m NIHMHHIIIHIIUHJUNINHIHHIHIHIIHWilli 1293 20154 9551 I turn A ‘56." Invalid—nan" A. I o v ‘ 4' .vapa'ldxr-rtun -t‘l‘ifi'fl U . .2, ”.31. I'I-‘VV‘WI‘Y This is to certify that the thesis entitled Evaluation of Cogranulated Urea-urea Phosphate as a Nitrogen Source for Crop Production presented by Richard Malcolm Johnson has been accepted towards fulfillment of the requirements for Master oi Science—degreein—Cnop—and—Soil Sciences Major professor Dat ‘5‘ qu 0-7639 MS U is an Affirmative Action/Equal Opportunity Institution MSU RETURNING MATERIALS: Place in book drop to remove this checkout from LIBRAR ”5-1:: your record. FINES will be charged if book is returned after the date stamped below. JAN ”013' 1992 3&2 EVALUATION OF COGRANULATED UREA-UREA PHOSPHATE AS A NITROGEN SOURCE FOR CROP PRODUCTION BY Richard Malcolm Johnson A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Crop and Soil Sciences 1986 ABSTRACT EVALUATION OF COGRANULATED UREA-UREA PHOSPHATE AS A NITROGEN SOURCE FOR CROP PRODUCTION BY Richard Malcolm Johnson Field, greenhouse and laboratory experiments were conducted to evaluate the relative N use efficiency of cogranulated. urea-urea phosphate (UUP) as. compared. to urea (U) and NH4NO3 (AN) . Performance was evaluated in conventional and reduced tillage systems. Estimates were made of the 3011's urea hydrolysis rate and of NH3 volatilization. Field experiments indicated a possible advantage to delaying N application 6-8 weeks after planting. Incorporation of UUP and U benefited crop yields. Greenhouse experiments showed that initial soil moisture content, residue level, N rate and N source all may influence N33 loss. No difference in urease activity was recorded due to treatment. Results from the field NH3 loss experiment suggested an advantage to UUP over U. The tentative ranking of N sources in terms of relative N use efficiency would be : AN>UUP>U. DEDICATION This thesis is dedicated to my grandparents Dorothy M. Johnson. and .Armand F. Joly. Their love and help throughout the years has been a constant source of inspiration. .Also, to my parents Robert and Carolyn. Johnson. Their encouragement and love has given me the determination to finish and succeed. ii ACKNOWLEDGEMENTS I wish to thank Dr. D.R. Christenson for his help in preparing this thesis and for serving as my major advisory His patience. and thoughtful suggestions have made the past several years enjoyable as well as productive I would also like to extend my appreciation to Dr. Ellis, Dr. Erickson and Dr. Poff for serving on. my guidance committee. Special thanks are extended to Calvin Bricker. His help and friendship throughout my graduate program made the long hours in the field and the lab much more bearable. Thanks also go to John Erwin, a good friend who showed me that computers are not that complicated after all. Finally, I would like to extend my sincere appreciation to Tennesse Valley Authority for assistance in funding and obtaining materials. iii TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES INTRODUCTION LITERATURE REVIEW Chemistry of Urea Chemistry of NH3 Loss: Non-Calcareous Soils Chemistry of NH3 Loss: Calcareous Soils Tillage System Factors Affecting Urea Hydrolysis Temperature Moisture Substrate Concentration Soil Reaction Organic Carbon, % Clay, CEC Cultivation Factors Affecting Ammonia Volatilization Temperature Soil Moisture Content Nitrogen Rate Method of Application Rate of Air-Flow & Atmospheric NH3 iv vii ix 04:4: 15 19 19 20 20 21 22 24 25 25 26 31 32 34 Texture, Organic Matter & CEC MATERIALS AND METHODS General Description of Soils Field Experiments Greenhouse Experiment Field Ammonia Volatilization Experiments Urea Hydrolysis Experiment Laboratory Analysis Plant Sample Treatment NH4 and N03 Extractions Total Nitrogen Determinations Soil Nitrate and Ammonium Determinations Statistical Analysis RESULTS AND DISCUSSION Residue Measurements Field Studies East Lansing Saginaw Greenhouse Cropping Study Crop Yield Nitrogen Uptake Urea Hydrolysis Measurements Field Samples Greenhouse Samples Field NH3 Loss Experiment V 35 39 4o 41 44 47 so 51 51 52 52 52 53 54 54 60 51 67 74 74 82 9o 91 93 95 Ammonia Loss Soil N03 and NH4 C02 Levels Soil and Air Temperature SUMMARY AND CONCLUSIONS LIST OF REFERENCES vi 95 99 99 101 104 108 10. 11. List of Tables Soil test values for field experiments. 41 Tillage systems for experiments 1984 and 1985. 42 Planting, harvesting and plant sampling dates for 44 field experiments conducted in 1984 and 1985. Soil test values for greenhouse and NH3 experiment.45 Residue cover as affected by tillage and 55 measurement method at East Lansing and Saginaw, 1984 and 1985. Effect of nitrogen source, nitrogen rate, method of62 application and time of application on yield of corn (averaged across two tillage systems) East Lansing, 1984 and 1985. Effect of nitrogen source, nitrogen rate, method of 54 application and time of application on ear leaf nitrogen concentration at tasseling (averaged across two tillage systems) East Lansing, 1984 and 1985. Effect of nitrogen source, nitrogen rate, method ofem application and time of application on total nitrogen uptake (averaged across two tillage systems) East Lansing, 1984 and 1985. Effect of nitrogen source, nitrogen rate, method of 53 application and time of application on yield of corn (averaged across two tillage systems) Saginaw, 1984 and 1985. Effect of nitrogen source, nitrogen rate, method of'n) application and time of application on ear leaf nitrogen concentration at tasseling (averaged across two tillage systems) Saginaw, 1984 and 1985. Effect of nitrogen source, nitrogen rate, method ofyq application and time of application on total nitrogen uptake (averaged across two tillage systems) Saginaw, 1984 and 1985. vii 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. Approximate probability of significance of the F 75 statistic for various sources of variance for three crops of corn in the greenhouse experiment, 1985. The effect of fertilizer source, fertilizer rate 78 and corn residue on crop yield in the greenhouse experiment, 1985. The effect of N rate and residue level on crop 80 yield in the greenhouse experiment, 1985. The effect of N source and N rate on crop yield in an the greenhouse experiment, 1985. The effect of N rate and residue level on nitrogen EB uptake by three crops in the greenhouse, 1985. The effect of initial moisture content and 85 fertilizer source on nitrogen uptake by three crops in the greenhouse, 1985. The effect of N source, N rate and residue level on86 nitrogen uptake by three crops of corn in the greenhouse, 1985. The effect of N source and N rate on nitrogen 88 uptake by three crops in the greenhouse, 1985. Effect of fertilizer rate, method of application, 92 tillage system and soil type on urease activity in corn plots receiving urea fertilizer, 1985. The effect of N rate and residue level on urease 94 activity of greenhouse samples, 1985. Relative humidity (Rh), minimum and maximum 95 temperature during selected 24 hour periods, temperature during measurement and C02 concentration during the field NH3 loss experiment, East Lansing, 1985. Effect of N fertilizer source, fertilizer rate and. 97 day of measurement on NH3 evolved from the soil in a field experiment, East Lansing, 1985. Effect of N fertilizer source and fertilizer rate 1m) on soil N03 and NH4 levels in samples taken at the termination of the NH3 loss field experiment, East Lansing, 1985. viii List of Figures The volatilization field sampler used in the 48 ammonia loss experiment, East Lansing, 1985. Relationship between percent cover as determined by 55 the photographic method and residue dry weight, 1984. Relationship between percent cover as determined by 57 the photographic method and residue dry weight, 1985. Relationship between percent cover as determined by 58 the line intercept method and residue dry weight, 1985. Soil and air temperatures during the NH3 loss 102 experiment, East Lansing, 1985. ix INTRODUCTION In 1984, over 11 million tons of N fertilizer were applied in the United States. Urea [CO(NH2)2], accounted for for almost 25 t of this total, with an application of 2.7 million tons. The amount of urea applied in the United States has increased steadily since 1955 (Hargett and Berry, 1984) . Several properties of this material have promoted its increased popularity. Urea contains 45- 46 t N, the highest percentage of N currently available in any solid material. It is also less expensive to manufacture than many other N fertilizers. Finally, urea is easy to handle and safe to store in both the solid or solution form. The principal drawback of urea is the possible loss of N due to NH3 volatilization. Although it is not an NH4 fertilizer in its applied form, urea is rapidly hydrolyzed by soil urease to (NH4)2CO3. This compound is unstable and rapidly decomposes. Several investigators have proposed the following reaction mechanisms for this process (Penn and Miyamoto, 1979; Terman and Hunt, 1964 and Chin and Kroontje, 1963): CO(NH2)2 + 2H2° ....... > (NH4)2CO3 (N34)2CO3 ---------- >2NH3 '1' C02 ' H20 214114011 It is apparent from examining the previous reactions that many physical, chemical and biological factors can affect the quantity of NH3 volatilized. Soil pH, soil moisture content, CEC, soil texture, soil urease levels, soil temperature, organic carbon and soil N all influence the amount of NH3 lost. In addition, atmospheric properties can give significant effects. The major factors include atmospheric NH3 concentration and relative humidity. Most of the properties just mentioned are difficult if not impossible to control in the field. However, some degree of control may be achieved through cultural practices. Nitrogen rate, N source, timing of application and incorporation of N fertilizers are all good management practices which can increase efficient use of N and reduce losses due to NH3 volatilization. Although these practices are effective in many cases, they are complicated even further when urea or urea-based materials are applied in reduced tillage systems. The principle drawback, with respect to NH3 volatilization, is associated with the layer of crop residue left on the surface in most reduced tillage systems. The residue can decrease the rate of fertilizer dissolution by preventing contact of the fertilizer with the soil. Also, urease activity is heightened in this residue layer. This would promote a rapid conversion of urea to (NH4)2CO3, thus increasing the volatilization potential. The objectives of this study were : 1) To investigate several methods in which N use efficiency may be increased in reduced tillage systems. These included variations in; N source, N application rate, method of application and timing of application; 2) To evaluate volatile N113 loss from urea and urea-urea phosphate in the field as influenced by N application rate, and 3) To estimate the soil's urea hydrolysis rate as influenced by N application rate, N application method, initial moisture content, tillage system and soil texture. Literature Review CHEMISTRY OF UREA Since the year 1773 when urea [CO (NH2)2] was first isolated from urine by Rouelle, scientists have studied its properties and benefits. However, it was not until 1920, when the availability of inexpensive raw materials made the manufacture of urea economically feasible, that it gained popularity as a N fertilizer. This source of raw materials came as a direct result of the implementation of the Haber process for the synthesis of NH3 (Hardesty,1955). Urea is produced by reacting NH3 and C02 gas under high pressure and temperature (19.2-27 .3 MPa , 180-195 C), in the presence of a suitable catalyst. Most often the catalyst is an iron-potassium aluminate mixture (Pesik, Stanford and Case, 1971) . The NH3 and C02 react to form ammonium carbamate, which is then decomposed to yield urea and water. The following reactions illustrate this process: 2NH3 + co2 -------- > unzcoonn4 (1) NHzCOONH4 -------- > NH2CONH2 '1' H20 (2) (Tisdale and Nelson, 1975) Urea, or carbamide, is the amide of carbonic acid. It is a nonionic nitrogen material used not only as a fertilizer, but also as a protein supplement in ruminant nutrition and in the manufacture of plastics. Urea is a white crystalline solid containing 45-46 % N in the most common fertilizer grade. It is very soluble in water, at 30 C , 100 ml of H20 dissolve 133 g of urea. This is compared with 242 g for ammonium nitrate and 78 g for ammonium. sulfate. If urea is mixed with NH4N03 the solubilities of both compounds are enhanced; 719 grams of urea and 845 grams of NH4N03 will dissolve in 100 m1 of water to yield 1,206 ml of solution (Gasser, 1964). These solubility characteristics of urea have helped to increase its popularity, especially for use in N solutions. Urea, in its crystalline form, is less hygroscopic than NH4N03, Ca(N03)2 and NaN03; but more hygroscopic than most other N fertilizers. Thus the material is likely to cake or deliquesce if stored in humid conditions. This is especially true if urea is mixed with other N. It is apparent that crystalline urea is more useful in making urea solutions, where granular or prilled urea, which is less hygroscopic, is of greater use as a solid fertilizer. This decrease in the hygroscopic behavior of urea may be due to a clay film that is generally present in the prilled or granulated products. Urea has numerous advantages for use as a N fertilizer. It has the highest percent N of any solid material and is easy and safe to handle and store. The high solubility of urea also make it ideal for solution application, alone or in combinations with other fertilizers or pesticides. There are also several disadvantages of application of urea as a fertilizer. Biuret [NHz-CO-NH-CO-NHZJ , may be formed in the synthesis or subsequent processing of urea. If present in sufficient quantities, the compound may exhibit toxic effects to crops. Free NH3, formed from the decomposition of urea, may damage the germination and early growth of crops. Finally, NH3 may be lost to the atmosphere upon the decomposition of surface applied fertilizer. This process is known as NH3 volatilization (Tisdale, Nelson and Beaton, 1985). Fertilizer manufacturers have developed many new N materials in an effort to decrease volatile losses of N from urea or urea-based fertilizers. Most try to combine the benefit of the high N analysis of urea with some technique to control the hydrolysis of urea or subsequent volatilization of NH3. Techniques to minimize losses include (1) reduction of pH (2) coating of urea