K MINERALEZAHON OF SOIL ORGANEC NITROGEN AS INFLUENCED BY ORGANIC MENUMENTS‘ MIME-GEN FERTSLEZER, CROP SEQUENCE AND TIME. Thus-£8 {out Hm D—eqc-co af M. 5. MICHIGAN STATE UNEVERSET‘! Frederick Au 1958 uunfluwg'xaummmnmmmml 1108 7156 - PPM“ Institution MSUIsMMMWMMVEMO "y W""" ' I~IEJE$§LIZATION OF SOIL ORG-$.13 IC NITROGEN AS INFLUEN CED BY ORGANIC fib’lEI-JDLENTS, NII‘IL‘GEN FERTILIZER, CROP SEQUENCE AND TIME BY FREDERICK AU AN ABSTRZCT Submitted to the College of Agriculture of Michigan State University of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Soil Science 1958 Approved 4 fl? 7/J’i,v;éf’ Frederick Au ABSTRACT Soil samples were taken in the fall of 1957 from field plots on which various residue treatments were initiated two to-six years ear- lier. These treatments were initiated prior to corn in a five-year rotation consisting of corn, beans, barley and two years of alfalfa- brome meadow. Five replications of each treatment were established each year on separate blocks of plots in five successive years, be- ginning in 1951. The four treatments included (1) the check and plots to which the following organic materials were incorporated into the soil prior to corn: (2) All hay grown during. the second year of alfalfa- brome meadow, (3) 24 tons of wheat straw, and (h) 35 tons of sawdust. One-half of each plot received supplemental nitrogen with the first three crops in the rotation. The soil was Sims clay loam. Yields of corn, beans and barley were depressed by the massive sawdust treatment. Supplemental nitrogen did not canpletely overcome this depression. Six years after sawdust treatment, corn yields were greater than for amr other treatment. Carbon and nitrogen retained after six years were greater for this than for any other treatment. Yields of corn were depressed following the four-ton straw appli- cation. Yields of later crops were unaffected. Retention of carbon and nitrogen in the soil was not materially greater than in the check. Two cuttings of alfalfa-brome returned to the soil prior to corn did not influence the yields of any crop in the rotation. Carbon and nitrogen contents of the soil after six years were only slightly greater than in the check. Release of nitrate and 002 from incubated soil samples reflected microbial immobilization of nitrogen for three years after the 35-ton Frederick Au application of sawdust. A surplus of energy carbon in these soils was reflected by extremely high microbial activity (002 evolution). These soils were characterized by Czfl ratios wider than normal for the soil and by C:N ratios of mineralization wider than the C:N ratio of the soil itself. Such.soils released nitrate during incubation in inverse proportion to the quantities of 002 produced. Soils in which surplus carbonaceous energy materials were largely dissipated tended to stabilize at a C:N ratio which appeared to be characteristic for this soil (11:1). CzN ratios ofxnineralization tended to be equal to or less than the C:N ratio of the soil itself. Nitrate was released from such soils in direct proportion to the quantities of 002 produced. MINERALIZATION OF SOIL OEANIC NINE-GEN AS INi'LUENCED BY ORGANIC AMENDI-IENTS, NITRG‘JEN FERTILIZER, CROP SEQUENCE AND TIME BY FREDERICK AU A THESIS Submitted to the College of Agriculture of Michigan State University of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Soil Science 1958 My. j“? _~ul ACKNOWIEDGMENT The author wishes to express his sincerest gratitude and apprec- iation to his major professor, Dr. A. R; welcott, for his unlimited assistance given during the author's course of study. He wishes to express his appreciation to Dr. R. L. Cook for the opportunity to work on this research problem, to Dr. J. F. Davis for his enlightening criticisms of this manuscript, and to his fellow graduate students for their advice and assistance rendered in the course of this research. TABLE OF CDN TEN'I’S DITRODUCTION.................. OBJECTIVES .................. REVIEMOFLITERATURE.............. NATERIALSANDME’IHODS ...... ..... .. Field Treatments and Cropping History . . . . laboratory Determinations . . . . . . . . . . HPERD/IENTALRESULTS.............. Laboratory Determinations . . . o . . . . . . Soil reaction . . . . . . . . . . . . . . . ReservephOSphorus............. Totalcarbon................ Extracted forms of soil nitrogen . . . . . . Totalnitrogen............... Soil CzN ratio 0 o . . . . . . . . . o . . o Mineralization of carbon . . o o o o o . . . Incubation mineralization of nitrogen . . . Relationship of laboratory Data to Crop Yields DISCUSSION................... SWRYA‘JDCDNCLUSIONS .o..._....... LI'IERATURECITED................ AppmmoonOQOOOOOQoooooooo Page 20 2o 22 2S 25 25 26 26 29 33 35 35 38 to 53 56 63 TABLE 10. Page Carbon evolved as carbon dioxide during a lh-day in- cubation period, as influenced by organic amendments, nitrogen fertilizer, crop sequence and time . . . . . . . 7O Nitrate nitrogen released during a lb-day incubation period, as influenced by organic amendments, nitrogen fertilizer, crop sequence and time . . . . . . . . . . . . 71 Ratio of carbon to nitrogen.mineralized during a lh—day incubation period, as influenced by organic amendments, nitrogen.fertilizer, crop sequence and time . .72 3. 7. 9. LIST CF TABLES Ammoniunextracted with.Mor;an's solution, as in- fluenced.by organic amendnents, nitrogen fertilizer, crop sequence and time . . . . . . . . . . . . . . . . . . Nitrate extracted with.Mor¢an's solution, as in- fluenced by organic amendments, nitrogen fertilizer, crop sequence and time . . . . . . . . . . . . . . . . . . Average crop yields over a six-year period following the addition of various organic amendments with and without supplemental nitrogen fertilizer . . . . . . . . . Soil reaction as influenced by organic amendments, nitrogen fertilizer, crop sequence and time . . . . . . . . Reserve phOSphorus extracted with 0.13N HCl, as in~ fluenced by organic amendments, nitrogen fertilizer, crop sequence and time . . . . . . . . . . . . . . . . . Total carbon determined by ignition method, as influenced by organic amendments, nitrogen fertilizer, crop sequence tinle0000000000000...cocooooooo' Pennanganate-soluble nitrogen, as influenced by organic amendments, nitrogen fertilizer, crop sequence and time . Total nitrogen (Kjeldahl), as influenced by organic amendments, nitrogen fertilizer, crop sequence and time . Soil CzN ratio, as influenced by organic amendments, nitrogen fertilizer, crop sequence and time . . . . . . . g Page 614 65 ‘67 #58 ‘69 1'" a I I j i b O I Q __ . O . \ r - n \ f0 FIGURE 1. 3. h. S. 7. LIST OF FIGURES Interlinkage of nitrogen and carbon cycles in mineralization and immobilization pro- cesses in the soil . . . . . . . . . . . . . . . . . Total carbon in soil at yearly intervals after addition 01' various organic amendments with and without supplemental nitrogen fertilizer . . . . . . ‘Ibtal nitrogen in soil at yearly intervals after addition of various organic amendments with and without Supplemental nitrogen fertilizer . . . . . . Carbon evolved as (132 during a lit-day incu- bation period from soil samples taken at yearly intervals after addition of various organic amendments with and without supplemental nitrogen fertilizer..................... Nitrifiable nitrogen released as nitrate during a Iii-day incubation period in soil samples taken at yearly intervals after addition of various organic amendments with and without supplemental nitrogen fertilizer...................... Nitrifiable nitrogen as related to soil C:N ratio and sawdusttreatment.................. Microbial activity measured as 002 evolved during a lit-day incubation period and its relation to soil cm ratioandsawdusttreatment.......o.o.o- Page 28 2; in A7 FIGURE 8. 