = mean?,m§ans fOllowed by none or the same letter are not different at P 05 . applies (Steel and Torrie, 1980). not significant at P(05) the tabulated LSDO5 For comparisons between surface soil (None) and subsoil in a given category of interactions, A7 Appendix A7. Forms of manganese in surface soil (0-25 cm) and subsoil (50-75 cm): effects of incubation treatments and long-term residual effects of nitrogen carriers and lime. Treatment ExtractabJLMn. N carrier Incubation NH40Ac 1 N_ H20 0.l N 1 N NH4OAc in field treatment KCl KCl KCl 087g soil - No lime Surface soil Sulfate None 4.0 4.2 0.38 10.5 - 8.7 +OM 7.3 5.0 0.58 10.5 - 13.6 +0M+N 8.4 5.7 5.5 9.6 - 13.0 Subsoil Sulfate None 24.6 19.1 0.98 15.4 - - Surface soil + Nitrate None 8.0 a 4.7 a 0.40 a 26.3 a — 35 a +OM 7.9 a 4.3 a 0.49 a 29.2 a - 44 ab +0M+N 75 b 53 b 10.3 b 46 b - 58 b Subsoil Nitrate None 2 64 1.52 0.46 2.19 - - Lime Surface soil Sulfate None 6.6 0.91 0.078 5.3 - 26.7 +0M 8.5 0.86 0.125 4.7 - 33 +0M+N 23.4 15.1 2.92 12.8 - 26.2 Subsoil Sulfate None 27.9 1.17 0.139 2.26 - - Surface soil Nitrate None 11.0 a 0.60 a 0.064 6.5 a - 36 a +0M 14.5 a 0.80 a 0.039 9.4 a - 62 b +0M+N 58 b 37 b 5.2 26.6 b - 54 b Subsoil Nitrate None 7.3 0.31 0.004 0.32 - - LSD05 Within N carrier ns* 3.08 ns 6.6 - - Between lime ns 4.15 ns 10.8 - a,b = Duncan's equivalent ranges. Within columns and subsets of three means, means fOllowed by none or the same letter are not different at P(05). For comparisons between surface soil (None) and subsoil in a given category of interactions, the tabulated LSD05 * applies (Steel and Torrie, 1980). ns not significant at P(05) A8 Forms of zinc in surface soil (0-25 cm) and subsoil (50-75 cm): effects of incubation treatments and longeterm residual effects of nitrogen carriers and lime. Appendix A8. Treatment Extragtable Zn Eragtjgnal extracting N carrier Incubation NH40Ac 1 N H20 0.1 N 1 N NH4OAc in field treatment KCl KCl KCl ug/g soil = No lime Surface soil + Sulfate None 0.45 0.32 0.022 a 0.51 - 0.46 +OM 0.56 0.31 0.034 a 0.50 - 0.52 +0M+N 0.60 0.35 0.30 b 0.45 - 0.47 Subsoil Sulfate None 0.38 0.263 0.012 0.107 - Surface soil Nitrate None 1.04 0.71 0.004 0.32 0.82 +OM 1.30 0.76 0.003 0.35 0.92 +0M+N 1.65 1.05 0.073 0.62 0.88 Subsoil Nitrate None 0.32 0.103 0.004 0.046 - Lime Surface soil Sulfate None 0.40 0.031 0.001 0.037 a 0.22 a +OM 2.19 0.017 0.003 0.019 a 0.20 a +0M+N 2.41 0.46 0.043 0.59 b 0.96 b Subsoil Sulfate None 0.77 0.072 0.003 0.034 - Surface soil Nitrate None 0.61 0.018 0.001 0.059 0.27 +OM 0.68 0.029 0.001 0.030 0.33 +0M+N 0.90 0.31 0.010 0.116 0.53 Subsoil Nitrate None 0.24 0.025 0.0005 0.023 - LSD05 Within N carrier ns* 0.36 ns 0.073 - - Between lime 0.267 0.44 0.015 0.213 - - +a,b = Duncan's equivalent ranges. Within columns and subsets of three means, means fOllowed by none or the same letter are not different at P(05). For comparisons between surface soil (None) and subsoil in a given category of interactions, the tabulated LSDOS applies (Steel and Torrie, 1980). ns = not significant at P(05) A9 Forms of aluminum in surface soil (0-25 cm) and subsoil (50-75 cm): effects of incubation treatments and long-term residual effects of nitrogen carriers and lime. Appendix A9. v Treatment Extractable Al Fractional extraction N carrier Incubation NH40Ac 1 N H20 0.l N 1 N, NH40Ac in field treatment KCl K01 K01 ug/g soil - No lime Surface soil + Sulfate None 168 182 0.3118 49 a 56 a +OM 177 184 0.3348 56 b 71 b +0M+N 177 189 1.87 b 56 b - 73 b Subsoil Sulfate None 289 166 0.187 39 - - Surface soil Nitrate None 115 a 46 a 0.105 1.998 - 37 a +OM 124 ab 46 a 0.155 2.818 - 42 a +0M+N 149 b 107 b 0.354 15.2 b - 54 b Subsoil Nitrate None 76 11.5 0.39 1.09 - - Lime Surface soil Sulfate None 56 a 0.72 0.063 0.047 - 15.5 a +0M 68 ab 1.59 0.089 0.148 - 17.1 a +0M+N 89 b 22.8 0.39 1.26 - 25.6 b Subsoil Sulfate None 88 18.3 0.0005 3.6 - - Surface soil Nitrate None 47 a 0.74 0.065 0.033 13.8 a +OM 54 ab 1.08 0.084 0.36 15.6 a +OMHN 72 b 14.8 0.064 0.75 - 23.6 b Subsoil Nitrate None 62 3.12 0.0005 0.29 - - LSD05 Within N carrier 47 ns* ns ns - Between lime 94 ns ns ns - .4. a,b "S = Duncan's equivalent ranges. a Within columns and subsets of three means, means followed by none or the sane letter are not different at P(05). For comparisons between surface soil (None) and subsoil in a given category of interactions, the tabulated LSD applies (Steel and Torrie, 1980). not significant at P(05) 05 Appendix A10. v A10 Forms of cadmium in surface soil (0-25 cm) and subsoil (50-75 cm): effects of incubation treatments and long-tenm residual effects of nitrogen carriers and lime. a Treatment Extractable Cd Fractional extraction N carrier Incubation NH4OAc 1 N H20 0.l N_ 1 N NH40Ac in field treatment KCl KCl KCl ug/g soil — No Time Surface soil + Sulfate None 0.020 0.011 0.003 0.032 b - 0.003 +OM 0.011 0.011 0.001 0.016 a - 0.002 +0M+N 0.015 0.015 0.006 0.018 a - 0.007 Subsoil Sulfate None 0.083 0.011 0.001 0.047 - - Surface soil Nitrate None 0.046 0.030 0.001 0.037 a - 0.002 a +0M 0.037 0.029 0.001 0.050 b - 0.053 c +0M+N 0.046 0.023 0.005 0.059 b - 0.037 b Subsoil Nitrate None 0.055 0.011 0.0005 0.027 - - Lime Surface soil Sulfate None 0.056 0.026 0.002 0.007 a - 0.002 a +OM 0.052 0.076 0.002 0.024 b - 0.024 b +0M+N 0.061 0.050 0.003 0.038 c - 0.018 ab Subsoil Sulfate None 0.126 0.023 0.0005 0.023 - - Surface soil Nitrate None 0.043 0.035 0.001 0.012 a - 0.004 a +0M 0.077 0.012 0.001 0.016 ab - 0.030 b +0M+N 0.064 0.011 0.001 0.025 b 0.023 ab Sufbsoi l Nitrate None 0.058 0.015 0.0005 0.020 - - LSD05 Within N carrier 0.068 ns* ns ns - - Between lime 0.087 ns ns nS - - a,b,c = Duncan's equivalent ranges. Within columns and subsets of three (15 means, means fOllowed by none or the same letter are not different at P(05). applies (Steel and Torrie, 1980). not significant at P(05) For comparisons between surface soil (None) and subsoil in a given category of interactions, the tabulated LSD05 Appendix All. A All Forms of lead in surface soil (0-25 cm) and subsoil (SO-75 cm): effects of incubation treatments and long—term residual effects of nitrogen carriers and lime. A Treatment Extractable Pb Fractional extraction N carrier Incubation NH40Ac 1 N H20 0.1 N 1 N NH40Ac in field treatment KCl K01 K01 ug/g soil - No Time Surface soil + Sulfate None 1.09 0.74 0.012 a 0.169 b - 0.44 +0M 1.23 0.86 0.028 a 0.109 a - 0.46 +0M+N 1.23 0.83 0.102 b 0.068 a - 0.45 Subsoil Sulfate None 1.55 1.48 0.060 0.41 - - Surface soil Nitrate None 1.54 a 1.11 0.008 a 0.34 b - 0.56 a +OM 1.75 ab 0.65 0.036 ab 0.34 b - 0.97 b +0M+N 2.02 b 0.76 0.106 b 0.265 a 0.80 ab Subsoil Nitrate None 1.28 1.18 0.041 0.41 - - Lime Surface soil Sulfate None 1.76 1.18 b 0.011 a 0.32 - 0.64 a +OM 1.97 0.38 a 0.041 a 0.37 1.26 b +0M+N 1.89 0.77 ab 0.132 b 0.31 1.02 b Subsoil Sulfate None 7.9 2.62 0.077 0.63 - - Surface soil Nitrate None 1.97 1.52 b 0.018 a 0.35 a - 0.60 a +OM 1.84 0.96 a 0.037 a 0.43 b - 1.48 b +0M+N 1.97 1.25 ab 0.158 b 0.40 b - 1.23 b Subsoil Nitrate None 3.9 2.93 0.074 0.72 - - LSD05 Within N carrier 6.0 ns* 0.048 ns - - Between lime ns ns 0.074 ns - - +a,b = Duncan's equivalent ranges. Within columns and subsets of three means, means fOllowed by none or the same letter are not different at P(05). For comparisons between surface soil (None) and subsoil in a given category of interactions, the tabulated LSD05 * applies (Steel and Torrie, 1980). ns = not significant at P(05) A12 Appendix A12. Forms of nickel in surface soil (0-25 cm) and subsoil (50-75 cm): effects of incubation treatments and long-term residual effects of nitrogen carriers and lime. k - Treatment Extractable Ni Eractjonal extraction N carrier Incubation NH OAc 1 N H 0 0.l N 1 N NH OAc in field treatment 4 KCl 2 KCl KCl 4 ug/g soil = NO lime Surface soil + Salfate None 0.30 a 0.226 0.021 0.208 b - 0.25 +0M 0.82 b 0.33 0.036 0.141 a - 0.168 +0M+N 0.61 ab 0.208 0.137 0.107 a - 0.168 Subsoil Sulfate None 0.56 0.36 0.046 0.202 - - Surface soil Nitrate None 0.51 0.254 0.011 0.122 b - 0.244 +OM 0.62 0.087 0.034 0.044 a — 0.137 +0M+N 0.74 0.144 0.027 0.100 b - 0.123 Subsoil Nitrate None 0.242 0.229 0.022 0.044 - - Lime Surface soil Sulfate None 0.36 0.087 0.008 0.084 b 1.15 +0M 0.43 0.029 0.026 0.019 a 0.094 +0M+N 0.42 0.054 0.039 0.018 a 0.107 Subsoil Sulfate None 0.96 0.195 0.015 0.046 - - Surface soil Nitrate None 1.34 b 0.093 0.008 0.081 b - 0.252 +OM 0.35 a 0.087 0.023 0.021 a - 0.109 +0M+N 0.41 a 0.104 0.042 0.050 ab - 0.135 Subsoil Nitrate None 0.57 0.179 0.033 0.037 - - LSD05 Within N carrier 0.43 0.073 0.028 ns* - Between lime ns 0.192 ns ns — +a,b = Duncan's equivalent ranges. Within columns and subsets of three means, means fOllowed by none or the same letter are not different at P(05). For comparisons between surface soil (None) and subsoil in a given category of interactions, the tabulated L3005 applies (Steel and Torrie, 1980). *ns = not significant at P(05) A13 APPENDIX B1. Hodunk Series The Hodunk series consists of moderately well drained Gray-Brown Podzolic (Ochreptic fragudalf) soils with fragipans which develOped on calcareous sandy loam glacial till. Hodunk soils are found in association with the well drained Hillsdale and moderately well drained Elmdale series which also develOped on calcareous sandy loam till. Soil Profile: A 0-17.5 p cm A2 17.5-40.0 cm B. 40.0-62.5 cm 1m B29 62.5-115 cm Hodunk sandy loam Dark grayish brown (10 YR 4/2) to very dark grayish brown (10 YR 3/2); sandy loam; moderately fine, granular structure; friable when moist and soft when dry; medium content of organic matter; medium to slightly acid; abrupt smooth boundary. 15 to 27.5 cm thick. Yellowish brown (10 YR 5/4); pale brown (10 YR 6/3) or light yellowish brown (10 YR 6/4); sandy loam; weak, fine, granular to weak, fine subangular blocky structure; very friable when moist and soft when dry; medium acid; abrupt wavy boundary. 15 to 25 cm thick. Brown (10 YR 5/3) to pale brown (10 YR 6/3); sandy loam to light sandy clay loam; massive to weak, thick, platy structure; firm when moist and brittle when dry; weak to moderately develOped fragipan; few thin clay folws; medium to strongly acid; clear wary boundary. 10 to 30 cm thick. Brown (10 YR 5/3) to yellowish brown (10 YR 5/4) mottled with yellowish brown (10 YR 5/8) and dark brown (7.5 YR 4/4), mottles are common, medium, distinct; sandy clay loam, heavy sandy loam, or light clay loam; few thin clay flows; weak, medium, subangular blocky structure; firm when moist, strongly to midium acid in the upper part and slightly acid in the lower part; abrupt irregular boundary. 32.5 to 75 cm thick. A14 C 115 cm+ Light yellowish brown (10 YR 6/4) to brown (10 YR 5/3) 9 mottled with yellowish brown (10 YR 5/6-5/8), mottles are common, medium, distinct; sandy loam; massive to very weak, coarse, subangular blocky structure; friable when moist and hard when dry; calcareous. TOpography: Gently to moderately STOping till plains and moraines. Drainage and Permeability: ‘ Moderately well drained. Surface runoff is slow to moderate. Permeability is moderate to slow depending upon the degree of develOpment of the fragipan. Natural Vegetation: Deciduous forest consisting of sugar maple, beech, oak, and hickories. Source: Schneider et al., 1967.