granule surfaces, or (3) reduction of NH3-loss through the precipitation of Ca and Mg carbonates (Terman, 1979). Urea forms addition products with many acids and salts. Examples would include urea nitrate (UN) [C0(NH2)2' HN03] and urea phosphate (UP) [C0(NH2)2' H3PO4]. Both of these materials possess a low solution pH and. theoretically' the reduction. of pH 'would decrease volatile loss of NH3. Gasser and Penny (1967) confirmed this theory and found UN and UP both to be efficient fertilizers. However, UN was found to result in injury to plant growth, where UP did not result in injury. Brenner and Douglas (1971) investigated UP further and found that it also reduced NH3 loss. The phosphoric acid formed in the decomposition of UP effectively retarded urease activity, thus reducing losses. Tennesse Valley Authority (TVA) has conducted considerable research using urea phosphates for N and P sources. Recently, they released several new experimental urea phosphate materials. These materials are generally cogranulated products of urea and urea phosphate. They are formed by adding supplemental urea during the production of urea phosphate. Urea-urea phosphate (UUP), has a flexible nutrient content and N:P205 ratios of 1-1- 0, 2-1-0 or 3-1-0 are possible. UUP represents an attempt at combining the high N analysis of urea with a compound that reduces NH3 loss. Other research has been conducted to further investigate the agronomic effectiveness of this material (Yerokun, 1984). Another property of urea fertilizers that may affect NH3 volatilization losses is granule size. Watkins et al. (1972) reported that varying urea granule size from 0.1 cm to 0.5 cm had no affect on NH3 losses. Nommik (l973a,b) found that the 1NH3 volatilization rate was retarded when the urea granule was increased from 0.003 g to 2.06 g. Mahli and Nyborg (1979) observed a significant decrease in the rate of urea hydrolysis by increasing urea granule size from 0.01 g to 2.26 g. They also noted that losses were even further suppressed by the inclusion of an urease inhibitor. Nommik (1973a,b) offered the following explanation for the inhibition of NH3 volatilization by increased urea granule size. He indicated that the rate of solute diffusion would be increased by increasing granule size . This was due to an increase in the concentration gradient. The urea or NH4 would thus diffuse to a greater depth in the soil and would be protected against further volatilization losses. Utilizing large granules would also decrease the rate of urea hydrolysis This is due to an uneven distribution of large granule urea, coupled with a limited supply of urease present directly on the soil surface. He also noted that further decreases in loss could be achieved if an urease inhibitor was added along with large granule urea. CHEMISTRY OF NH3 LOSS: NON-CALCAREOUS SOILS When urea is applied to the soil surface, it is quickly hydrolyzed to ‘(NH4)2C03. This reaction is catalyzed by the enzyme urease. Urease is found universally in soils. Bacteria, fungi and actinomycetes all produce ‘urease and. thus decompose. urea (Tisdale, Nelson and Beaton, 1985) . Certain soil properties will encourage an increase in the level of the enzyme. Increases in the soil microbe population will usually be associated with increased urease activity. Other properties related to heightened urease activity would include an increase in; organic carbon, soil nitrogen, clay content and CEC (Zantua et al., 1977; Dalal, 19753McGarity and Meyers, 1967 and Reynolds, Wolf and Armbruster, 1985). Ammonium carbonate is unstable in the soil and rapidly decomposes to NH3 and C02 gas. The following equations illustrate this process: CO(NH2)2 ‘1' H20 ------- > (NH4)2C03 (3) (NH4)2CO3 +H20 ------- > 2 3 '1’ C02 +1220 (4) 2 4011 10 Upon the decomposition of (NH4 )2 CO , the following equilibrium situation may govern ghe NH3-N loss: NH4+<---->NH3 (aq) + H+ (5) In non-calcareous soils, NH3 volatilization occurs as the result of the dissociation of NH4+ to “33(aq) and H+.The following relationships describe this phenomenon. [NH3(a ) [H - Rd - 10'9-5 (6) [“34 1 log NH3(a - -9.5 + pH (7) [“34 1 “3(aq) is the concentration of NH3 in the soil solution. This concentration will increase with increasing pH. For example, at pH values of 5, 7, and 9 the concentration of NH3(aq) will be 0.0032, 0.32 and 32 t of the total ammonical N in the soil solution (Nelson, 1982). The amount of NH3 lost from the soil solution will be a function of the partial pressure of NH3 over the solution. Henry's law states that the amount of a gas dissolved in a liquid at constant temperature is directly proportional to the partial pressure of the gas above the solution. Thus, at equilibrium: [N33(aq)] "' KHPNH3 (3) If the concentration of NH3(aq) is changed, by addition of NH4+ or by increasing the pH, the equilibrium between NH3(aq) and PNHB will be changed. The net result 11 of this change in equilibrium will be a loss of NH3 to the atmosphere. NH3044) """" > NH3(air) (9) This NH3 loss, or volatilization, will continue until limited by NH4+ concentration or by pH. As previously stated, NH4+ upon dissociation will liberate a H+ ion. Thus in an unbuffered system, NH3 volatilization would lead to an acidification of the medium and cease after a short time. Avnimelech and Laher (1977) found that the buffering capacity of the system against an increase in pH is extremely important in determining the quantity of NH3 volatilized. If the system is well buffered, volatilization may proceed for long periods even though large quantities of H+ ion are released. This conclusion was verified by Vlek and Stumps (1978) , who found that in an aqueous system the buffering capacity frequently determined the amount of NH3 lost. Ferguson et al. (1984) stated "the potential for NH3 volatilization is decreased with increased H1" ion buffering capacity.” These findings imply that the amount of NH3 volatilized by a soil may also depend on that soil's ability to supply H+ ions and thus resist an increase in pH. CBEMISTRY OF NH3 LOSS: CALCAREOUS SOILS Jewitt (1942) postulated that the loss of ammonia in alkaline soils occurred by the following mechanism: 12 NH4+ + OH- < ------ > NH3 + H20 (10) At high pH values, the activity of the OH“ ion would be increased, thus driving the reaction to the right and promoting increased loss of NH3. Increasing the concentration of NH? would have the same effect. This equilibrium was also suggested by several later investigators (Duplessis and Kroontje, 1964: Wahhab et al., 1957 and Ernst and Massey, 1960). Ernst and Massey (1960) were among the first to suggest that calcium compounds play an important role in NH3 volatilization. They suggested that an increase in the degree of Ca-saturation of the soil exchange complex would occur with an increase in pH. This would lead to a decreased adsorption of NH4+ and increase the probability of volatilization losses. Larsen and Gunary (1962) investigated several ammonium fertilizers in an attempt to identify the mechanism of NH3 volatilization. They postulated that the different rates of NH3-loss could be explained by examining the solubilities of the reaction products of different NH4+-N sources with Ca compounds in the soil. They investigated NH4NO3, (NH4)ZSO4 and several ammonium phosphates. Although they did not achieve conclusive results, it was apparent from their work that several factors other than pH were effecting NH3 volatilization. 13 Foremost of these was the formation of low solubility Ca salts when NH4+-N sources reacted with CaCO3 in the soil. Terman and Hunt (1964) conducted a more complete investigation of this phenomenon. They concluded that NH3-volatilization losses increased with decreasing solubility of reaction products. They suggested that the following reactions occur in calcareous soils: AS AN As previously discussed, the (NH4)2C03 formed is unstable and will decompose yielding NH3 and C02 gas. Both DAP and AS react with CaC03 to form Ca-salts of low solubility. AN forms a soluble compound. If the reaction products are of low solubility , a more complete formation of (NH4)2CO3 will occur. This will allow greater volatilization of NHL Conversely, if the reaction products are soluble and stable, less (NH4)2CO3 will form and losses will be decreased. It should be noted that the hydrolysis of urea is not dependant on reaction with calcium compounds. It only requires H20 and urease. Penn and Kissel (1973) described a series of reactions which they suggested represented the mechanism 14 of NH3 volatilization in calcareous soils. When NH4 compounds are applied to the soil surface they react with CaCO3 to form Ca-salts of varying solubilities: x(NH4)zY + nCaCO3 < ------- > n(NH4)2CO3 + Can!x (14) (where Y represents the anion associated with the NH4+ cation) (NH4)2C03 is unstable and decomposes as follows: (NH4)2CO3 + H20 < -------- > 211113 + H20 + co2 (15) 2NH4OH If Can!x is insoluble, more (NH4)2C03 will be formed. If Can!x is soluble and does not form a precipitate (NH4)2CO3 formation will be minimized. The anions investigated that form insoluble precipitates with calcium.*were: Ca (F',SO42',HPO42' ). The other salts investigated. formed soluble reaction. products, Ca (NO ,Cl,I)and gave low NH losses. 3 3 As (NH4)2CO3 decomposes according to this reaction C02 is lost from solution at a greater rate than NH3. This allows for the production of additional OH’ and leads to greater NH3 loss by the following reaction: NH4+ + on‘ < ------ > NH4OH < ------ > NH3 + H20 (15) This equilibrium is pH dependant with lower pH's favoring the NH4+ forms. 15 Although this mechanism described loss of NH3 for NH4+-N compounds, it offered little insight as to how the loss of NH3 from urea might be controlled. This problem was addressed by Penn et a1. (1979) . Their technique involved the application of Ca and Mg nitrates or chlorides with surface applied solutions of urea. They discovered that the N113 was dramatically reduced when this procedure was used. They described the process with the following equation: (NH4)2CO3 + Ca(NO3)2 [or Mg(NO3)2] ----- > oaco3 (or MgCOa) + 2NH4NO3 (17) As the soil pH increases from the hydrolysis of urea to AC, CaCO3 and MgCO3 were precipitated. This reduced the AC concentration and decreased NH3 loss. Tillage System Many reduced tillage practices have significant effects on the soil environment. Changes can occur in soil physical, chemical and. biological properties (Baeumer and Bakermans, 1973: Griffith et al., 1977 and Phillips et al., 1980 ). These changes have necessitated a re-evaluation of many agricultural practices. For generations, agriculture has relied on the moldboard plow as the principle form of primary tillage. Prior to its adoption, economic control of weeds was not possible. It was not until the development of plant growth regulators 16 in the 1940's that reduced tillage systems became feasible ( Phillips et al., 1980 ). Herbicides allowed the farmer to reap the benefits of reduced tillage agriculture, while avoiding the problems caused by weed competition. In 1985, 31 % of the cropland in the United States was planted in some form of conservation tillage system (Cons. Till. Inf. Cen., 1986 ). 'This represents an increase of almost 2 2 over 1984 and from all indications this trend will continue. Because of this increase in popularity, an increased percentage of agricultural research has addressed problems associated with reduced tillage systems. Reduced tillage systems leave a layer of crop residue on the soil surface. This may lead to an increase in soil moisture content and a decrease in soil temperature ( Moody et al., 1963: Thomas et al., 1973: Van Doren and Triplett, 1973: Van Wijk et al., 1959). The increase in soil moisture is most likely due to reduced evaporation and increased infiltration of rainfall. The decrease in temperature is related. to the insulating effect of the residue and a change in surface albedo, which reflect radiation rather than adsorb it (Thomas and Frye, 1984) as well as the increased moisture content. 17 A. reduced tillage system is generally’ a cooler, wetter environment. Nitrogen mineralization will tend to be slower in these systems. This is because the soil is undisturbed and the organic residues are left on the surface where decomposition is slower. Denitrification is also a greater concern in reduced tillage systems. Rice and Smith (1982) found that the rate of denitrification is directly related to the soil moisture content. Reduced tillage systems should therefore, exhibit a greater rate of denitrification because they possess a higher soil :moisture content. Nitrate leaching losses tend to be higher in these systems. This is due to several factors. Evaporation is negligible in tillage systems that leave significant quantities of residue on the surface. Therefore, the upward movement of water and salts is practically stopped. This is coupled with decreased rainfall infiltration. The result is downward movement of rain and N03. Also, there exists a greater percentage of large pores in an undisturbed soil. The main flow of water will be through these large pores and not through the bulk of the soil. This will yield a deeper penetration of both water and N03 (Thomas et al., 1973 and Thomas and Frye, 1984) Finally, NH3 volatilization losses can be very high in reduced tillage systems (Bandel et al., 1980; Fox and 18 Hoffman, 1981 and Mengel et al., 1982). This is especially true if urea or other NH4+-N sources are surface broadcast. Touchton and Hargrove (1982) reported that both yield and N uptake were significantly reduced when a UAN solution was applied as a surface broadcast application. This treatment was notedly less efficient than surface broadcast solid urea. Thomas and Frye (1984) explained the increased loss of NH3 from UAN solutions by noting that contact with plant residues (and urease) is more likely in this form. Torello and Wehner (1983) found that urease activity in bluegrass clippings was 18 to 25 times greater than in the underlying soil. Klein and Koths (1980) showed that the urease activity was significantly higher in a no-till corn production system as compared to a conventionally tilled system. It is thus apparent that considerable caution must be exercised when applying urea as a surface broadcast application in a reduced tillage system. Several researchers have tried incorporating urea fertilizers into the soil ' (Touchton and Hargrove, 1982: Mengel et al., 1982 and Grove et al., 1983). Itwas postulated that this practice would decrease N113 volatilization losses. Ernst