9. Page Ratio of mineralization of carbon to mineralization of nitrogen as related to soil CzN ratio and sawdust treatment . . . . . . . A8 Nitrifiable nitrogen as related to carbon evolved as 002 during a 1h-day incubation periodatBSOC.................o 50 INTRODUCTION Much work has been done on methods for estimating the nitrogen supplying power of the soil. Nunerous methods proposed have met with limited or local success. The advent of an increased use of nitrogen fertilizers danands as thorough as possible a re-evaluaticn and/or augnentation of our present knowledge concerning this subject. In the present study, interest has been centered on the in- fluence of organic mendments, nitrOgen fertilizers, crop rotation, and tine on biological and chanical properties of the soil and their relation to crop yields. The rotation emeriment which was used as the basis for the study was established in 1951. laboratory investi- gations were initiated in 1957. The first year's data were the subject of a master's thesis by J. A. Mora (39). The results of a second year 's continuation of this research are reported here. ,Q 1. OBJECTIVES The two objectives of this work are: lb obtain chenical and biological data which may be used in later studies to correlate the nitrogen supplying power of the soil with crop yield. 'Ib evaluate the effects of residue treatments with and with- out supplemental nitrogen on chemical and biological properties of Sims clay loam through the course of a five-year rotation. REVE‘ 0F LITERATURE lhe$composition of organic materials in soil is a complex process which has engaged the research efforts of nunerous investigators for the past two centuries. As experience accrued, it became apparent that de- composition processes were responsible for the dynamic nature of soil as a natural bocb‘. The processes themselves and their end products were found to influence the chemical, physical, and biological proper- ties of soils in cmplex ways. Because of this complexity, the litera- ture in this area is full of apparent contradictions. Some general patterns of behavior have carried through much of this work, however, and have been loosely formalized and accepted as guiding principles of soil management. The review which follows is concerned principally with the relationships between decanposition processes and nitrogen transformations in soil, particularly as these relate to the avail- ability of nitrogen to crops. Effects on Crop Growth of Organic Materials Added to 80118 It has long been known that the addition of green manure or other organic materials to soil may result in either beneficial or deleterious effects on crops. In 1915 Wright (88), studying the effect of tuning under organic materia]; in an undecauposed form, found that the amount of available nitrogen in soil decreased when mature plant materials were used. On the other hand, no depressive effect occurred when succulent green, manure was used. He concluded that, for maintenance of nitrogen supply in soil, green manuring was a good practice. Later workers (72) showed that the incorporation of carbonaceous organic materials, such as sawdust and straw, into soils depressed the level of nitrate in soils. Addition of such materials also retarded tree growth and decreased crop f‘ '(3 {I (1 yields (16,h8,6l). It was shown that the amounts added and the particle size of these materials governed the degree to which these effects were expressed in a soil (b0,58,61). Ezqalanations for nitrate depression by certain organic materials were not apparent at first. It was known that incorporation of manurial materials into soil produced a‘more vigorous growth and activity of soil microorganisms (3h, 81). anith (62) found that disappearance of nitrates in soil was undoubtedly due to assimilation by microorganisms. He dis-- covered that when straw was added to soil, the rate of assimilation of nitrates by microorganisms was greater than the rate of production, and that crops suffered fran an actual lack of nitrates. Allison (2) obtained similar results when he added readily decomposable materials of wide CxN ratios to soils. Although the nitrogen content was maintained at higher levels in soil receiving more resistant materials, very little of the nitrogen was present in the ammonia: or nitrate form. Peev; at a; (h?) believed that this stabilization of nitrogen by materials more resistant to microbial decanposition was due to the formation of "lignoprotein' complexes. Hocesses which result in the tying up of mineral nitrogen in organic canbinations which are unavailable to plants have come to be known by the general term, “immobilization.“ Processes which result in the release of mineral forms of nitrogen which are available to plants are grouped under the term, mineralization" (7,210. A diagramatic representation of the recognised processes involved in mineralization and imobilisaticn of nitrogen in soils is presented in Figure 1, Page 5. Figure 1 shows that the immobilization and mineralization of nitrogen and carbon are associated. The principal agents in the immobilization of (1 re I - l ‘\' 1' -—.——-~—~—u ‘L?’ L. 1;». 'rP-~ “ 1 h. 4 3 - 9 ’ x 5989 *4 "'5".be ’ .{ filly”; Od-LTNK ’ . ., ‘O .1, . “M x (\ -;s '50 "4V ?/\.-:\ ‘5‘ ’-.';;;<() . 1‘: "‘. I L N _J \ M‘LTC‘DH“ Cei‘g . . - m... f“— < “‘ Elf.“ Pr‘oducrs I u 2-H ; . t \ “O I Dy .- an Synflsfiis) X x ‘. .,‘ 0 ,fi\ . a \ _ i "“2 ’3‘ . A Hun us, ' —- 4\ \ l l ' pl'J ‘I V ' 1mm N. N I FROG EN CY 3'.-. 1". . C:‘..'iii() Tx' CYC 1.6 Figure 1. Interlinkage of nitrogen and carbon cycles in mineralisation and imobilisation processes in the soil. nitrogen and carbon are the higher plants. Microorganism are prin- cipally responsible for the mineralization or release of nitrogen and carbon as nitrate and 002 , in which forms they are again available to higher plants. Microbial immobilization is incidental to the decasposition of plant materials. The absolute quantities of nitrogen and carbon which may be immobilized in the form of microbial cells at any given time is a function of the nunber of cells, or the size of the microbial population. When plant materials which contain a large proportion of carbon to nitrogen are added to soils, the size of the decay population and the quantity of nitrogen immobilized in microbial cell materials may increase temporarily to levels which seriously deplete the soil of the mineral forms (principally nitrate) which are essential for plant growth. Stojanorvic, 33 22-. (67). using nitrate and ammoniun salts labelled with N15 found that, in the presence of corn leaves, 19 pounds of nitrogen per acre per day was immobilized. During the same period, 27 pounds per acre per day was mineralized, resulting in a net increase in mineral