and Massey (1960), in a laboratory experiment, found a decrease in NH3 loss with an increase in depth of incorporation. Preliminary results indicate that an increase in efficiency is achieved by incorporating urea fertilizers. Reductions in 19 nitrogen immobilization may also be observed when nitrogen materials are incorporated and placed away from the concentration of residues at the surface (Mengel et al., 1982). Finally, incorporation of fertilizers will allow for a more rapid and complete dissolution of the material. A surface residue may impede this process and thus increase NH3 loss. FACTORS AFFECTING UREA HYDROLYSIS Temperature It is generally accepted that urease activity increases with increasing temperature (Conrad, 1940: Fisher and Parks, 1958: Broadbent et al., 1958 and Simpson and Melsted, 1963).More recently several investigators have reported that an optimum temperature range exists. Further increases in temperature above this level will result in deactivation of the enzyme and thus a decrease in activity. The temperature range generally accepted as an optimum is 10 to 70 C (Bremner and Mulvaney, 1978: Petit et al., 1976: Chin and Kroontje, 1963 and Gould et al.,1973). Several investigators have studied the persistence of urease in soils at subzero temperatures. It was found that temperatures of -10 to -33 C did not destroy soil urease (Bremner and Zantua, 1975: Speir and Ross, 1975 and Zantua and Bremner, 1975). Furthermore, it was 20 reported that urease activity was detected in soils at - 10 to -20 C (Bremner and Zantua, 1975). This activity was postulated to have resulted from enzyme-substrate interactions in unfrozen water at the surfaces of soil particles. This conclusion was evidenced by experiments showing that urea hydrolysis did not occur in autoclaved soils or in the absence of clay minerals. Moisture Several investigators have suggested that the soil moisture content had no effect on soil urease activity (Gould et al., 1973 and Zantua and Bremner, 1979). These reports are contrasted by other workers who have found that urease activity is increased (Stojanovic, 1959 and Vasilenko, 1962) or decreased (Simpson and Melsted, 1963) by increasing soil moisture content. It is likely that the effects of soil moisture on urease activity are generally small, as compared to soil temperature. Hydrolysis rates are probably highest at soil moisture contents in the readily available range to plants. Substrate Concentration Many researchers have reported increases in urease activity with an increase in substrate (urea) concentration (Fisher and Parks, 1958: Simpson and Melsted, 1963: Overrein and Moe, 1967 and Gould et al., 1973) . Other investigators have observed no response in 21 urease activity to urea applications (Zantua and Bremner, 1976 and Lloyd and Sheaffe, 1973). An explanation to this controversy was offered by Bremner and Mulvaney (1978) . The rate of hydrolysis by urease increases with increasing urea concentration until the enzyme becomes saturated with the substrate. Further additions of urea over this level of saturation will give no increase in activity. Earlier work reporting an increase in urease activity with an increase in urea concentration was conducted in systems in which the substrate was limiting. More recent experiments have demonstrated that if urea is not limiting, an increase in activity will not be observed. SOIL REACTION Several investigators have attempted to determine the effect that pH has on urease activity (Vasilenko, 1962: Simpson and Melsted, 1963 and Petit et al., 1976). Much of this work utilized different buffer solutions and urea concentrations so comparisons between studies are difficult to make. Also, results are variable. Some workers have reported an optimum pH range of 6.5 to 7.0 (Vasilenko, 1962 and Petit et al., 1976) while others have reported a range of 8.8 to 9.0 (Tabatabai and Bremner, 1972). 22 Organic Carbon, % Clay, CEC Numerous researchers have determined that soil urease activity is highly correlated with organic carbon in soils (Gibson, 1930: Conrad, 1940: Chin and Kroontje, 1963: McGarity and Meyers, 1967: Moe, 1967: Lloyd and Sheaffe, 1973: Zantua and Bremner, 1976: Zantua et al., 1977 and Reynolds et al., 1985). These studies also found that additions of organic compounds to a soil may temporarily lead to an increase in urease activity. The increased levels may persist for several weeks, but eventually they will become identical to that of unamended soil. These findings suggest that the soil constituents protect urease against microbial degradation and inactivation. Each soil seems to have a stable level of urease activity, depending on its soil constituents (Zantua and Bremner, 1976). Paulson and Kurtz (1969) attempted to determine the locus of urease in soils. They were curious as to the percentage of activity that arose from urease directly associated with microorganisms and that from urease adsorbed to soil colloids. They determined that 79-89 % of the urease activity was due to urease adsorbed on soil colloids. Burns et al. (1972) obtained similar results and postulated that most urease activity was associated with the organo-mineral complex. This would give urease a degree of resistance against the activities of proteolytic enzymes and help to account for the persistence and stability of urease in the soil. A 23 carbon addition would yield a temporary burst in activity, but when the source was depleted the excess, free urease would be degraded or inactivated. The activity would then return to its stable level. Urease activity is also highly correlated with clay content and soil CEC (Zantua et al., 1977 and Reynolds et al., 1985). These findings would be explained by the aforementioned mechanisms. A high soil CEO or clay content would also be reflected by an increase in the quantity of urease adsorbed by the soil colloids. Boyd and Mortland (l985a,b, 1986) found that urease or other enzymes adsorbed by hydrophobic bonding to smectite-organic complexes exhibited varying degrees of activity depending on the nature of the adsorbed organic species and the nature of the enzyme. The immobilized enzymes tended to exhibit a decreased thermal stability and were more susceptible to degradation by proteolytic enzymes. This was explained by noting that both urease and protease bind to the surface of the clay. The close proximity of these enzymes would increase proteolysis over the level observed in a homogeneous solution. It was also stated that this mechanism may play a minor role in soils due to the scarcity of hydrophobic bonding sites. 24 Cultivation Gibson (1930) was perhaps the first to demonstrate that cultural practices may have significant effects on soil urease activity. In a survey of soils of Scotland he reported greater activity in soils from pastures than in cultivated land. He also noted that forest soils were generally' more active than cultivated soils. He attributed this increase in activity to the greater content of organic matter present in this soils. Speir et al. (1980) compared. urease in pots activity' under a perennial ryegrass sod and in pots left fallow. Activity was initially similar in both treatments. However, the activity in the fallow pots was found to decrease steadily. This was attributed to a decrease in the microbial population and .a temperature dependent denaturation of the enzyme. Reynolds et al. (1985) reported higher urease activities in pasture samples as compared to cultivated samples of the same soil type. They noted that pasture samples generally possessed higher organic carbon, CEC and microbial numbers. This may have accounted for the increased activity. Klein and Koths (1980) found greater activity in no-tillage corn plots as compared to conventional tillage plots. It is possible that the increased surface residue present in no-tillage plots could have accounted for this increase in activity. 25 FACTORS AFFECTING AMMONIA VOLATILIZATION Temperature Most investigators have reported increased loss of NH3 ‘with increasing ‘temperatures (Martin. and Chapman, 1951: Volk, 1959: Ernst and Massey, 1960: Wahhab et al., 1960: Watkins et al., 1972 and Harper et al., 1983). This increase in NH3 loss could result from an increase in evaporation. This would occur as the temperature was increased and could have served as a driving force to NH3 volatilization. Also, an increase in the equilibrium constant (K in E0 6) would occur with increasing temperature. This results in a higher proportion of the ammonical N present as NH3(aq). Increases in temperature would also increase diffusion of NH3 from the soil and allow a more rapid conversion of NH3(aq) to NH3(air),thus increasing the volatilization potential (Vlek and Stumpe, 1978 and Nelson, 1982). Meyer et al. (1961) reported a decrease in NH3 loss with increasing temperature. They explained this by noting the decrease in nitrification that would occur at lower temperatures.They also pointed out that urease is active at low temperatures. This would lead to a rapid increase in the soil NH4 concentration. The soil exchange complex could not adsorb this surge of additional NH4 and it 'would be available for loss. Gasser (1964) in a similar experiment, reported a temperature response that 26 was closely associated with soil CEC. If the soil CEC was low results similar to Meyer et a1. would occur. This was because the NH4 present was in excess of the soil's ability to adsorb it. However, if the soil CEC was high, the NH4 could be adsorbed and the increased loss at low temperature might not be observed. Fenn and Kissel (1974) conducted a thorough investigation on the effect of temperature on NH3 loss from applied NH4+-N fertilizers. They concluded that the effect of temperature on NH3 loss depends on the presence of CaC03 in the soil and on the type of NH4+-N compound. If the compound reacts with CaCO3 to form an insoluble precipitate, the effect of temperature was slight. They observed 'that. a. high. temperature ‘would yield. a high initial loss, but would decrease with time. A low initial temperature would give a low initial loss, but increase with time. If the NH4+-N compound formed a soluble reaction product with CaCO3, the temperature response was notedly different. No response was observed to N114+ rates , but an increase in total loss and the rate of NH3 loss was reported with an increase in temperature. Soil Moisture Content Many researchers report that NH3 loss will increase with increasing soil moisture content up to field 27 capacity (Volk, 1959: Ernst and Massey, 1960: Wahhab et al.,l960: Kresge and Satchell, 1960: Baligar and Patil, 1968 and Harper et al., 1983). This will allow for maximum rates of hydrolysis and still permit rapid soil drying. A low soil moisture content will cause a decrease in hydrolysis and allow less drying to occur. Other researchers have suggested that NH3-N losses decrease with increasing soil moisture content (Martin and Chapman, 1951: Wahhab et al., 1957: Fenn and Escarzaga, 1976 and Yerokun, 1984). It is possible that when the soil moisture content is increased over the level needed to solubilize added fertilizer, an inhibition of (NH4)2CO3 formation occurs. It should be noted that the majority of this research was conducted at soil moisture levels at or approaching field capacity. This work was also conducted in closed dynamic systems in which relative humidity could have effected the results. Ernst and Massey (1960) reported a decrease in NH3 loss with increases in relative humidity. Hargrove et al. (1977) in a field experiment, observed a diurnal fluctuation of NH3 loss. They explained this behavior by noting a similar diurnal fluctuation in atmospheric relative humidity. The major effect of an increase in relative humidity would be a decrease in the rate of soil drying. However, Brown and Bartholomew (1963) reported that there existed a competition between NH3 and aqueous vapor for sorption 28 sites on clays. All moist clays tended to adsorb less NH3 than dry clays in the range of NH3 pressures from 1 to 60 mm Hg. An increase in adsorption did occur with increases in pNH3 greater than 60 mm Hg. Jewitt ( 1942) suggested a mechanism for NH3 loss that was closely associated with the concentration of NH4 salt in the soil and the loss of moisture from the soil. The initial soil moisture content had a relatively small effect. The following equilibrium was considered: + - NH4 '1' on < --------- > NH3 + H20 (20) In this situation, both NH3 and H20 would have their own. partial pressures and ‘would evaporate at a rate proportional to their respective molar concentrations. The soil is buffered somewhat with respect to OH' concentration. The soil solution is also buffered for changes in NH4+ concentration. This is due to the reversible reaction of free NH3 with the soil exchange complex. The result is that the equilibrium between NH3, NH4+ and OH' is maintained as a constant. Thus the ratio of NH3 loss to H20 loss should be a constant. However, the ratio falls off slowly indicating a depletion of NH4 ion from the soil exchange complex. Wahhab et al. (1957) reported similar results a constant ratio of NH3 loss to H20 loss, limited by the NH4 ion in the exchange complex. 29 Chao and Kroontje (1964) conducted a more complete investigation of this relationship. They discovered that NH3 loss and H20 loss are not linearly related. They occur at different rates and may be described by different functions. The rate of NH3 loss decreased with time, while the rate of H20 loss remained constant to near air-dry conditions. Several other researchers have also looked at the relationship between moisture loss and NH3 loss (Martin and Chapman, 1951:Kresge and Satchell, 1960: Lauer et al., 1976 and Ferguson and Kissel, 1986). In most of these experiments evaporation was thought to act as the driving force behind NH3 volatilization. Experiments utilizing urea have revealed that the rate of drying frequently determines the quantity of NH3 lost. The rates of loss are generally not related directly and are not constant. If conditions of rapid drying exist, NH3 loss may be at a minimum. This is because the soil may have approached air-dry before the hydrolysis of urea was complete. Maximum losses have occurred under conditions of gradual drying, in which sufficient moisture is present to allow complete hydrolysis of urea (Ernst and Massey, 1960: Volk, 1959: Fenn and Escarzaga, 1976 and McInnes et al., 1985). Several workers have showed a decreased loss of NH3 if sufficient rainfall or irrigation is present to move 30 the fertilizer into the soil (Meyer et al., 1961: Oberle and Bundy, 1984 and Bouwmeester et al., 1985 ). Fox and Hoffman (1981) have suggested the following guidelines for application of unincorporated urea in no-till corn: (1) If at least 10 mm of rain falls within 2 days of application of urea, no NH3 loss will occur . (2) If 10 mm or more rain falls 3 days after the urea is applied, NH3 losses will be slight ( < 10 8 ). (3) If 3-5 mm of rain falls within 5 days or 7- 9 mm within 9 days NH3 will be moderate ( 10 to 30%). (4) If no rain falls within 6 days, the NH3 loss can be substantial ( > 30 % ). Increased H20 applications reduce NH3 loss due to the fact that undissociated urea moves into the soil. Fenn and Miyamoto (1979) have demonstrated this effect. They suggest that urea moves slightly behind the wetting front. This explains the high NH3 loss normally observed in conditions of high evaporation. If the wetting front moves the urea below the depth of possible capillary movement losses will be greatly reduced. 31 Nitrogen Rate Many researchers have investigated the effect of an increased application rate of NH4+-N or urea fertilizers on NH3 volatilization losses. Most researchers have found that the total quantity of NH3 volatilized increases with increasing application rate. This would be predicted by Eq's [3] and [4]. An increase in NH4+ would shift the equilibrium to the right and a greater quantity of NH3 would be available to be volatilized. The principle point of disagreement in this area of research concerns the rate of NH3 volatilization. Some workers have found that the rate of NH3 loss increases with increasing rate of application (Wahhab et al., 1957 and Baligar and Patil, 1968b) . Other investigators have reported that the rate of NH3 loss is constant (Jewitt, 1942: Martin and Chapman, 1951: DuPlessis and Kroontje, 1964 and Hargrove et al., 1977 ). Fenn and Kissel (1976) found that the rate of NH3 loss may also depend on the NH4+-N source. Using (NH4)ZSO4, they observed an increase in percentage loss of NH3 with increasing application rate. With NH4N03 they reported a decrease in percentage loss. They attributed the decrease in percentage loss with NH4NO3 to a decrease in pH. They noted that (NH4)ZSO4 should produce a greater decrease in pH, but this was not the case in a calcareous soil. The percentage loss with (NH4)2S04 was also found to rapidly 32 decrease with decreasing application rate. This could be explained by a greater degree of adsorption of NH4+ on the soil exchange complex. Method of Application Surface application of urea or other NH4+-N fertilizers results in an increased NH3 volatilization potential (Ernst and Massey, 1960: Meyer et al., 1961 and Gasser, 1964). This loss potential is generally increased if the soil surface is moist (Fenn and Kissel, 1976 and Fenn and Escarzaga, 1976). With urea this increased loss is due primarily to an increase in urease activity in moist soils. The applied urea fertilizer will be rapidly hydrolyzed and there is a greater likelihood that the material will volatilize before it can be moved into the soil by rainfall. With other NH4+-N sources, the primary problem is a more rapid dissolution of the fertilizer when applied to a moist soil (Fenn and Escarzaga, 1976). This will allow for a greater period of time in which volatilization can occur. One strategy used to decrease these losses is incorporating the fertilizer into the soil. Several researchers have demonstrated that this practice can significantly reduce volatilization losses . Jackson and Burton (1962) reported that urea and NH4N03 resulted in equal yields when incorporated 15 cm below the soil surface. Steinberg (1944) reported that the NH3 loss from 33 (NH4)ZSO4 could be reduced to 0 when the fertilizer was covered with 6 cm of soil. Ernst and Massey (1960) applied urea to the soil surface and then covered it with soil layers of: 0.6, 1.3 and 3.8 cm. Although the differences among treatments were not large, there was a trend of decreasing NH3 loss with increasing depth of incorporation. Fenn and Kissel (1976) observed that a dry soil cover was more effective than a moist soil cover. They postulated that a dry soil cover would stop evaporation of water from the soil. This would account for the decreased losses of NH4+-N fertilizers that they observed. A dry soil cover would also decrease the rate of urease activity, this would account for the decreased losses of urea and urea-based fertilizers. Another technique that is frequently used in an attempt to decrease volatilization losses is timing of fertilizer application. Mills et al. (1974) reported that rapidly growing corn seedlings were effective in diminishing NH3 volatilization losses. They attributed these reductions to a rapid crop uptake of NH4+. Also, they noted that acidification of the soil, especially in the rhizosphere, after NH4+ adsorption may aid in its retention by the soil. It is important that the application rate not exceed the needs of the plant. In this case plants may exhibit NH3 toxicity symptoms. Meyer et a1. (1961) also observed an increase in N use efficiency when they applied urea as a summer sidedress. 34 They cited a more efficient uptake by an actively growing crop as the reason for decreased NH3 losses. Rate of Air-Flow 8 Atmospheric NH3 Several investigators have reported an increase in NH3 losses with an increase in the rate of air-flow over the soil. In a field experiment this would. mean an increase in wind speed. In a laboratory experiment this would be an increase in the rate air exchange (Chao and Kroontje, 1964: Overrein and Moe, 1967: Watkins et al., 1972: Kissel et al., 1977 and Lauer et al, 1977 ).The rate of NH3 loss increases until an air-exchange rate of 15-20 volumes per minute is achieved (Kissel et al., 1977). This rate of air-exchange is equivalent to a wind velocity of approximately 0.26 km/hr. Wind velocities of this magnitude are almost always present at the soil surface. However, in a crop canopy this velocity may be significantly reduced. Many laboratory experiments have utilized an exchange rate of 10 volumes per minute or less. In this range, NH3 loss increases linearly with increases in the air-exchange rate. Not only are these experiments under-estimating the NH3 loss, they are also increasing variability. Fenn and Kissel (1973) reduced loss variability to 10 % or less, with an air-exchange rate of 14-16 volumes per minute. A rapid flow of air across the soil surface will decrease the partial pressure of NH3 in the immediate 35 vicinity. This will permit rapid diffusion of NH3 from the soil, due to a large partial pressure gradient (Nelson, 1982). Avnimelech and Laher (1977) stressed the importance of this factor and stated that: " One can easily demonstrate that if no NH3 is present in the air, all the NH3 in the soil will eventually disappear.” They also commented that this factor is highly variable and is affected by industrial and urban fumes, as well as by the neighboring fertilized fields. A number of researchers have investigated atmospheric NH3 concentrations (Leubs et al., 1974: Denmead et al., 1974: Denmead et al., 1976 and Denmead et a1, 1978). Another consequence of rapid air flow across the soil surface is an increase in the soil moisture loss. This is especially true if the air is of low relative humidity. As previously stated, the rate of soil drying can have significant effects on the quantity of NH3 lost from a soil. Texture, Organic Matter & CEC Several researchers have demonstrated that soil texture may be an important factor in determining the quantity of NH3 lost from a soil. Wahhab et al. (1957) observed significantly higher losses in a sandy soil, as compared to a sandy-loam soil. This trend was also demonstrated by Chao and Kroontje (1964) . Greater losses were recorded on a coarse textured soil. Ryan and Keeney 36 (1975) reported similar results with surface applications of a wastewater sludge. A decrease in NH3 loss was observed with an increase in clay content of the soil. It is generally accepted that soils covered with surface residues or a grass sod will lose more N due to NH3 volatilization than bare soils. Volk (1959, 1961) reported a NH3 loss of 25 % for a bare soil, 29 8 for an unlimed turf and 39 % for a limed turf. Meyer et al. (1961) observed increased NH3 loss when a straw residue covered the soil surface. A similar increase in volatilization losses was also observed in forest soils. N113 loss ranged from 6-30 2 when urea was applied to a bare soil and from 27-46 8 when the surface was covered with a forest litter (Watkins et al., 1972). Rashid (1977) showed an increase in NH3 volatilization losses when organic residues having a high C/N ratio were present. He explained these losses by noting' that a decrease in nitrification would occur at high C/N ratios. This would leave a larger concentration of NH4+-N to be potentially volatilized. Immobilization of nitrogen was not considered. Increased NH3 losses due to surface residues are generally explained by one of two mechanisms. Urease activity is consistently higher in organic residues. Torello and Wehner (1983) observed urease activity 18 to 25 times higher in turfgrass clippings as compared to the 37 underlying soil. Increased urease activity would lead to a greater degree of urea hydrolysis and possibly, increased volatilization losses. Also, residues possess a limited CEC. This would permit them to retain less NH4+ on exchange sites and more NH3 could be lost. The greater volatilization potential of coarse textured soils could also be explained by a limited CEC. A number of researchers have investigated the effect that the soil exchange complex can have on NH3 volatilization. Conrad and Adams (1940) reported that the soil adsorbed both urea and the NH4+ produced in the hydrolysis of urea. Urea was generally retained to a lesser extent that the NH4+. The positively charged NH4+ ion would be attracted to the net negative charge of the clay lattice. Urea is an uncharged molecule and would thus move with the soil water. Adsorption of NH4+ would decrease volatilization losses. Several other workers have also arrived at this conclusion. An increase in CEC should result in a decrease in NH3 losses (Martin and Chapman, 1951: Ernst and Massey, 1960 and Baligar and Patil, 1968). Gasser (1964) indicated that a soil's base exchange capacity (BEC) was the most important factor in determining the quantity of NH3 lost from applied (NH4)ZSO4. Soils with a BBC of less than 10 me/100 g were found to volatilize 20 2 or more of the added fertilizer. Losses decreased to less than 10 % when the BEC increased to 20 me/100 g. Fenn and Kissel (1976) confirmed earlier 38 opinion by finding that, in general, NH3 loss decreased as CEC increased. They also noticed that the percentage decrease in NH3 loss,from an increase in CEC, was greater for an application rate of 110 kg/ha than at 550 kg/ha. They' explained this occurrence. by’ postulating' that a greater percentage of the NH4+ would be adsorbed at a lower application rate. MATERIALS AND METHODS The objectives of this research were discussed in the previous section. This section will describe the experimental procedures. Field experiments were conducted in 1984 and 1985. They were conducted at two different locations to evaluate the effect of soil texture on relative nitrogen-use efficiency. One site was located at the Saginaw Valley Bean and Sugar Beet Research Farm. The soil type was a Charity clay. The other site was located in East Lansing on a Conover loam. At both locations a conventional tillage system ‘was compared. ‘with a conservation tillage system. Fertilizer performance was also evaluated as determined by yield, N uptake and % N content at silking. A greenhouse experiment was conducted in the winter of 1985 to determine crop response under controlled conditions. In the summer of 1985 a field experiment was initiated to determine quantitatively, the amount of NH3 lost by volatilization. This experiment was conducted at the field site in East Lansing. In 1985, the urease activity was determined at both field sites and in greenhouse soils. Factors including 39 40 tillage system, soil texture, nitrogen application rate and method of application were evaluated. GENERAL DESCRIPTION of SOILS Field experiments were conducted on both the Charity clay and the Conover loam. In the greenhouse and ammonia loss experiment only the Conover loam was utilized. The soils are described below. Charity Clay This soil is classified as Aeric, Haplaquept, fine, illitic, mesic with 80 g kg'1 sand, 280 g kg'1 silt and 640 g kg'1 clay. The cation exchange capacity was estimated to be 270 to 290 mmol (p+)'kg'1 soil (Zielke, 1983 and Yerokun, 1984). CODOVOI‘ Loam This soil is classified as Udollic, Ochraqualf, fine, loamy, mixed, mesic with 380 g kg'1 sand, 450 g kg“ 1 silt and 170 g kg"1 clay. The cation exchange capacity was estimated to be 105 mmol (p+) kg’l soil (Yerokun, 1984). 41 Soil test values are presented in Table 1. Table 1. Soil test values for field experiments. 3132521. _Exshanssabls ggj] prg pH K §§____MQ __ ------- mg kg‘1 ---- Charity 7.2 63 354 7215 1281 Conover 7.4 60 150 2122 363 Field Experiments In this study two tillage systems were established in each (location (Table 2). A conventional tillage program consisted of fall disking and fall moldboard plowing and a conservation tillage program of fall disking and fall chisel plowing in Saginaw and no-till in East lensing. In the conventional tillage programs and in the conservation tillage plots in Saginaw in 1984, the spring seedbed was prepared with a single pass of a spring and spike-tooth harrow. The remaining conservation tillage plots received no spring tillage. 