nitrogen of 8 pounds per day. In the presence of wheat straw, 95 per cent of added amonim, or 38 pounds per acre per day, was im- mobilized while 18 pounds per day was mineralized from soil organic matter. This resulted in a net immobilization of 20 pounds per day. From their calculations, it was apparent that fertilizer nitrogen can be ab- sorbed by soil organians at much faster rates than by growing crops. However, where soil cmditions are favorable, net immobilization is a rather transient phencnenon, and nitrogen immobilized by microorganisms is again released very rapidly. They found that nitrogen utilized by /1 I“ I" /O plants came partly from soil organic matter, even when fertilizer nitrogen had been added to the soil. Carbon-nitrogen ratios of organic materials control to a great degree the rate of mineralization of the nitrogen they contain. As a general rule, materials with wide ratios have their nitrogen less immediately available to plants than those with narrow ratios. For example, a fresh plant material with an 8031 ratio decomposes more slowly than one with a )40:1 ratio because in such materials nitrOgen is the first limiting factor in the size and activity of the microbial population responsible for decay. Materials with ratios less than 20 or 30, contain more nitro- gen than is required by the microorganisms so that the excess is liberated as ammonia which is later nitrified to nitrates (9). Waksman, at El (.80), found, as a general rule, that plant material with a nitmgen'con- tent of 1.7 Per cent has a sufficient quantity to allow microbial decompos- itiOn -to proceed rapidly without net immobilization or depression of nitrogen availability. However, the chemical composition, other than nitrogen content, plays an important role in the decomposability of organic materials and the min- eralizability of nitrogen (12). Rubins, at 22-. (55), found that as a rule, materials of low protein content snowed correspondingly high proportions of hemicelluloses, celluloses, and lignin. It was the relative ease with which the first two compounds decanposed that rendered the nitrogen of these low-protein content materials relatively unavailable. These investi- gations showed that the C:N ratio did not always indicate the availability of nitrogen in organic materials. The type of carbon associated with {V 1" nitrogen in the materials could influence the release of available nitro- gen. For example, castor panace with a 0:1! ratio of 9:36 and a lignin content of 32.2 per cent released nitrogen more rapidly than cottonseed meal with a Och ratio of 5.h0 and a lignin content of 5.1; per cent. Since the carbon in lignin is rather resistant to microbial attack, it repre- sents an energr source of low availability and, consequently, plays little or no role in depressing net mineralization of nitrogen. As a result, castor panace, with its high lignin content, behaved like a material of lower 0:1! ratio, having a greater availability of its nitrogen. The most important constituents of natural organic materials added to soil are carbohydrates (celluloses, hanicelluloses, sugars, and starches), tannins, fats, waxes, lignins, proteins and their derivatives. These various chemical constituents are attacked at different rates. Sugars and starches, some hemicelluloses and some proteins undergo a most rapid decanposition by a great variety of microorganisms. Cellulose, certain hanicelluloses, some fats, and oils are decomposed more slowly, and by specific organisms. Lignins, some waxes and tannins are most resis- tant to decanposition. However, the content of available nitrogen in the soil was found to be the most important factor controlling cellulose decanposition (77). The composition of organic materials varies with the degree of maturity of plants, and maturity influences the rate of decomposition. Green plants are rich in soluble sugars and soluble nitrogenous canpounds; mature plants are rich in henicelluloses, celluloses, and lignins (73). Wakanan, gt 5.1.: (80), found that mature plants decanposed more slowly than younger plants due to differences in the proportions of their chenical caistituents. I1 IV I) Under normal soil conditims, the initial decomposition of plant material such as straw or hay is more or less rapid, depending upon species and age and the relative proportions of easily and difficulty decomposable constituents. As decanposition progresses and the more readily decom- posable constituents disappear, the process becomes much slower until a certain level is reached, when the residual mass changes frau brown to black (71;). This residue is huuus. It includes the slowly decomposing resistant constituents of the original plant materials and of successive past generations of microbial cells. The formation of chemical complexes between lignaceous and nitrogenous constituents may contribute to the low decanposability of hunusfil, 36, ’47, 78). In spite of its resistant nature, hunus is decanposed slowly and continuously. Because of its high nitrogen content and low energy availability, decanposition of hunus is accompanied by net mineralization of nitrogen. This slow but continuous release of nitrogen from hunus represents the principle nitrogen supply for plants in nature and for most crops under cultivation. Environmental factors which may affect decomposition and availability of nitrogen in soils can be briefly mentioned. Russell, 33 a; (5?), pointed out that temperature, moisture and dissolved oxygen in rain water were important in the biochemical decomposition of soil organic matter. They noted that decomposition did proceed noticeably below 5° C., and that rainfall was unusually effective in initiating decomposition. Temperature effects on soil were studied by Panganiban (143) who found that ammoni- fication took place between 15° and 600 0., the rate increasing with rise mtanperature. Nitrification occurred between 15° and 110° C. The optimum tenperature for nitrification was 35° C. or slightly higher. More recently, Rothwell, gt 5; (51;), have found that nitrification proceeds at a low but significant rate at temperatures as low as 5° C. 0n the other hand, [1 fi [’4 ,‘I 1'? 