42 Table 2. Tillage systems for experiments 1984 and 1985. ,______ Conventional Conservation. location—Isa: Saginaw 1984 FD-FMP-SC-P FD-FCP-SC-P 1985 " FD-FCP-P E. Lansing 1984 FD-FMP-SC-P P 1985 " P + FD-fall disk, FMP-fall moldboard plow, PCP-fall chisel plow,SC-spring cultivation, P-plant. The quantity of residue present on the soil surface was measured after spring tillage but before planting. In both years, two procedures were used. The photographic method of Hartwig and Laflen (1978) was used to obtain an estimate of percent cover. Also, a quantitative measurement of the actual amount of residue present was made using the procedure of Whitfield et al. (1962). In 1985, the line transect method of Laflen et al. (1981) was used to obtain an additional estimate of percent COVOI‘ . The nitrogen treatments consisted of three sources [urea (U), urea-urea phosphate (UUP) and ammonium nitrate (AN)] three rates (0, 134, 179 kg N ha'l), two placements (surface broadcast and banded 5 cm deep below the soil surface between rows between rows) and two application 43 dates. In Saginaw the application dates were: May 9 and June 26 in 1984 and May 11 and June 11 in 1985. In East Lansing, the dates were May 11 and June 12 in 1984 and May 2 and June 7 in 1985. Treatments were arranged in a split plot design with four replications. Tillage was the main plot and nitrogen treatment the sub-plot. In Saginaw, P fertilizer was applied at rates of 106 kg 1:205 ha'1 in 1984 and 157 kg P205 ha'1 in 1.935. In East Lansing, P and K were applied at 27 kg P205 ha"1 and 107 kg K20 ha'l, respectively. Weeds were controlled by preemergence application of 1.96 kg ha’1 Cyanazine and 2.8 kg‘ ha'1 .Alachlor. Preplant control of quackgrass (Agrgpyrgn_:§p§n§ L.) was accomplished with 1.68 kg ha'1 of Glyphosate and post emergence control of Canada thistle (W L.) was accomplished with 0.84 kg ha'l of Bentazon. Corn (Zea_may§ L.) was planted with a John Deere no-till planter, at 54,000 seeds ha'1 in 76 cm I'OWB . Ear leaf samples were collected at silking by removing ten leaves per plot. In addition, whole plant samples were collected at maturity. This was done by harvesting ten plants (less roots) per plot. The ears were removed from the stalks. Weights of stover and ears were determined. Subsamples of both grain and stover were collected for moisture determination. Nitrogen 44 determinations were made on grain and stover so that N uptake could be calculated. Yields were determined by hand harvesting two 7.01 1:: rows. All yields were corrected to 15.5 % moisture. Planting dates, harvest dates and plant tissue sampling dates are presented in Table 3. Table 3. Planting, harvesting and plant sampling dates for field experiments conducted in 1984 and 1985. Planting Harvest Ear Whole Location Year Date Date Leaf Plant Saginaw 1984 5/9 10/2 7/27 9/20 1985 5/7 10/1 8/2 9/17 E.Lansing 1984 5/11 10/5 7/15 9/13 1985 5/2 9/24 7/22 9/22 Greenhouse Experiment A greenhouse experiment was conducted in the winter of 1985 using Conover loam soil to measure crop response to N fertilizers. Soil test levels are given in Table 4. Treatment combinations included three N sources (U,UUP and AN), three N rates (0,75 and 150 mg N kg"l soil), two 45 levels of surface applied corn residue (0 and 336 g m'z) and two moisture regimes (surface moistened and surface dry) arranged in a randomized complete block with four replications. Table 4. Soil test values for greenhouse and NH3 experiment. MW Soil Type pH P K Ca Mg Zn CEC ---- kg ha’1 --- mg kg'1 Conover loam 7.2 164 314 4267 591 5 105 + cmmol (p') kg 1 soil The soil taken from the field was air dried, mixed and screened to pass a 4.75 mm sieve. One thousand eight hundred eighty g of soil was then placed in tared pots lined with plastic. Each pot received 50 mg P kg'l soil and as KHZPO4, K levels were adjusted to 100 mg H kg'l soil with KCl. In addition 20 mg Zn kg'l soil was applied before planting the second crop. The moisture content was then adjusted to 0.15 kg H20 kg'l soil and ten corn seeds were planted to a depth of 2.54 cm. These were later thinned to three plants per pot. Residue treatments were applied after planting. Fertilizer was applied 46 immediately after planting for those pots receiving the high initial moisture treatment. These pots then received three 0.025 L moisture applications as a spray treatment over the next ten days. This is equivalent to a 1.3 mm rainfall event. The surface of the pots receiving the low initial moisture treatment was permitted to dry for a period of three days before fertilizer was applied. These pots received no additional moisture for seven days. After this period 0.100 L of water was applied to the surface to leach the fertilizer into the soil. After the initial ten day period, all pots were put on a regular watering schedule and maintained at 0.15 kg H20 kg'l soil. Three consecutive crops of corn (My; var. Pioneer 3901) were grown for a period of five weeks each. Daytime temperature ranged from 20 to 29 C. Night temperature was controlled at 20 C. Supplemental irradiation of 20 umol m'2 '1 s was provided for a period of 16 hr day'l. Moisture was adjusted daily to 80 1: of field capacity (0.15 kg H20 kg‘1 soil) by gravimetric means . At the end of five weeks plants were harvested by clipping the above ground portion at the soil surface. Soil from the no residue treatments was sieved to pass a 47 4.75 mm screen, repotted and replanted. Where residue was applied seeds were "punch" planted to a depth of 2.54 cm. At the end of the experiment soil samples were taken from each pot for laboratory analysis. Field Ammonia Vo1atilization Experiments Field measurements of NH3 volatilization were made in East Lansing on a Conover loam soil. The objective of this experiment was to quantify the NH3 lost from applied urea and urea based fertilizers with a minimum disturbance to the soil field environment. This was done by constructing a vacuum driven aeration apparatus similar to that of Kissel et al. (1977). A diagram of the basic apparatus is given in (Figure 1). The experimental variables consisted of: N source (U, UUP) and N rate (0, 84, 168, 252 kg N ha'l). These treatments were replicated three times, with two samples per replication. Metal containers with a diameter of 15.5 cm were driven into the soil. When covered, the air volume enclosed above the soil surface was 1 liter. The containers were connected to boric acid (20 g L'l) traps and to the vacuum pump with tygon tubing. The air flow rate was adjusted to 0.083 L 5'1. This air exchange rate is equivalent to a wind velocity of 0.05 km h'l' The 48 oi Hobaono easemeefleuaso> O 0 I'll. , .mmas .meeaeag seem .ucoafiquXe mmoH cacoEEm ecu ca pom: uoaoamm madam cowumufiawumao> venom one .fi ouswfim oops swam owuom ofism aboom> 49 volatilization chambers were covered only when measuring NH3 loss to minimize disturbance to the soil environment. NH3 loss was measured for a period of 1800 seconds on alternate days for an eight day period. Trapped NH3 was measured by titration with standard H2804. In addition to NH3 measurements, C02 and soil and air temperature measurements were made inside the volatilization chambers. A C02 sampling port was installed in the system after the volatilization chamber and preceding the chemical trap (Figure 1). Two 0.003 L samples were taken from each chamber during NH3 measurement. These samples were stored in a cooler and analyzed the same day using an infared gas analyzer system as described by Schumacher and Smucker (1983). Soil and air temperatures were recorded during NH3 measurement and for 600 seconds prior to this time. A system of copper constantan thermocouples was installed and temperature readings were recorded once per 60 s using a computerized data collection system. Environmental data including relative humidity, air temperature and rainfall were collected every day prior to NH3 measurements. It should be noted that the chambers were covered during rainfall events, but uncovered immediately after. At the termination of the experiment, soil samples were collected from each of the chambers and saved for N03 and NH4 analysis. 50 Urea Hydrolysis Experiment In the summer of 1985 selected soil samples were collected from the field experiments in Saginaw and East Lansing. Soils were sampled to a depth of 2.54 cm and 20 probes per plot were taken. These samples were sealed in plastic bags and immediately placed in a cold room at 4 C until urea hydrolysis measurements could be performed. The urea treatments evaluated from the field experiments included: nitrogen rate (0, 134, 179 kg N’ ha '1), method of placement (surface broadcast or sidedress) and tillage system (conventional or conservation). In addition, selected urea treatments from the greenhouse were evaluated. They included: nitrogen rate (0, 75, 150 mg N kg'l soil),and residue level (0, 3.36 mg kg'1 soil). Treatments were arranged in a randomized complete block design with four replications. Before urea hydrolysis rates were determined, background. urea levels in the soil was measured. No detectable urea was found. Urea hydrolysis was determined using the buffer method of Zantua and Bremner (1975) . This consisted of incubating 5 g of soil with 0.010 L of a urea solution containing 5,000 ug urea-N. Incubations were conducted in 51 stoppered 0.125 L Erlenmyer flasks on a rotary shaker at 50 rpm. Incubations were carried out at 25' + 1 C for 5 hours. Soils were then extracted with 0.040 L of 2M 331 containing 50 mg L'1 phenylmercuric acetate at 200 rpm on a rotary shaker. After extraction, an aliquot of the extract was analyzed for urea-N by the procedure described by Douglas and Bremner (1970). Urea hydrolysis was' calculated by subtracting urea remaining from the initial urea added. Rates were reported as ug urea hydrolyzed g soil"1 h'l. Laboratory Analyses Plant Sample Treatment All. plant samples from. the field and greenhouse studies were dried at 60 C for 48 hours. Dry weights were then obtained. Samples were thereafter ground to pass a 0.425 mm sieve and total Kjeldahl N content was determined. 52 NH4 and N03 Extractions Soil samples were placed in plastic bags immediately after sampling and then stored in a cold room at 4 C until analyses could be performed. Extractions for N03- and NH4+-N were performed by using a 10:1 w/v ratio of 2M KCl. Total Nitrogen Determinations Total N on plant tissue was determined by the micro Kjeldahl 'method similar* to that. described. by Bremner (1965). Two hundred milligrams of plant tissue were digested in 0.100 L Kjeldahl flasks, using .003 L 72 M H2804 and 1.3 g catalyst mixture (100:10:1 mixture of K2804:CuSO4:Se). The samples were digested for 1.5 hours. After cooling, 0.10 L of distilled water was added to each. sample. Ammonia. was liberated. by alkaline steam distillation into boric acid and was determined by titration with standard H2504. All determinations were made in duplicate and are reported as 1: N in oven dry plant tissue. Soil Nitrate and Ammonium Determinations Nitrate and NH4 in the soil was determined by automated colorimetric procedures. Nitrate was determined by the Cd reduction method (Technicon, 1973a) and ammonium by the alkaline phenate method (Technicon, 53 1973b) . Nitrate and NH4 in the extracts was determined individually using a Technicon Autoanalyzer System II. Values obtained are reported as ppm NO3'-N and NH4I-N in air dry soil. Statistical Analyses Statistical methods described by Steel and Torrie (1980) and Little and Hill (1983) were used for pertinent statistical analyses. RESULTS AND DISCUSSION Residue Measurements After the tillage treatments were applied and prior to planting, the levels of residue present on the experimental plots were determined. This was done because the amount of residue present on the surface of the soil was thought to play a major role in the tillage fertilizer interaction. In 1984 and 1985, two different methods of estimating residue were used. The first method was a quantitative measurement in which the residue present in a 4 m2 area was collected, washed free of soil, oven dried and weighed. In addition, two estimates of percent cover were made. The photographic procedure of Hartwig and Laflen (1978) was used in both 1984 and 1985. A second estimate of percent cover was made in 1985. This was the line-intercept method of Laflen et al. (1981). These data are presented in Table 5. In both locations and years the reduced tillage systems left an appreciably greater amount of residue on the surface. The no tillage systems tended to possess a greater level of residue than the chisel plowed 54 55 Table 5. Residue cover as affected by tillage and measurement method at East Lansing and Saginaw, 1984 and 1985. _ELQW___ _Chisal_ _219x 1984 1985 1984 1985 1984 1985 1984 1985 Line transect (%) - 75 - 5 - 45 - 14 Photo- graphic (3) 84 85 45 13 63 73 21 18 Residue Dry Wt. (Mg hanl) 3.01 2.92 0.27 0.16 3.16 2.14 0.97 0.58 56 .emoH .unmaos_huv snowmen use vosuoa.ownomuw nobono emu an vocaahouoo as uo>ou accused cooauob oesocouusaom mm>oo pzmommm o.oo~ 8.5» o.¢e o.a. o.o~ b no.9 u m E) 1949 END e mm.o + xvmoé n .N ouswam I 1 1 1 0'8 1 » T 0'? 0'1 0'2 tun/ow) 1H513M 0'9 57 0.2: .nmoH .unmaos hut snowmen was cosmos canomuwouono ecu an vocHEhouoo mm uo>oo accused comaumb ofinmcowumfiom .m muswwm mm>ou ezmummm 98 o.om o4; o.o.e. 900 . _ . _ r. _ . _ I. 9 9G 09 o 3 1 .II -zm ..ui N S .ImuMmW . w av Au . .1 “O O I m l.’( e mu ao.o . xumo.o u > . a. 58 .mmmH .unwwos asp mangoes tom ponuos unouumucfi mafia osu an pocfiEhouoo mm He>ou unwound cooauob owzmcowumaom .q ouswfim mm>ou Fzmummm o.oo— ofwo 9mm 9m; 9mm 0. 0 6 96 @m9% 0 0 0 I 1m. 2 Ju r 18 .0 r 5.: u m b. 6 1mu vumé + 835 u » 1 av 0'9 (UH/0N] lHOIBM 59 treatments. The moldboard plowed treatments exhibited similar results for both years and locations. Linear regressions were performed to investigate the relationship between residue dry weight and percent cover. The correlation coefficient (r) for the photographic method was 0.85 in 1984 (Figure 2) and 0.83 in 1985 (Figure 3). For the line intercept method, the correlation coefficient was 0.87 (Figure 4). These statistics indicate that there is a good positive correlation residue dry weight and percent cover. They also indicate that there exists little difference in the methods used to determine percent cover. The final choice of a method may be based on the labor input and not on the accuracy of the determination. For this reason the line intercept may be- favored. Measurements can be collected in a short period of time and the results are immediately available. This procedure would seem to be very useful for growers and extension scientists who may require information quickly in the field. The photographic procedure is also very useful, however this method requires a longer period of time before data can be evaluated. 60 Field Studies The field experiments were conducted at Saginaw and East Lansing in 1984 and 1985. The experiments were not combined because the variances were found to be nonhomogeneous. This was determined by Bartlett's test (Little and Hills,1978) . Consequently, the experiments will be discussed separately. Significant effects due to tillage were not found and the interaction between tillage system and N treatment was also not significant. Therefore all the field. data was averaged across tillage systems. Therefore, under the conditions of this experiment tillage system. did not influence the relative N use efficiency. Several researchers have reported a decrease in both yield and N uptake with reduced tillage systems (Fox and Hoffman, 1981: Bandel et al., 1980 and Mengel et al., 1982). A similar response was not observed in these field experiments. It is possible that temperature differences may account for these discrepancies in results. Ammonia loss increases with increasing temperature (Ernst and Massey, 1960). The results which reported an increase in NH3 loss with reduced tillage systems were conducted in Pennsylvania, Maryland and Indiana, respectively. These areas may possess a higher average temperature at the time of fertilizer application. Also, the experiments were in production for 61 a longer period of time than those presented here. Both of these occurrences would help to explain the differences in results. East Lansing In 1984 and 1985 the experiment consisted of three N fertilizer sources, three N application rates, two methods of application and two times of application. In 1984, there was not a consistent response in crop yield to the N treatments (Table 6). There was an increase in yield with the early application date at the 134 kg ha'1 rate. This response could possibly be explained by climatic data. There was a rainfall of 1.42 cm two days after the early N treatment was applied. After the late application date only trace rainfall occurred in the two weeks following fertilizer application. Fox and Hoffman (1981) suggested that if 10 mm of rain falls within two days of fertilizer application, no NH3 will be lost. This amount of moisture will be sufficient to move the fertilizer into the soil where NH3 loss will be less likely. The apparent reduction in yield at the 179 kg rate is not explained. There was a trend for increasing yields with increasing N rate for all sources at the late application, but the response was not significant. Any Table 6. Effect of nitrogen source, nitrogen rate, method of application and time of application on yield of corn (averaged across two tillage systems) East Lansing, 1984 and 1985. 1984 1985 m 8133* W 8148: _Isrtilizsr_ Source Rate 5/11 6/12 6/12 5/2 6/7 6/7 kg N ha’1 ----------------- Mg ha'1 ---------------- 0 ------ 6.68 ------------ 5.95 ------ AN## 134 - 7.50 - - 10.8 - 179 - 7.74 7.07 - 10.8 10.9 U 134 8.05 7.25 7.50 10.7 10.8 10.4 179 7.81 7.79 ‘7.55 10.5 10.8 10.5 UUP 134 9.21 5.93 7.07 10.5 10.4 10.0 179 7.23 7.41 7.54 11.2 10.5 10.5 LSD (5%) 1.31 0.81 # placed75 cm below the soil surface in a band between ws. 5 AN-ammonium nitrate, U-urea, UUP-urea-urea phosphate. 63 loss of N from the broadcast application at this date was not apparent in the yields obtained since the sidedressed treatments yield the same as the broadcast treatments. In 1985, there was a significant yield response to applied N regardless of source, time or method of application (Table 6). While there tended to be an increase in yield at the 179 kg rate over the 134 kg rate, the differences were not significant. Again, there was no advantage to the sidedressed treatment. It is apparent that the residual N level in this soil prevented a yield response to applied N in 1984. In 1985, there was a response to applied N, but the 134 kg rate was sufficient. Yield levels in 1985 were sufficiently high so that a yield response at the 179 kg rate would have been expected had the residual level not produced such a high yield level. The control in both years yielded nearly twice the expected yield of 3 to 4 Mg ha'l. In 1984 the ear leaf N concentrations reflect the lack of yield response in that these levels do not increase with N rate (Table 7). The level of N in the control was not significantly less than any of the N treatments. The levels of N present in all samples were determined to be in the "low" to "very low" range of sufficiency. This would indicate that a possible stress 64 Table 7. Effect of nitrogen source, nitrogen rate, method of application and time of application on ear leaf nitrogen concentration at tasseling (averaged across two tillage systems) East Lansing, 1984 and 1985. 1984 1985 m 8.158 mm* Source Rate 5/11 5/12 5/12 5/2 5/7 5/7 kg N ha‘1 ------------------ 3 N ------------------ 0 ------- 2.15 ------------- 1.82 ------ AN** 134 - 2.11 - - 2.22 - 179 - 2.00 2.04 - 2.26 2.21 U 134 2.09 2.36 2.10 2.18 2.13 2.09 179 2.15 2.17 2.29 2.24 2.25 1.94 UUP 134 2.11 2.25 2.04 2.11 2.13 2.15 179 2.21 2.20 2.19 2.28 2.29 2.14 LSD (5%) NS 0.19 # placed5cm below the soil surface in a band between ws. 5 AN-ammonium nitrate, U-urea, UUP-urea-urea phosphate. 65 to the plants may have existed, however, this effect was not reflected in the yield or uptake data. In 1985, the ear leaf N concentrations reflect the yield response observed (Table 7). There was also a trend indicating an increase in St N with increasing N application rate. This effect was observed in the broadcast treatments at both application dates. There was no apparent effect of N source, method or timing of application. The N levels in these samples were also in the ”low" to "very low" range. The N uptake data for 1984 and 1985 closely reflected the yield data (Table 8). In 1984 there was a significant response to the early application date at the 134 kg N ha'1 rate. At the 179 kg N ha'1 rate, a response was suggested, but it was not significant. There was also a trend suggesting that an increase in N uptake occurred with increasing N rate. This effect was only noted at the late application date. In 1985 there was a significant response due to increased N rate in the early broadcast UUP and in the late broadcast AN. This response was also suggested with the late broadcast U and UUP and the sidedressed UUP. There were no apparent advantages to N’ source, timing or method of application. 66 Table 8. Effect of nitrogen source, nitrogen rate, method of application and time of application on total nitrogen uptake (averaged across two tillage systems) East Lansing, 1984 and 1985. 1984 1985 _Broadsast__ §i§§# _Broadcast_ 8192* Source Rate 5/11 6/12 6/12 5/2 6/7 6/7 kg N ha‘1 ---------------- g/lo plants -------------- -- 0 ------- 24.5 ------------- 22.2 -------- AN## 134 - 33.9 - 47.0 - U 134 42.5 34.0 35.2 47.9 48.1 179 41.7 37.7 31.9 48.2 47.8 UUP 134 39.4 32.3 30.8 45.9 45.7 179 41.5 35.7 35.0 49.0 49.2 LSD (5%) 5.73 5.42 t placed 5ICm below the soil surface in a band between rows. # AN-ammonium nitrate, U-urea, UUP-urea-urea phosphate. 67 Saginaw In 1984 the experiment conducted in Saginaw was identical to the experiment run in East Lansing in 1984 and 1985. In 1985 three additional factorial combinations of treatment variables were added. The yield data from 1984 demonstrated a significant response due to N fertilization (Table 9). This was indicated by a significant difference in yield between the control and treated plots. All N sources performed similarly. Time of application had an apparent effect in 1984. Although the differences were not significant, yields were generally lower for the early application date. These differences could not be explained by environmental conditions. It is suggested that the rapidly growing corn plants present at the later application date were effectively absorbing N thus decreasing the N available for loss. There is also an apparent benefit from sidedressing the fertilizer. There was a general increase in yield from the 134 kg rate at the early application date and from the sidedressed treatment, but none of the differences were significant. Yields of the treated plots in 1985 were significantly greater than the control (Table 9) . The extremely low values for the 0 N treatment suggest that the levels of residual N in this soil is very low. This is in sharp contrast with the soils in East Lansing. Table 9 . 68 Effect of nitrogen source, of application and time of application on yield of nitrogen rate, method (averaged across two tillage systems) Saginaw, 1984 and 1985. 1984 .1985 M4444. 5144* Images. 4144* 1911111221.; Source Rate 5/11 6/12 6/12 5/2 6/7 6/7 kg N ha'1 ------------------ Mg ha'1 ------------------ 0 ------ 4 . 01 ------------ 3 . 05 ------ AN** 134 - 8.52 - 9.38 7.79 7.81 179 - 9.06 10.5 10.0 8.61 8.70 U 134 7.30 9.24 9.43 8.41 8.53 8.07 179 8.50 9.19 10.7 9.52 8.68 8.83 UUP 134 8.50 9.02 8.71 8.88 7.89 8.07 179 9.50 8.59 9.23 9.68 8.86 8.51 LSD (5%) 1.70 1.01 i placed 5 cm below the soil surface in a band between rows. # AN-ammonium nitrate, U-urea, UUP-urea-urea phosphate. 69 There was no response to N source or method of application, however, there was a uniform if not significant response to increasing N rate. Also there was an apparent benefit to the early application date. This effect cannot be conveniently explained by environmental data. Sufficient moisture occurred after both application dates to leach the fertilizer into the soil. The differences observed were, most probably, a consequence of the severe N depletion of the soil. At the time of the second N application the plants were under severe stress. All plants that had not received N applications were chlorotic and severely stunted. The yield depression observed at the late application date was most likely a result of this stress. Plants that received the early N application did not exhibit stress symptoms. In 1984 there was a significant increase in ear leaf N concentration due to N fertilization (Table 10). At the 179 kg N ha’1 rate, on the late application date, NH4N03 performed better than the U broadcast treatment and better than the U and UUP sidedressed treatments. This was the expected response of NH4NO3. Ammonium nitrate forms a soluble reaction product when applied to calcareous soils, thus decreasing' NH3 loss (Penn and Kissel, 1973). There was a positive response to the N rate at the early application date. Finally, sidedressing the fertilizer also increased the ear leaf N concentration. These results reflected the responses Table 10. 70 Effect of nitrogen source, nitrogen rate, method of application and time of application on ear leaf nitrogen concentration at tasseling (averaged across two tillage systems) Saginaw, 1984 and 1985. 1984 1985 _nroadcas&__ Sids# _Brsadsa§:_ Sids* .Esrfilizsz_ Source Rate 5/11 6/12 6/12 5/2 6/7 6/7 kg N ha'1 ------------------ 2 N ---------------------- 0 ------- 0095 ------------- 1031 -------- AN## 134 - 2.46 - 2.11 1.86 1.89 179 - 2.62 2.85 2.22 1.99 2.18 U 134 1.90 2.37 2.59 2.12 1.75 1.94 179 2.10 2.44 2.62 2.06 1.99 2.07 UUP 134 1.84 2.39 2.58 2.22 1.98 2.04 179 2.21 2.54 2.63 2.08 1.97 1.95 LSD (5%) 0.17 0.32 3 placed 5 cm below the soil surface in a band between rows. # AN-ammonium nitrate, U-urea, UUP-urea-urea phosphate. 71 noted in the yield data. The principle advantage to the late application date was considered to be a more efficient uptake of N by the rapidly growing plants present at that time. The increases due to sidedressing were probably a result of increased contact of fertilizer with the soil, thus increasing the probability of NH4 becoming adsorbed on the exchange complex. Results from 1985 showed a significant benefit from N fertilization, but no advantage to a specific N treatment as determined by N uptake (Table 11) . There was, however an apparent advantage to early application of N. This response was significant at the 134 kg N ha'1 rate of U. This effect was probably a result of the severe N depletion of the soil. Plants receiving the early N application did not exhibit signs of N deficiencies. There was also a trend indicating a benefit to sidedressing. There were no differences due to N source in 1984 as determined by N uptake (Table ll). There were differences due to N application rate, method and timing of application. There was a uniform trend throughout the data indicating a benefit due to increased N rate. This response was significant for the late broadcast of UUP and the sidedressed U and UUP. There was also an indication in the data that there was a benefit to the sidedressed treatments, however this was significant only 72 Table 11. Effect of nitrogen source, nitrogen rate, method of application and time of application on total nitrogen uptake (averaged across two tillage systems) Saginaw, 1984 and 1985. 1984 ______;uuuL_ _nroadsast__ 5138* _Broadsast_ 3108* .zsrtilize:_ ' Source Rate 5/11 6/12 6/12 5/2 6/7 6/7 kg N ha'1 ---------------- g/lo plants ---------------- 0 ------- 15.8 ------------- 15.1 -------- AN## 134 - 34.6 - 28.7 25.7 28.1 179 - 38.0 42.8 29.7 25.9 29.4 U 134 23.5 31.2 32.9 29.6 22.7 33.0 179 27.1 34.1 39.3 30.2 31.4 27.8 UUP 134 24.2 30.8 34.6 32.4 26.6 30.9 179 26.3 38.3 39.5 31.4 29.1 32.0 LSD (5%) 4.75 - 7.10 # placed 5 cm below the soil surface in a band between rows. # AN-ammonium nitrate, U-urea, UUP-urea-urea phosphate. 73 for the 179 kg N ha"1 rate of U and AN. There was also a uniform significant response to time of application. The early application date yielded a lower N uptake. This data would confirm the hypothesis that at the later application date, the plants more efficiently utilized the applied N. The N uptake data for 1985 reflected the ear leaf N concentration data and the yield data (Table 11) . There is an apparent advantage due to early application of N and increased N rate. sidedressing of N also resulted in apparent yield increases, this was significant for the 134 kg N ha"1 rate of U. There was no benefit due to N SOUICB . It is apparent from the experimental results that environmental effects can play a significant role in fertilizer performance. Results were affected by the environment in both years in East Lansing and in 1985 in Saginaw. In 1984 in Saginaw, environmental factors contributed only sl ightly . Experimental results demonstrated a positive effect to a later fertilizer application date. This was attributed to a more efficient uptake of N by the rapidly growing plants present at the later date. In addition, benefits were suggested due to increased N rate and sidedressing of N. Both of these factors would serve to effectively increase the 74 concentration of NH4 on the exchange surface. No effects were observed due to N source. Greenhouse Cropping Study The treatments in this experiment consisted of three N rates, three N sources, two rates of corn residue and two initial moisture regimes. Factorial combinations of these treatment were arranged in a randomized complete block design with four replications. Three crops of corn were grown. Yield and total Kjeldahl N were determined so N uptake could be calculated. Relative N use efficiency will be estimated from these data. Approximate significance of the F statistic for the various factors and interactions is presented in Table 12. Crop Yield The yield of the first crop was affected only by the rate of corn residue applied to the surface . Yield of the 0 residue was 2.65 g/pot compared to 2.84 where residues were applied [LSD (5%)= 0.12]. This was not the 75 Table 12. Approximate probability of significance of the P statistic for various sources of variance for three crops of corn in the greenhouse experiment, 1985. Source of 2101* Yld2 2103 Upt1## Upt2 Upt3 UptT Yarianss Source (3) - 9* 99 9* 9* *9 9* Rggidu.