10 microbial immobilization of nitrogen may beccme increasingly important at low temperatures: Greaves, gt a}. (22), showed that larger nunbers of of microorganisms developed in soils stored at 10° c. than at higher temperatures. Bollen (9) observed that 60 percent moisture saturation capacity was optimtm for armonification and nitrification. However, 75 per cent of estimation capacity was found to be the optimum moisture content for carbon dioxide evolution in prolonged respiration experiments. Waksnmn, gt a (79), found that soil moisture equivalent to 50 per cent moisture- holding capacity was optimum for Carbon dion'de evolution. Noteworthy work on the effects of oxygen and carbon dionde con- centrations on soil nitrification and ammonification has been reported by Plunmer (L19). He fo md that the relationship between carbon dioxide concentration and nitrate production was governed by the partial pressure of oxygen. When the oxygen in the soil atmosphere was reduced below two per cent, denitrification occurred. The extensive changes which occur when a soil is subjected to drying and remoistening have been noted by nunerous workers. Ellison (1) con- eluded that air-drying caused a decided decrease in bacterial numbers. Lebedjantzev (33) observed that air-drying of soil brought about an extremely large increase in ammonium and amide nitrogen, a sharp decrease in numbers of microorganisms, and a large increase in solubility of organic substances. Waksnan (75) reported an increased liberation of ammoniun when any change was imposed upon physical, chemical or microbiological equilibria of a soil. Among the treatments which were found to bring about such changes was drying, followed by moistening. Birch, it. al (8), found evidence that clay played a role in the effects of drying and remoistening. They noted that the drying effect was one of liberation from the clay of rapidly decomposable material which, under steadily moist conditions, was protected by the clay frau microbial attack. The kind and amount of clay and the amount of organic material associated with it were also important. An adequate supply of nutrients, notably phOSphorus, is also needed for rapid decomposition. Kaila (30) concluded that phOSphorus equivalent to 0.1 to 0J4 per cent of the dry weight was required for decanposition of natural organic material. As an average, 0.2 per cent phosphorus appeared to be the critical level below which decomposition was retarded and immobilization of mineral phOSphate occurred. Chang (11;) reported a marked increase in rate of decomposition of straw when dipotassiun phosphate was applied to mature straw compost. Soil reaction is another factor that influences the patterns of nitrogen transformation in soil. Potter, et a1 (50), showed a greater gain in soil nitrogen in limed than unlined soils. Fraps, gt a}; (19), found that addition of 03003 stimulated nitrification. Jensen (28), study- ing the mineralization of organic nitrogen, observed that the critical C:N ratio of added materials was strongly influenced by soil reaction. In an acid soil, pea pod meal with a CzN ratio of 13.321 was the only material that showed an increase in inorganic nitrogen. No mineralization occurred in an alkaline soil where the CxN ratio was 26:1 or above. Below this, the release of nitrate increased rapidly with decreasing CxN ratio. Seasonal variations in carbon dioxide production, nitrate accumulation and bacterial numbers have been reported in soil. Russell, _e_t_. a}. (57 ), found very little activity during the winter months. Bacterial nunbers, carbon dioxide production and nitrate accunulation all increased with II (1 temperature above SOC. In general they observed successive periods of spring activity, sumner sluggishness, autunn activity, and winter in- ertness. The presence ofroots of growing craps and the sequence of crepe in a rotation have been reported to affect mineralization of nitrogen and soluble phosphorus content. Kubota (32) found that a rotation which in- cluded alfalfa reduced soluble phOSphorus content of soil more than a canparable rotation without a legume. Brown (13) observed that rotation of crops resulted in, greater nunbers of soil organisms as well as greater ammonifying and nitrifying powers in soil than continuous cropping to corn or clover. Carbonaceous matter exuded or abraded from roots of growing plants has been shown to favor development of nitrate construing organians in soil with a consequent transformation of nitrates to insoluble organic forms (35). Lyons, 93 El (35), pointed out that certain plants differ in their ability to take up nitrogen from the soil because of characteristic differences in the amount or composition of the organic matter liberated by their roots. Goring, gt a]; (21), investigating the influence of crop growth on mineralization of nitrogen in soil, concluded that less mineral nitrogen accumulated in cropped soils than in fallow 80118. They believed that nitrogen unaccounted for in the cropped soils was immobilized in the soil and was not lost to the air. The depressive effects of plant materials on crop growth are prin- cipally due to depletion of the available soil nitrogen, and sometimes phosphorus, by microbial assimilation, rather to toxic constituents of these materials (h). Crops may be benefited in a number of ways by the addition of plant {I f‘ ,‘b materials to soils. Murray (ho) found that incorporating green manures into a soil, not too low in humus, brought about a priming action which promoted an intensified mineralization of soil hunic nitrogen. Birch gt a}. (8) ,attributed this priming action, not to an increase in microbial activity, but to an exchange displacement of organic compounds, already associated with and protected by clay, by similar organic compounds re- leased during the decomposition of the freshly added organic materials. Hallam, _e_i_:. a]; (23), concluded that green manure crops served the most use- ful purpose as immobilizers and conservers of plant nutrients, or, in the case of legumes, as possible contributors of nitrogen. Incorporation of materials with narrow CzN ratios can have imxnediate beneficial effects on crop growth. However, microbial immobilization of nitrogen by materials with a wide C:N ratio has been reported to have useful residual effects. Allison (2) noted that an immediate harmful effect resulted from adding materials of a wide C:N ratio to soil. How- ever, the ultimate effect of this action was beneficial, provided suf- ficient time was allowed for the nitrate supply to return to normal. He attributed these results to a temporary increase in biological activity, followed by a slowing up of his activity until a point was reached where the proteins assimilated in microbial cells were made available to plants through their death and the subsequent amonification and nitrification of the microbial remains. Plants take up from the soil through their roots a nunber of different elanents in the form of mineral salts. One way in which these elements are added to the soil is through plant residues. Essential plant nutrients, including sulphur, phosphorus, magnesiun, calcim, copper, zinc, boron, (4 (I 1h manganese, are held in plant residues largely in insoluble forms which are unavailable to the succeeding crop. These are released in soluble mineral forms by microbial mineralization in a manner similar to that previously described for nitrogen and carbon. Tottingham, gt _a_l_ (69), fcund that, when.manure was added to soil, the rapidly growing bacteria caused an. impressive decrease in the water-soluble phOSphOI'uB of the manure and transfomed it into organic phosphorus. Eventually, this was released in an available form as a_ result of bacterial action on dead microbial cells, after the more available energylmaterials were used up. Inorganic acids, such as H2003, 112801,, HNO3, and organic acids are formed during the decomposition of plant residues. These serve to solubilize soil minerals, releasing essential nutrients in forms available to plants. Plant materials are sources of carbon and nitrogen for the main- tenance of soil organic matter or bonus. The latter displays typical properties of hydrophilic colloids: Its ability to absorb considerable quantities of water makes it an important factor in determining the water- holding capacity of the