(R) 9* *9 *9 - 99 *9 99 Rate (Rt) - 99 9* *9 *9 99 *9 Moisture (M) - t - - 99 - 99 “*3 - - - - - - - M*S - - - - *9 - 9* H*Rt - - - - - - - R*S - 9* - - 9* - 9 R*Rt — 99 9 *9 *9 9* - S*Rt - - *9 — *9 *9 *9 M*R*S - - - - - - - R*S*Rt - *9 - - *9 - *9 M*R*RT - - - - - - - M*S*Rt - - - - - - - M*R*S*Rt - - - - - - - * significance at 0.05. ** significance at 0.01. # Crop Yield 1,2 and 3. ## N uptake 1,2,3 and total. 76 effect of the residue that was expected. It was postulated that the residue would cause an increase in N113 loss and thus a depression in yield and N concentration in the plant tissue. The residue should prevent the fertilizer from moving into the soil where it could become adsorbed on the soil exchange complex. Residues possess a limited exchange capacity, thus adsorption is unlikely. In addition, residues generally possess a higher urease activity (Torrello and Wehner, 1983). which should also increase N33 loss. Because the residue did not cause a decrease in yield or in N concentration, a different explanation must be offered. The observed effect is perhaps a result of moisture stress. The residue present on the surface of the soil would tend to decrease evapotranspiration, thus maintaining a higher moisture content in the soil. This should normally not be a concern in the greenhouse. However, it could have occurred as a result of the initial moisture treatments investigated in this study. During the initial treatment period, the crops receiving the low moisture treatment were subjected to a ten day moisture stress period. This was an unavoidable consequence of the experiment and may have influenced yields. In the second crop the simple effect of moisture was significant. In addition, the interaction of N rate, N source and residue level was also significant. 77 An increase in yield was observed for those pots receiving the low initial moisture treatment. The yield for the low moisture treatment was 1.95 compared to 1.83 g/pot for the high moisture treatment [LSD (5%)- 0.11]. A low surface moisture content would effectively limit the hydrolysis of urea, thus decreasing NH3 loss. The high moisture treatment, in which several wetting and drying cycles were simulated, would tend to increase urea hydrolysis, evaporation and thus NH3 loss. The net effect of the high moisture treatment should be a decrease in N use efficiency and yield. This was observed in the second cropping. The three way interaction of N rate, N source and residue level was significant (Table 13) for the second crop. When residues were present on the surface of the soil there was a significant yield response to all N rates for all three sources. However, in the absence of residue yields were affected by increasing N rates only for U and UUP. In addition, the only N source differences noted were between the 75 mg N kg'l soil rates of U and AN. It appears that the effect of the residues is to increase the NH3 loss to the extent that the differences due to N rate and N source are increased. When residue is not present, these factors contribute less to the total N33 loss. Further evidence of this effect is presented in the section on N uptake. 78 Table 13. The effect of fertilizer source, fertilizer rate and corn residue on crop yield in the greenhouse experiment, 1985. 4. Source Rate Yldl Y1d2 Y1d3 Yldl Y1d2 Y1d3 mg N kg'1--- --------------- g pot'l ------------------ soil 0 2.03 1.36 2.37 1.84 0.86 1.90 U 75 2.73 1.72 2.41 2.75 1.24 2.49 150 2.69 2.06 3.16 2.97 1.79 2.77 UUP 75 2.61 1.79 2.64 2.78 1.28 2.50 150 2.51 2.15 3.39 2.72 2.34 3.14 AN 75 2.71 2.01 2.72 2.80 1.60 2.65 150 2.64 2.03 3.93 3.05 2.67 4.09 L80 (5%) NS 0.27 NS NS 0.27 NS 47336 g m‘2 79 In the third crop, the two way interactions of N rate by residue and N source by residue were significant (Table 12). Yields were generally higher for the third crop than would be expected. A decrease should be observed due to N depletion of the soil. The higher than expected yield level might be explained by a one week longer growing period. Also this crop was grown later in the year, so photosynthesis was probably higher due to increased light levels. There was a significant increase in yield for crop three with increased N rate at both levels of residue (Table 14). There was also a significant depression in yield in the high residue treatment . The lower yields for the high residue treatment could be a result of increased N113 loss, due to a heightened urease activity in the residues. The limited CEC of the residue would also permit a greater loss of NH3. There were no significant differences between N sources at the 75 mg kg’1 soil rate (Table 15). There was a response to N source at the 150 mg kg‘1 soil rate. Ammonium nitrate performed better than both of the urea fertilizers. This may be a reflection of the lower volatilization potential of AN. There was also a difference between the urea fertilizers, UUP performed better than U. This may be a result of a decrease in urease activity and NH3 loss due to the decrease in pH 80 Table 14. The effect of N rate and residue level on crop yield in the greenhouse experiment, 1985. WM‘ Fertilizer Rate Yldl## 2162 2163 Yldl Yld2 2163 mg N kg'l soil --------------- g pot'l -------------- 0 2.03 1.36 2.37 1.84 0.86 1.90 75 2.68 1.84 2.87 2.77 1.37 2.27 150 2.61 2.08 3.86 2.91 2.27 2.96 LSD (5%) Ns ### 0.19 Ns ### 0.19 # 336 g m-2 ## crop yield 1,2 and 3. ### Source*rate*residue interaction significant, see Table 13. 81 Table 15. The effect of N source and N rate on crop yield in the greenhouse experiment, 1985. Source Rate Yldl# 2162 Yld3 mg N kg'l soil ------ g pot'l ...... 0 1.94 1.11 2.14 U 75 2.74 1.48 2.45 150 2.83 1.92 2.97 UUP 75 2.70 1.53 2.57 150 2.62 2.25 3.27 AN 75 2.75 1.80 2.69 150 2.85 2.35 4.01 LSD (5%) Ns NS 0.23 # Crop yield 1,2 and 3. 82 usually associated with UUP. The crop yields for all sources at the 75 mg kg'1 soil rate were approaching that of the ON control. This would indicate that the N is becoming depleted in the soil. It is suggested that three crops were sufficient to remove the N from the soil, in the 75 mg kg’1 soil rate treatment to background levels. Nitrogen Uptake The N uptake data generally reflected that of the yield data, with the exception that a moisture by source interaction became important for the uptake of crop two and the total uptake. The simple effect of N source was significant for uptake one. The N uptake values were 78.1, 75.0 and 82.8 mg N/pot for U, UUP and AN respectively [LSD (5%)- 3.72]. There was a difference between AN and the urea materials, but not between the urea fertilizers themselves. There ‘was also a significant residue by N rate interaction for the N uptake of crop one (Table 16). There was a significant difference in N uptake between the control and the 75 mg kg"1 soil rate for both levels of residue, however, only the residue treated pots showed a difference between the 75 mg kg'1 soil and the 150 mg -1 kg soil rates. In addition, the N uptake was lower in the residue treated pots for the 0 N and the 75 mg kg"1 83 Table 16. The effect of N rate and residue level on nitrogen uptake by three crops in the greenhouse, 1985. OResiszne—__ Fertilizer Upt1# Upt2 Upt3 Tot ____33§§ + Resigneflfi Uptl Upt2 Upt3 Tot mg N kg'l ------------ mg N pot‘l ------------- soil 0 35.4 18.0 16.2 69.6 26.8 12.9 13.8 53.6 75 78.1 37.9 22.8 139 69.2 21.2 17.3 107 150 82.0 60.0 43.1 185 85.3 52.2 24.1 162 LSD (5%) 4.30 ### 4.71 NS 4.30 ### 4.71 NS # N uptake 1,2,3 and total. ## 336 g m' . ### Source*rate*residue interaction significant, see Table 18. 84 soil rates. This difference was not significant for the 150 mg kg'1 soil rate. In crop two there were significant moisture by N source and residue by N source by N rate interactions. In the moisture by N source interaction two trends became apparent (Table 17). There is a decrease in N uptake for U and UUP, but not for AN. This would further serve to substantiate the premise that the decreased efficiency of the urea fertilizers is due to an increase in NH3 volatilization losses. There was also a trend indicating that there was a benefit to UUP over U. This was significant with the high initial moisture treatment. In the three way interaction significant effects due to N rate, N source and residue level were all recorded (Table 18). There was a uniform increase in N uptake with increased N rate. Also, there was a significant decrease in N uptake with the high residue rate. This trend was observed for all treatments except the 150 mg kg'1 soil rate of AN. Differences between U and UUP were noted for the 150 mg kg'1 soil rate , but not for the 75 mg kg'1 soil rate. The N uptake for AN was significantly greater than U in all cases, however, it was significantly greater than UUP only at the 150 mg kg":L soil rate of the residue treated pots. This effect may be suggesting a benefit to UUP over U. 85 Table 17. The effect of initial moisture content and fertilizer source on nitrogen uptake by 3 crops in the greenhouse, 1985. Fertilizer 9:02 1 9:99.: "_Q:Qn_;__ __19§§1__ Source Wet Dry Wet Dry Wet Dry Wet Dry -------------------- mg N/pot --------------- Urea 75.5 80.7 29.6 41.0 20.4 25.1 125 146 Urea-urea phosphate 74.1 76.0 38.6 45.2 24.3 25.0 136 146 Ammonium nitrate 81.9 83.7 51.7 50.8 32.9 33.3 166 167 LSD (5%) Ns 4.6 NS 6.6 86 Table 18. The effect of N source, N rate and residue level on nitrogen uptake by three crops of corn in the greenhouse, 1985. .Iertilizer. ____#_0_Besi§ue______ +3Resi§ue______3 Source Rate Uptl Upt2 Upt3 Tot Uptl Upt2 Upt3 Tot mg N kg'l ----------------- mg N pot-1 --------------- soil 0 35.4 18.0 16.2 69.6 26.8 12.9 13.8 53.6 t1“H 75 78.2 34.1 20.4 132 64.7 18.0 17.3 100 150 83.2 53.6 34.8 172 86.1 35.4 18.4 140 UUP 75 75.3 36.7 25.3 137 68.4 20.1 16.7 105 150 78.8 62.3 34.5 176 77.6 48.5 22.1 148 AN 75 80.8 42.8 22.6 146 74.5 25.6 17.9 118 150 83.9 64.0 60.0 208 92.1 72.6 31.9 197 Lso(5%) 6.5 9.3 6.5 9.3 # N uptake for crop 1,2,3 and total. ## 336 g m‘2 ### Uaurea, UUP=urea-urea phosphate, ANsammonium nitrate. 87 In the third cropping there were two significant interactions noted, a residue by N rate interaction and a N source by N rate interaction. There was a positive response in N uptake to increased N rate (Table 16). This was significant in all cases except the 75 mg kg'1 soil rate of the residue treated pots. The lack of response in this case probably indicates that N in the soil is becoming depleted. There was also a response showing a decrease in uptake when residues were present. This effect was significant for the 75 mg kg’1 soil and 150 mg kg'1 soil rates, but not for the 0 N control. The N rate by N source interaction for the third crop (Table 19) shows an increase in N uptake with increased N rate for all sources. However, at the 75 mg kg'1 soil rate there was not a significant difference due to N source. In addition only UUP ‘was significantly different from the control. This is further evidence that three crops was sufficient to remove the N from the soil. At the 150 mg kg"1 soil rate AN possessed a significantly greater N uptake than both of the urea fertilizers. There was not a significant difference between U and UUP. In the data for total N uptake there were significant interactions of moisture. by' N source and residue by N rate by N source. The trends exhibited in crop two continued to hold in the moisture by N source interaction (Table 17). There was evidence of N loss from 88 Table 19. The effect of N source and N rate on nitrogen uptake by three crops in the greenhouse, 1985. Source Rate Uptl# Upt2 Upt3 Total mg N kg'l soil ---------- mg N pot‘l ------- 0 31.1 15.5 15.0 61.6 U 75 71.5 26.1 18.9 116 150 84.7 44.5 26.6 155 UUP 75 71.8 28.4 21.0 121 150 78.2 55.4 28.3 161 AN 75 77.6 34.2 20.2 132 150 83.6 68.3 46.0 202 LSD (5%) NS ## 5.76 ## # N uptake for crop 1,2,3 and total. ## Source*rate*residue interaction significant, see Table 18. 89 the urea fertilizers applied to moist soils. This is probably a result of an increase in the rate of urea hydrolysis. Several researchers have reported an increase in N33 loss with an increase in initial moisture content (Martin and Chapman, 1951 and Ferguson and Kissel, 1986). Moist soils are also more prone to evaporative losses. This is an important factor in this experiment, because the high moisture treatment also included several wetting and drying cycles. When urea is applied to a dry surface hydrolysis is inhibited and NH3 loss is decreased. The N uptake for U and UUP was similar when applied to a dry soil, however, when applied to a moist soil UUP performed better than U. This would indicate a possible increase in N use efficiency due to UUP. As previously mentioned, this effect is most likely a result of a decrease in the rate of urea hydrolysis. This effect was also reported by Bremner and Douglas (1971) . The N uptake for AN was significantly greater than the urea fertilizers. Also AN showed no response to the moisture treatments. This would suggest that the increased losses due to the high moisture treatment with the urea fertilizers was probably due to NH3 volatilization. Nitrogen uptake generally decreased with each successive crop until at the third cropping the N uptake from treated pots was not appreciably greater than that of the ON control (Table 18). When residue was present on the soil surface, N uptake decreased. This effect was 90 significant for all treatments for total N uptake. In addition there was a significant increase in total N uptake due to an increase in N rate. There was not a significant difference in total N uptake between U and UUP, although a benefit was suggested from UUP. Both U and UUP had a significantly lower total N uptake than AN. Soil samples collected at th end of the study showed 7.0 mg kg'1 mineral N (No3 + NH4) for the N treated soils and 6.7 mg for the control, suggesting that the applied N was depleted. Urea Hydrolysis Measurements The urease activity of selected field and greenhouse soils was determined to evaluate the relative rate of urea hydrolysis in response to several factors. In the field. samples variables included; N' application rate, method of application soil type and tillage system. In the greenhouse samples the variables included; N rate and residue rate. Only treatments that received urea applications were evaluated. 