soil; it takes part in base exchange reactions; and it is subjected to diapersion and flocculation phenomena which play an important role in crumb formation and aggregate stabilization (56). As has been noted ( p. 9) the slow but continuous release of nitrogen through decomposition of hunus represents the principle source of nitrogen for many crops in most soils. Residue Management and Maintenance of Hunus It is generally accepted that equilibrim levels of soil organic matter are much lower under cultivation than in virgin soils under forest or grass vegetation (6,56). This is due to the fact that all soil {‘1 z") management practices which pranote optimum crop growth also promote higher nunbers of microorganisms, higier levels of microbial activity, hence more rapid decomposition of soil organic matter (hh). Livestock manures and legune grass sods have proven most effective in maintaining soil organic matter levels in the face of the depletive influence of cultivation (56). Livestock systems of farming are being replaced by cash crop systans on many farms. This has prompted renewed interest in research designed to re-evaluate other organic materials in terms of their usefulness in maintaining soil organic matter. Some of these investigations are con- cerned with normal cash crop residues, such as corn stalks, or cereal straw,- materials which can be grown without sacrifice of cash return in any year of the rotation (37). The use of catch crops and cover crops falls in this same category (52,53). Other investigations have involved the use of extraneous organic materials, such as sawdust, large quantities of which are frequently produced as waste by-products in areas where trans- port distances might not prohibit their economic transfer onto agricultur- a1 land (11,10). Among the concepts which have been and are being tested in such studies is the role of supplemental nitrogen in stabilizing carbon and promoting greater retention of resistant materials in the form of humus. (In general, such increases in organic matter level as have been observed as a result of nitrogen fertilization have been attributable to increased residue production due to increased crop growth rather than to any sparing action of nitrogen on carbon loss (3). However, it does appear that such a sparing action may be expressed with materials high in lignin or at advanced stages of decomposition of other materials (29). Such a result [1 (‘t {I 16 would be expected on theoretical-grounds, considering the high degree of resistance to microbial attack which has been shown for chemical complexes of ammoniun and proteins with lignin or soil hunic materials (11). Numerous investigators have shown increasing retention of carbon in soils with increasing nitrogen treatment (10,27,59,7l). It is know: that soil organic matter can fix large quantities of ammonia by strictly chanical processes (36,63). To what extent this phenanenon influences the efficiency of monitor fertilizers and the stability and prqierties of humus is not known. ' Estimating Availabiliiof Soil Nitrogen Under intensive systems of culture, as in the yeenhouse, it has been found possible to relate crop growth to the level of nitrate in the soil (61:). Under field Conditions, however, nitrates are subject to unpre- dictable losses due to leaching, crop removal, denitrification and bio- logical or chemical immobilization. As a result, nitrate revealed by chenical test at any given time may or may not bear a relation to crop per- romance. Approximately 98 per cent of the nitrogen in soil is present in organic materials,-p1ant residues, microbial cells or humus. The soil's nitrOgen supplying power is primarily a function of the quantity of this oanbined nitrogen which is present and the rate at which it is released or mineralized by microbial decomposition. Procedures for estimating nitro- gen supplying power in soils have, therefore, been based on methods for estimating either total nitrogen or mineralization rate, or both. These procedures may be classified into three general categories: 1. Estimation of total organic nitrogen; 2. Measurement of labile fraction of soil I“. 17 organic nitrogen; 3. Microbiological methods for e stimating mineralization rate or nfineralizability. Gainey (20) observed that a very close and direct relationship existed betwaen nitrogen content of soils and their nitrate accumulating ability. He obtained a correlation of 0.99030.012 for a "nonfertile" series of soils and 0.98810.0006 for a "fertile" series. Fraps (18) concluded frcm results with pot experiments that the average size of the crop, and the nitrogen withdrawn from the soil, increased with the total nitrogen content of the soil. Allison, _e_t_ 31 (5), showed that a positive correlation existed between total soil nitrogen and nitrate formed fran soil organic matter at all incubation periods for lined and unlined soils. Woodruff (87) was able to estimate the rate of nitrogen delivery to crops from a chemical determination of soil organic matter. The organic matter determination was actually ane stimate of total nitrogen, since a uniform nitrogen content was assuned. An empirical mineralization factor was calculated using yield data from long term fertility experiments. Truog (70) proposed a method for extracting a labile fraction of the nitrogen frcm soil organic matter by partially oxidizing the latter with alkaline permanganate. It was presumed that the solution attacked the readily oxidizable portion of the soil organic matter. By this oxidation, nitrogen is released as ammonia and is measured together with exchangeable ammonia. Thus, it appeared that the method would provide a direct measm‘e of the two most immediately available sources of ammonia for the nitri- fication in soils. These, in turn, should be related in a straight-forward manner to a 80118 ability to release nitrate for crop use. Fitts, at g; (17). found that under Iowa conditions nitrate produced during incubation under standardized conditions provided a basis for 18 predicting the nitrogen requirements of corn. They obtained a negative correlation between nitrifiable nitrogen and yield response of corn to nitrogen fertilization on Iowa soils. Saunder, it; a_l_ (60), used this technique as a means for estimating available nitrogen on sane Southern Rhodesia soils. They found that the nitrogen mineralized in laboratory incubated soils, sampled towards the end of the dry season, appeared to provide a good index of the nitrogen likely to be available for crop use under field conditions during the subsequent growing season. The authors noted a tine-lag betwaen the start of ammonification and the start of nitrification. This lag was still noticeable after 5 weeks of incubation. Thus, it was necessary to determine the time when nitrification began for each soil type and for different environmental conditions before a reliable estimate of mineralization or nitrification rate could be made. Waksman, gt a}. (77), believed that the cellulose-decomposing power of a soil could yield information on the total availability of soil nitro- gen. Whereas nitrification was found to be a good index of soil fertility, sumonification was not (76). Since a principal end-product of aerobic heterotrophic micro- biological processes is carbon dioxide, the evolution of carbon dioxide from incubating soils has been used as an index of the respiratory activity of soil microorganisms. Waksman, et al (79), obtained data which indicated that the determination of the amount of carbon dioxide evolved from in- cubating soils, as well as estimates of the nunber of microorganisms and of the nitrification yield of the soil, could be used as indices of soil fertility (76). Stoklasa (68), in 1912, stated that where there was greatest nitrification, there was found the greatest production of carbon dioxide. However, Patrick (145). attanpted to find out why crops following {1' /\ red clover should yield more than those following timothy or corn. He found that high carbon dioxide evolution was correlated closely with nitrate depression. These conflicting results were probably due to differences in composition of the crop residues remaining in the soiIS. Oxygen uptake may be used to measure microbiological activities, and current attempts are being made to use it as a basis for e stimating soil fertility levels (31). 20 MATERIALS AND PIE IHODS Field Treatments and Cropping History Forty soil samples, composited by treatment and by year of establishment fran the "Michigan rotation" plots at the Ferden farm in Saginaw County were used in this experiment. The soil is classified as Sims clay loam. The "Michigan rotation" was originally established to determine the effect on crop yields of the addition of large amounts of sawdust in comparison with more nomal qmntities and types of residues. ‘Ihe five-year crop sequence .for the rotation was corn, beans, barley, and twa years of alfalfa-bronze. Fertilizer treatments were as follows: Corn: 100 pounds 5-20-10 per acre Beans: 200 pounds 0-20-10 per acre Barley: 210 pounds 5-20-10 per acre Alfalfa-bran: no fertilizer either year This crop sequence had been in effect for three canplete cycles of the rotation before the following residue treatments were initiated: l. Two-year-old alfalfa—brome (two cuttings of hay removed), followed by corn, beans, barley and two years of alfalfa. brome. 2. Sane as treatment one, except that neither cutting of the second year of alfalfa-brome was ranoved. 3. Same as treatment one, except that 35 tons of sawdust per acre was applied after renoval of the second cutting of hay on the second year of alfalfa-brome. l‘ [1 21 1;. Same as treatment three, except that four tons of wheat straw was applied instead of sawdust. Residue treatments were initiated on a complete block of plots in each of the following years: 1951, 1952, 1953, 1951;, and 1955. The soil block which was cropped in 1956 was the same as that on which treatments were established in 1951. On this block, in 1955, two cuttings of alfalfa-bronze hay were returned on treatment two, and four tons of wheat straw was applied on treatment four, as was done in 1951 before the first cycle of the rotation was initiated. However, the application of sawdust on treatment three was not repeated in 1955 Preceding the second cycle of the rotation which began on this soil block in 1956. Through the first cycle of the rotation, one-half of each plot plant- ed to corn received a supplemental side-dressing of 1:0 pounds per acre of nitrogen, except for the sawdust treated plots which received 120 pounds. Beginning with the second cycle of the rotation 31.1957, one-half of all corn plots received 100 pounds of supplanental nitrogen per acre. One-half of each plot planted to beans has consistently received 140 pounds of nitrogen per acre. 0n one-half of each plot of barley, 20 pounds of supplemental nitrogen was applied as topdressing,except in 1952 when to pounds was used and in 1956 and 1957 when the supplanental application was discontinued due to lodging which accompanied a change in the variety Planted. The treatments were replicated five times on the soil block which was establiShed each year. Soil sanples were taken from each of these treated plots. mplicates were then composited by treatment and by year of es- tablishnent. The samples for the present study were taken in September n 22 1957. They were obtained from plots which represented the second, third fourth, fifth and sixth years after the initial residue treatments were made. laboratory Determinations The following laboratory deteminations were made on air-dried soil samples: - A. Soil pH was determined with a glass electrode, using a 1:1 soil-water suspension. B. Ammoniun and nitrate nitrogen were measured in Morgan's extracting solution using procedures described by Peach and English (146). Immoniun determinations were made by direct nesslerization. Nitrates were determined by the phenoldisulfonic acid method (15). C. Alkaline permanganate oxidation, as described by Trucg (70), was used to determine “available organic soil nitrogen,a including ammoniscal soil nitrogen. D.. "Nitrifiable nitrogen" was estimated as nitrate formed dur- ing a two-week incubation period, following a slight modification of the procedure developed by Stanford, et a1 (66). Ten-gram soil samples were used., The soil was mixed with an equal volune of vermiculite- The mix- ture was placed on another layer of vermiculite over a glass wool pad in a carbon filter tube. A covering layer of vermiculite was placed on top. Nine m1. of a 0.2 per cent water solution of synthetic soil con- clitiener1 was added and allowed to remain in contact with each sanple for 1"Ioamaker," a proprietary product of the Monsanto Zhemical Co. was used. ("I 23 15 minutes before beginning the initial leaching. This was done to assure clear leachates. The moisture was adjusted by applying suction. The tubes were stoppered with one-hole rubber stoppers. Samples were incubated at 35° C. in a chamber adjusted to a constant relative humidity of 98 per cent. The latter was achieved by exposing shallow pans filled with 2 per cent 11251))J in the bottom of the incubator. Nitrate produced was deter- mined after two weeks by the phenoldisulfonic acid method (25) in water extracts cleared of colored organic materials by filtration in the presence of dry Ca(OH)2. E. Total carbon was calculated firm the weight lost during ignition, according to the procedure described by Mitchell (38). Dup- licate 25-to-bD g. samples were ignited in a muffle furnace at 380° C. for 8 hours. Total carbon was also determined by dry canbustion in a carbon train on duplicate samples of 17 soils. The error regression of total carbon on ignition weight loss for the paired duplicates of this group of 17 soils was used as the basis for calculating total carbon fran weight loss for all soils: y- 0.30 + 0.14152; r " 0.9km». where: I " total carbon 2 - ignition weight loss .1? - correlation coefficient (significant at l per cent) F. Total nitrogen was determined by standard Kjeldahl procedure as modified by Prince (51). The catalyst was a mixture of CuSOh, Ego and xgsoh. Methyl red (0.1 per cent in 95 per cent ethanol) was selected as the indicator. G. Rate ofrespiration was measured by the simultaneous 002 absorption method of Norman, 33 2.; (1&1). One-hundred-grams of soil was placed in two quart Mason jars, and moisture adjusted to 70 per cent of 21: water holding capacity. The soils were incubated for two weeks at 35° C. The soils were aerated every four days to supply sufficient oxygen for maximun 002 production. Vials containing 0.5N NaOH were placed in each jar to collect (Dz at one-to four-day intervals, depending on rate of 002 evolution. Carbon dioxide produced was calculated after titrating the contents of each vial with standard H01 in the presence of an excess of 3.1012. Phenolphthalein was used as indicator. H. Reserve phosphorus (P205) was determined in soils by. the method developed by Spurway and Lawton (65 )c All determinations were in duplicate, except for incubation 002 mass urenents . 