91 Field Samples There were no significant differences in the field samples due to treatment, however, there were two interesting’ trends in the data (Table 20). The most distinctive one was an apparent higher urease activity in the Charity clay soil as compared to the Conover loam. Several researchers have reported an increase in urease activity with increasing clay content of the soil (Zantua et al.,l977 and Reynolds et al.,l985). This has been attributed to an increased adsorption of urease to the soil colloids. There was also an indication of a decreased urease activity in reduced tillage systems. This effect cannot be easily explained. Some workers have reported increases in urease activity in reduced tillage systems (Kleins and Roth, 1980). Other investigators have shown a decrease in urease activity in ground left fallow (Speir et al., 1980) . The reduced tillage systems investigated in this research were in production for only two growing seasons. This is probably an insufficient. period of time for microbial levels to build up in the soil to a level where they might affect urease activity. The plots evaluated by Kleins and Roth (1980) were in no-till production for approximately ten years. It is possible that the response observed in the present experiment was a result of a general decrease in enzyme activity. This might occur in 92 Table 20. Effect of fertilizer rate, method of application, tillage system and soil type on urease activity in corn plots receiving urea fertilizer, 1985. Soil Tvp§_3 ‘_ Method Fertilizer angyg: of Rate Conv Red### Conv. Red. Application Kg N ha'1 ug urea hydrolyzed h'1 g'1 soil 0 388 366 411 410 Surface 134 381 297 416 403 broadcast 179 377 335 419 414 Sidedressed# 134 329 308 413 414 179 389 341 416 411 LSD (5%) NS NS NS NS 3—ilaced in a band 576m below the soil surface between rows ## Fall moldboard plow plus spring cultivation, on both soils. ### No-till on Conover. Chisel plow plus spring cultivation on Charity. 93 a reduced tillage system because the enzymes and microbes are no longer mixed throughout the profile by tillage procedures. Speir et al. (1980) attributed similar decreases in urease activity to a decrease in microbial population and a temperature dependent denaturation of the enzyme. The effect due to tillage system was less pronounced for the clay soil. This was probably a result of an increased concentration of urease bound to the soil colloidal fraction of the clay soil. There was no consistent effect due to N rate or method of application. Greenhouse Samples There were no significant effects due to treatment in the greenhouse (Table 21). There was apparent increase due to increased levels of residue and increased N rate, but these differences were slight. 94 Table 21. The effect of N rate and residue level on urease activity of greenhouse samples, 1985. Fertilizer LEE-119.93 W Rate mg N ha'1 soil ug urea hydrolyzed h'1 g’1 soil 0 394 396 75 395 405 150 403 406 LSD(5%) NS # 336 g m'2 95 Field NH3 Loss Experiment Field measurements of NH3 volatilization were made in the summer of 1985. This was done in an attempt to quantify the actual amount of NH3 lost from U and UUP in a field situation. The experimental variables included two N sources and four N rates. The experiment. was conducted in East Lansing on the Conover loam soil. The NH3 volatilized was measured five times over a seven day period. In addition C02 and soil and air temperatures were made during NH3 loss measurements. Environmental data including relative humidity and air temperature readings were collected at the beginning of each day that NH3 measurements were made (Table 22). Soil samples were collected at the end of the experiment and N03 and NH4 determined. Ammonia Loss There were significant differences in NH3 loss due to day, N treatment and the interaction of day and N treatment (Table 23). The NH3 loss increased over time to a maximum value and then decreased to background levels. Losses on the first day of the experiment were not 96 Table 22. Relative humidity (Rh), minimum and maximum temperature during selected 24 hour periods, temperature during measurement and C02 concentration during the field NH3 loss experiment, East Lansing, 1985. Date Rh t Cg; minimum maximum ambient (%) ----------- C ----------- umole fraction 8/16/85 58 - - 28 - 8/17/85 64 29 14 27 461 8/18/86 - 31 22 28 492 8/19/86 67 27 12 22 475 8/21/86 61 28 13 26 455 8/23/85 56 28 12 26 508 97 Table 23. Effect of N fertilizer source, fertilizer rate and day of measurement on NH3 evolved from the soil in a field experiment, East Lansing, 1985. ___£srtili§sr__ 3085813 _ Sce mg N/C Rate 8/17 8/18 8/19 8/21 8/23 Total Kg N ha'1 ---------------- mg N## ---------- ### - 0 0 0 0 4.2 0 0 4.2 U 160 84 0 25 0 0 0 30 320 168 0 63 100 6 3.2 171 480 252 0 82 196 20 2.5 301 UUP 0 0 0 0 0 0 0 0 160 84 0 0 19 0 0 19 320 168 0 35 59 3.3 0 96 480 252 0 0 82 12 31 126 LSD (5%) 43 78 # mg N applied to an individual chamber. ## mg N volatilized in 9,000 s. ### mg N volatilized in 1,800 s. 98 recorded. This was expected due to the time required for urea hydrolysis to occur. On the second and third day of measurement significant losses occurred in both U and UUP treated plots. Significant responses due to increased N rate were were recorded on days two, three and in the total N volatilized. An increase in N rate would directly affect the equilibrium illustrated in Eq's [5] and [9] on pages 9 and 10. The net effect would be an increase in NH3(aq) and thus N33(air)v which would increase the volatilization potential. This effect would be more pronounced in the loam soil, because the lower CEC will allow for less NH4 adsorption. There was a trend throughout the data suggesting that a decrease in NH3 loss may occur if UUP is selected as the N source rather than U'. This effect was highly significant at the 252 kg N ha"1 rate. There was also a delay in volatilization when UUP was the N source. Bremner and Douglas (1971) reported a decrease in urease activity with urea phosphate (UP). They postulated that the phosphoric acid formed in the decomposition of UP effectively retarded urease activity. This is the effect suggested by the experimental results. 99 Soil N03 and NH4 There was a significant difference in No3 levels in the soil, but not in the levels of NH4 (Table 24). The quantity of No3 increased with increasing N rate for both U and UUP, however, UUP tended to have higher No3 levels than U. This would suggest that UUP resulted in lower NH3 losses. As previously mentioned, UUP is thought to decrease urease activity. A decrease in urease activity would allow for a greater period of time in which nitrification could occur. This trend is suggested by the higher levels of soil N03 present in the UUP plots. CO2 Levels The end products of urea hydrolysis are NH3, C02 and H20. This is illustrated in Eq's [3] and [4] on page 9.- The levels of CO2 were determined in each volatilization chamber during NH3 measurements. It was postulated that this would be an estimate of the relative rate of urea hydrolysis. No effects were recorded due to N treatment. The C02 levels in the treated chambers was not significantly greater than that of the 0 N controls. It is apparent that there exists a significant background 100 Table 24. Effect of N fertilizer source and fertilizer rate on soil N03 and NH4 levels in samples taken at the termination of the NH3 loss field experiment, East Lansing, 1985. ____Isrtilizer____ N03 NH4 Source Rate -1 --- -l --- Kg N ha mg kg soil Urea 0 ' 34.9 0.80 84 103.8 1.22 168 210.5 0.83 252 247.4 0.89 Urea-urea 0 29.1 0.68 phosphate 84 130.1 0.70 168 209.3 0.83 252 313.0 1.18 LSD (5%) 68.5 Ns 101 CO2 flux from the soil. This is most likely a reflection of soil microbial activity. Soil and Air Temperatures Soil and air temperatures were recorded during NH3 measurements (Figure 5). Insufficient equipment was available to take readings in each chamber, therefore statistical inferences could not be made. It is possible to make generalizations about the environment inside the volatilization fichamber. It was hoped that the NH3 sampling device would not seriously alter the soil environment. If large increases in temperature occurred during sampling, this may introduce a bias into the NH3 loss readings. On the third day of measurement maximum NH3 loss occurred, the soil and air temperature readings for this day are presented in Figure 5. It is apparent from examining this figure that an initial increase in air temperature occurred. This is followed, with minor exceptions, by a gradual decrease in temperature. Soil temperature stayed relatively constant, although an slight decrease is apparent. A short period of time passed before the aeration apparatus was turned on, it is suggested that the initial increase in air temperature is 102 .mmaH .wcamcou ummm .ucoaaquXm mmoH mmz one wcwuso monousuodseu new use Hwom .m musmfim o.o9 o.cm o.o~ o.o« o.o - b L b b n p L b — b n b n — P n n n a ..u a 1 7.. .7. .3 In“? .4 0 a) z .3. TS \ J 0 . r z 19 nu w dam e 4 e e In... Sc 0 s e e .0 0'83 (3] BHHiHHBdNI-Ji '103 a result of this lag period. The following decrease is probably a result of increased evaporation of H20 from the soil. This would be stimulated by the negative partial pressure of H20 ,‘ created over the soil by the increased air flow. The effect of the temperature inside the volatilization chamber is probably minor. There was an overall decrease in temperature over the course of the experiment. This would most probably yield a decrease in NH3 loss. SUMMARY AND CONCLUSIONS Field and greenhouse experiments were conducted to investigate several methods by which N use efficiency may be increased in reduced tillage systems. In the field, these included variations in; N source, N application rate, method of application and timing of application. In the greenhouse experiments were designed to evaluate the effects of N source, N application rate, corn residue levels and initial moisture content. Results from. the field experiments, although affected by environmental variability, indicate that there may be a positive effect on crop yield and N uptake from a delayed application of N fertilizer. This delay was 6 to 8 weeks in the reported experiments. This effect is probably due to a more efficient uptake of N by the rapidly growing crops present at the time of the late application date. There were indications that an increased N application rate and sidedressing of N fertilizer may result in benefits to crop yield and N uptake. Both of these factors would serve to increase the concentration of NH4 on the soil exchange complex. Adsorbed NH4 would have a lower volatilization potential. 104 105 There were no consistent effects observed due to N 8011163 . In the greenhouse study there were indications that both initial moisture content and residue present on the surface of the soil could influence NH3 volatilization losses. A high initial soil moisture would promote a rapid, sustained urea hydrolysis rate. An increased level of residue on the soil surface could also result in increased urease activity. Heightened urease activity may result in higher NH3 losses. Crop yield and N uptake generally increased with N application rate. Significant differences were observed between AN and the urea based fertilizers. Ammonium nitrate has a lower volatilization potential due to the formation of a soluble reaction product in the soil. This would yield a decreased concentration of (NH4)2CO3 in the soil and thus decreased N113 loss. Although not uniformly significant, there was a trend indicating that UUP would result in an increased N use efficiency as compared to U. This may be explained by a decrease in pH in the immediate vicinity of the fertilizer application, resulting in suppressed urease activity. A decreased OH" ion activity also will tend to result in lower NH3 losses. A laboratory experiment was conducted to evaluate the urease activity of soils in selected treatments. The non-buffer method of Zantua and Bremner (1975) was 106 utilized because it was reported to be a better index of urease activity under natural conditions than the buffer methods. Measurements were conducted in both conventional and reduced tillage systems. In addition, measurements were made of greenhouse samples. Experimental variables included: N application rate, N application method soil texture and residue level. The results of this experiment indicated that urea hydrolysis was probably not limiting in any of the field or greenhouse experiments. There were no consistent significant effects due to treatment in the greenhouse or field samples. There were,however, indications that soil type and tillage system may influence urease activity in some situations. Higher urease activity was associated with the clay soil. This effect may be a result of increased adsorption of urease on the soil colloids which might decrease its denaturation by proteolytic enzymes. The reduced tillage systems were also associated with a decreased urease activity. This may be caused by a general decrease in microbial levels due to a fallowing effect. Data from the NH3 loss experiment suggested that there may be an advantage to UUP as a N source as compared to U. This effect is more pronounced at higher application rates. The benefit of UUP is probably 107 associated with a delay and subsequent decrease in the urea hydrolysis rate. In conclusion, an increase in N use efficiency may be gained if fertilizer application is delayed until the plants are at a stage of growth in which they could efficiently uptake the N. This would probably entail a 6 week delay in application. Sidedressing of N fertilizer decreased apparent NH3 loss in some instances, but this trend was not uniform throughout the data. Due to the increase in NH3 loss observed with a high initial moisture content, it would seem a good practice not to apply urea fertilizers to a moist soil. If the material is applied to a dry soil, urea hydrolysis may be delayed until sufficient rainfall occurs to leach the fertilizer into the soil. There is some indication from greenhouse and field experiments that there is an advantage to using UUP over U. This effect, however was not expressed uniformly’ throughout the data. 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