25 EXPER DEN TAL RESULTS In the experimental design of the present work, three factors were confounded in such a way as to make it'impossible to relate experimental results directly to initial treatments in a cause and effect relationship. The time elapsed since treatment was confounded with soil differences between blocks of treatments which were established in different years. Both of these factors were systematically related to the sequence of crops in the rotation. Thus, a single year's data provide no basis for dis- tinguishing between: (8.) Soil effects; (b) effects ofresidues frau immediately preceding crops; and (c) the changing intensity with time of Specific effects associated with the initial treatments. The comparison of one year's data with another, however, does permit some separation of these confounded effects. This will be true, par- ticularly, when data for a complete 5-year rotational sequence are available. To facilitate such canparisons, all data have been tabulated so as to permit ready identification of soil blocks with years after treatment and the crop grown during the sampling year. In the discussion, a tentative canparison is mde with the previous year's data as reported by Mara (39). ‘ laboratorLDeteminations Soil reaction ( PH) Table 1; (Appendix) contains the results of determinations for soil reaction. The lowest PH was found the second year after treatment. In the previous work (39), this soil block was found to be lower in pH than the others. Thus, the principle differences in soil reaction observed in Table I; appear to have been related to soil differences rather than to treatment or the immediately preceding crop sequence. {1 {J ,‘I ['3 26 Reserve phOSphorus (P205) Determinations for phOSphorus extracted with 0.13N H01 (65) are presented in Table 5 (Appendix). The smallest amounts recovered were from the second year and the largest from the fourth year after treatment, The same soil blocks were low and high in the previous year's data. The results tend to indicate that soil variation was more of a factor than treatment or cropping sequence in determining the level of reserve phOSphOI‘uS in these soils. Total carbon The results of total carbon determinations by the ignition method are recorded in Table 6 (Appendix). The actual values obtained were of the order of 30 per cent greater than those obtained the previous year by Mora, who used the wet combustion method ( 51). The values reported here for total carbon are undoubtedly erroneously high because an undetermined quantity of water of hydration was included in the weight lost during ig- nition. 1n the group of 17 soils for which carbon determinations were made using both dry ccmbustion and loss on ignition, the ratio of ignition weight loss to total carbon was approximately 2.11:1. This is considerably greater than the factor 1.72 which is conventionally used to convert total carbon to organic matter. Loss on ignition was selected as the method for determining carbon in the present study because it was felt that the use of large samples of soil would minimize sampling error, particularly in the sawdust-treated soil. However, this eiqaectation was not fulfilled. Using 25-to hO-g samples, a standardized ignition tenperature of 380°C. and a standard 8-hour ignition period, the precision of the determinations was such as to yield a relative standard deviation between duplicates of 6.5 per cent 27 of the mean. Using one-to two-gram samples in the carbon train the relative standard deviation was 1.8 per cent of the mean. The rather low precision and the erroneously high carbon recovery of the loss-onpignition determinations were not recognized early enough to permit the use of a different procedure. The results are presented here because they are the only data available. Some interpretations can be drawn from the relative differences between treatments and years. The total carbon data are presented graphically in Figure 2. As ‘would be expected, the highest levels oftotal carbon were found in the sawdust-treated.plots. TWO years after sawdust application, there were seven to ten tons more carbon in the sawdust-treated plots than in the checks. By the sixth year after treatment, this difference had been re- duced to three to five tons. However, there was no evidence that the level of total carbon in the sawdust—treated.plots had declined from the second to the sixth year, as would have been expected from progressive decomposition of the large sawdust application. 0n the contrary, there was a marked tendency for car- bon to increase. A.similar increasing tendency was observed for all other treatments. Canparison of the data in Figure 2 with the data reported by More (39) indicates that the observed trends were more closely related to crop sequence than to soil differences between blocks established in different years. Both years' data reflect a tendency for carbon to be depleted by the three tilled crops in the rotation (com, beans and barley) and a strong tendency for carbon levels to be restored by the two years of alfalfa-brome meadow. In the present year's data, these effects tend to appear as residual effects which were observed in the year following a given crop. The values for the plots which were in barley (3rd year after 28 (2 cuttings of 2nd yr hay) Alfalfa-Brome 335 with supplemental N Check 33. <)='without supplemental N N 31* email. 1mm 7.. vi a. U .. 6 ar. Mu. (a I...“ .1 N m30uv-ifid V O .navu‘ul.) .JZJH II} m «Sum Rewind {4 0 $qu, Fwd 5 h * 3 Years after treatment (h tons/A) Straw L l. .I'oly’rllli II: Ila- ll. 5 11 Q/ 71 H9 3 2 2 2 2 mm. .axOHV r marommwnmusgunq. OH Mum) 62.. 5 NF! uzoemaiirms h Nmmw:r‘ rmnmcm 3 9 7 5 2 2 . 2 meow mom cognac Hence mace Years after treatment (35 tonS/A) Sawdust 151M » p m lo}.- .«I..- III 3 l 9 2 2 l 31L m who.“ an“. . ,..r,u.3:.h J n. .731.an e, «02“ r . L storm instant . O lama.) v.3 Nee a, 50.3 -;z|1:, {J r J whizrcwm or... J lib h-) ‘L'll I IalIL Q/ 7; H) 3 1 Q/ 2 2 2 2 2 1 II. n _ «xwtud 3-:!|i§-!5§ifil p.10 um-ndw._€u4w3 5.! iii}?! .a em», SIN reflmmufléuit :1!!! t ..1 .avw» 3” I I . >wna1 r s . P. L. ..-..-” 3 Q/ S l 7.. 3 9 5 14 1M 14 3 3. 2 v; .. -1-....---11-11...----- -..---..1.-- 111.6 .44... ..UOU . 1; . .-- A.......... m .0:- e ...-.... ”...-...... 5 m 1 1 1 1 1. 1.-.-1...u- -.1...11N.11 t a a. - . - .1111 m -.:w a umw- <1.qieq +u Q_ - - 14am». unm h - 1 1-1 ..-----..-111- 1.1.1111. r . e 1------ n N -1.11 . OT- -- -1 - - 1 111. in NH- T .1412 Wu.— 0W1wa..w. . . . _ . b L -1r'111r111e11llf111p11111r1 3 9 .5 l 7 3 9 H.) 1.14 h 14 3 3 2 577 t4= with supplemental N meow use 2 Hmpop .mpao (h tons/A) Straw n.1,???-..-1a....maqfiq Or11rl .0... m». p1 N I. 'll’ ..".i|ln Ill-(IDI-I I.‘Tl!l:‘t‘1 1-1.1.1.}. 1.1.1-1114 r11. :4d4m .11114HUW.1- -..--11 1nflhmMqumm4q-mj Cr- -- 1. - 11 e we.;uzw h... . .---11.-. -1 men-....mm.-1mm1m..14<, Du Chi-1111.1 1...- 11-11-,- -1.----d<.wn,1m.m.m11 4w -111:.1-111.11111..-1-:-111111 m m Emmi $3.4m. ...u .d We) . 3 SC om. wzqwm up! w ”(J _ Ob/ r1119 F _.l1.— ..11 a“ 1.1 b 3 up a; La La Le a; a) mu once you 2 Hugo» .mpeo S 6 1. Years after treatment 5 ' 6 Years after treatment 1.1 3 Total nitrogen in soil at yearly intervals after addition of various organic amendmentS'with and without supple- mental nitrogen fertilizer. 2 Figure 3 o 35 build-up under alfalfa-brome may have been due to the larger quantities of carbonaceous residues left as roots, stubble and stover from the larger crops of corn, beans, and barley which were produced where supplemental nitrogen was used. (See P. 29.) These could have exerted an. immobiliz- ing or stabilizing influence on nitrogen, similar to that exerted by sawdust. The residual build-up of nitrogen late in the rotation, after early supplemental applications of nitrogen, was most marked in the sawdust- treated.plots. Soil CAN ratio 'As has been-pointed out (P. 26), the total carbon figures reported here are not quantitatively reliable. Their value lies in the fact that they provide a basis for relative comparisons among treatments or soil ‘ blocks. This applies equally to the calculated CaN ratiOS'whiCh are recorded in Table 9 (Appendix). As far’as relative differences are concerned, two similaritieS‘were Observed in.these data and those reported by Hora. The widest C:N ratios 'sere found in the block of plots established in 1955.- This was due to the low nitrogen content of the soils in this block. (Cf. Figure 3. and Table 8.) Among the residue treatments, the widest C:N ratios were found during the first two years following the lassive application of sawdust. This*was due to the higher carbon content of these soils. (Cf. Figure 2 and Table 6.) * There'was a tendency for CzN ratios in 1957 to stabilize at a value around 1111. Ih.Mora's work, this stable ratio, which.appeared to be characteristic for the soil and cropping system, was 9:1. Mineralization of carbon Carbon evolved during incubation for 1h days at 35° C. is recorded '\ 36 for the various soil samples in Table 10 (Appendix). { The value of this determination is that it provides a measure of the availability to micro- organisms of carbonaceous energy materials remaining in the soil at the time the soil samples were taken. These energy materials include root renains fran the immediately preceding crop, as well as true soil organic matter (or hunus) and such partially transformed organic materials as may remain from previous crop residues or organic amendments. The incubation respiration data are plotted graphically in Figure )4. The amount of (1)2 produced from soil two years after sawdust application was approximately double that for any later year or any other treatment. There was little difference among soils sampled three to six years after sawdust was added. These results are essentially the same as those reported by Mora (39). It would appear that surplus energy materials (cellulose, hemicellulose, etc.) in the sawdust were largely dissipated during the first two or three seasons after application. The more resistant materials which remained were relatively stable and (hid not change appreciably in energy availability during subsequent years. The quantities of‘mz produced frcm soils three to six years after sawdust application were consistently higher than for soils which had received the other residue treatments. This might be due, at least in part, to the fact that residual organic materials fron the sawdust treat- ment may have been qualitatively different than residual materials fran the otter treatments. However, total carbon was much higher in the soils which received the massive sawdust treatment. (Cf. Figure 2 and Table 6.) The greater production of 002 would appear to be more closely related to total quantity of substrates remaining after extensive decomposition. These terminal substrates themselves would appear to be essentially similar regardless of initial treatment. C:N ratios were found to be very similar 37 Alfalfa-Brome (2 cuttings of 2nd yr hay) Check ' 5 wa ..xp ,uu...<.f.<_. 0A.- dun; .3: I!‘al|.|-r-l1 . h 3 I. ll. OleasamM . r ‘IclllI‘IIl.l\l. 3 'Years after treatment u é 20' 15 12} I 8.- .' M. 0 -- [Rah/IE NEIIII . ouplxwm 2 ---.Qu-.--.-.- .. «#55:? o-O..U...1L..- 1 1 without supplemental N N‘with supplemental N Years after treatment :3 f IO! 2 ANOH xvmnom pom Um>Ho>o nonnmo mpnsom LIN—N. .74 r_.U\(/\ 6 wumvflfi- 4 an ..m my--:Wmm,-ww.q,m A it a Nhlumwflnqua m a ......mfiv. h t r M .. m V m? Ln uqml 3 .m n 0.. ..I -... m m _ s r M a t ..u m. . .. e .Olbiltll‘lrslt. .... _.O . r. . . my ,0 9. .l m +u a n m V r a a .d.t m w» s s(~. - m . liltir 0mm. vi lulrlllooLuuu I‘ rill. grill... r1: 1.-..13. I.........--...rLl!|l-lL 20 16' 12 8 h ANOH Nvoaow and Um>Ho>m sesame mpnsom from soil samples taken at yearly intervals after addition of various organic amendments with and without supplemental Carbon evolved as C02 during a lh-day incubation period nitrogen fertilizer. Figure he 38 in all soils four or five years after initial treatment. The (1)2 production fran soils following the check, alfalfa-brow, and straw treatments were essentially the same during any given year. The normal rates of application represented by the alfalfa-brome and straw treatments, were not high enough to alter the biological properties of these soils in this six-year period. Incubation mineralization of nitrogen Nitrate-nitrogen produced during a lit-day incubation at 35° c. is tabulated by soil blocks and treatments in Table 11 (Appendix). The values are quantitatively of the same order as those reported by Nora (39) for the previous year's soil samples. The data are plotted in the histograms in Figure 5. The results with sawdust treatment were essentially the same as those for the previous year. There was marked suppression of incubation mineralization rate during the early years after treatment. This was due to microbial immobilization of nitrogen in the presence of excess energy materials contributed by the saw- dust. There was a progressive recovery in net mineralization rate, year after year, through the sixth year after treatment. This recovery was enhanced by applications of supplemental nitrogen. By the third year after treatment, nitrate production was essentially equivalent to that for the check soil. In More's data this equivalence was not achieved until four years after treatment. During the fifth year, nitrate pro- duced by the sawdust treated soil was substantially geater than the check, and this high rate was maintained through the sixth year. More had found that supplenental nitrogen treatment depressed incu- bation mineralization of nitrogen during the early years of the rotation in the check soils and those which received the alfalfa-brome and straw treat- m ants . 39 (2 cuttings of 2nd yr hay) Alfalfa-Brome Check without supplemental N O N ,II-4I-4IIIIIII. N4 L noun. 2.3 6 ...... 2.036 m. J. , m . m mm): Iflu Iununq e N wwszrbvfimm! un.oz~.d/m , -,r