The Enigma of Soil Nitrogen George Rehm, University of Minnesota

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1 The Enigma of Soil Nitrogen George Rehm, University of Minnesota 1. Introduction Throughout the northern and western Corn Belt, nitrogen (N) is the most dominant nutrient in the world of plant nutrition. We ve learned from research through the years that management of this essential nutrient has a major impact on profits realized from crop production. A shortage of N reduces yield and subsequent profits. Excessive rates increase the cost of the fertilizer program without increasing yield and potential profit is lost. The agronomic and economic consequences of effective and efficient management of N are, without doubt, the major consideration. The environmental consequences, however, cannot be overlooked (2). This lesson will describe the important transformations of N in soils. Nitrogen is unlike other nutrients because it is transient and transformations in soil are highly influenced by biological organisms. The important forms of N in soil and transformations from one form to another will be discussed. 2. The Nitrogen Cycle 2a. Plant Uptake The complex nature of N in soils is shown in what s commonly referred to as the nitrogen cycle (see Figure 1). Since N cycles in soils, it s difficult to know where to start and where to end. One logical approach is to look at additions and withdrawals. Figure 1. Nitrogen is added to soils from the following sources: commercial fertilizer animal manures crop and plant residues fixation by legumes the atmosphere

2 The amount supplied from commercial fertilizers is variable and a function of crop and production environment. Nitrogen supplied from livestock manures is also variable and is a function of species, method of handling, and method used in application to the soil. Manure nutrient content and associated losses are discussed in other Extension publications (1) and will not be repeated in this lesson. Nitrogen exists in crop residues as water soluble forms as well as complex organic molecules that decompose over various periods of time. The process of release of N from organic sources is summarized in the term mineralization that will be described later in this lesson. The total N content of various crop residues is summarized in Table 1. T able 1. Approximate amounts of N in grain and residue of common Minnesota crops. Crop Component lb. N/unit of Harvest Corn Grain 0.9 lb./bu. Corn Stover 22.2 lb./ton Wheat Grain 1.25 lb./bu Wheat Straw 13.3 lb./ton Soybeans Grain 3.75 lb./bu Alfalfa Whole plants 45 lb. ton In general, the numbers in this table do not have an impact on guidelines for rates of fertilizer N to apply. Several research projects have focused on predicting the amount of N released from crop residue and soil organic matter. These efforts, however, have not always been definitive. Soil organic matter is the end result of adding crop residues and other residues to the soil. Some organic N compounds in soil organic matter decompose and release N rather rapidly. Others are more resistant to decomposition and release N over a longer period of time. Nitrogen supplied to the soil from legumes can be an important source to meet the needs of crop production. Total amount of N added to the soil from various legume crops is summarized in Table 2. The values in this table are not the N credits for these crops when using N rate guidelines. Instead, these values are the amounts added to the soil system when the various legume crops are incorporated into the soil with tillage. Table 2. Approximate amounts of nitrogen added to the soil system by various legume crops. Legume Crops N Added lb./acre Alfalfa 194 Red Clover 114 White Clover 103 Peas 72 Soybeans 58

3 The numbers in Table 2 are not associated with a particular yield level. These numbers are to be used for relative comparisons only and not to arrive at the rate of N to apply when a grain crop follows a legume in rotation. Various N compounds are present in the atmosphere and are returned to the earth in rainfall. The N in rainfall is present as ammonium, nitrate, nitrous oxide and organic combinations. Industry is the source of the ammonium. The nitrate is attributed to lightning.. Total amounts of N are estimated to range between 1 and 50 lb. per acre annually, depending on location. Nitrogen in rainfall is generally higher around areas of intense industrial activity and cannot be counted on as a constant source for crop production. Nitrogen leaves the soil in various ways and forms. These are: crop removal denitrification leaching volatilizations fixed by clay sized particles fixation in soil microorganisms Nitrogen, of course, is a component of the harvested and nonharvested crops, except when removed as biomass. the N in residue is returned to the soil system to be recycled. The N in the harvested crops must be replaced from a combination of sources described above. A small amount of N in the ammonium form can be trapped or fixed between the layers in various clay sized particles. This N is not permanently lost; it can return to be available to a growing crop. However, amounts of N in this form in the soil are relatively low. Volatilization is defined as the loss of free ammonia from soil. Volatilization losses are associated with the application of anhydrous ammonia to soils that do not seal. This loss can occur at either the time of application or in a few days following applications. Delayed losses are slow and usually cannot be detected by odor. Volatilization losses are almost impossible to measure. Accurate measurements have not been reported. Losses by denitrification and leaching will be discussed in sections that follow. The discussion of the equilibrium between mineralization and immobilization will describe the fixation of N by soil microorganisms. 2a. Plant Uptake of N Absorption by plants is a major avenue by which N is removed from the soil system. Plants take up N in both the ammonium (NH + 4 ) and nitrate (NO 3 ) form (see Figure 2). A combination of both forms is needed for optimum plant growth. Plant growth will be curtailed when the N source is restricted to either NH + 4 or NO 3.

4 In this soil system, NH + 4 is supplied from: 1) plant residues; 2) animal manures; and, 3) commercial fertilizers. The NO 3 is supplied from: 1) animal manures; 2) commercial fertilizers; and, 3) the atmosphere. The conversion of NH + 4 to NO 3 (nitrification) will be discussed later in this lesson. Figure 2. Once absorbed by plants, both NH 4 + and NO 3 must be converted into compounds that are used for growth and development. Through various physiological reactions controlled by enzymes, both the NH 4 + and NO 3 are first incorporated into amino acids. The enzyme, nitrate reductase, is responsible for conversions of NO 3 first to NH 4 +, then into amino acids. The amino acids are then combined in a wide variety of physiological reactions to form proteins. Proteins, simple and complex, are necessary to maintain plant structures as well as normal plant metabolism. 3. N Transformations in Soils So far, this discussion has focused on the addition of N to soils and pathways for loss. In soils, N is transformed in many ways. Some are very important for consideration in N management practices and crop growth. Many of these transformations do not occur in isolation. They are interdependent on each other. Mineralization and nitrification are two major processes that take place in soils and they are related to each other. Mineralization is an umbrella process in which N in the organic form is converted in the end to NO 3. Organic N is first converted to NH The NH 4 is converted to NO3 in the reaction called nitrification. Of the total amount of N in soils, approximately 90% is stored in the soil organic matter. Only a small percentage of the organic N is converted to NO 3 N each year. This conversion is driven by soil microorganisms; it is not a chemicalonly reaction. If moisture and temperature are favorable, 1% to 3% of the N in soil organic matter will be converted each year to forms of N that can be used for plant growth. Although the subject has been extensively researched over the years, the release of N from soil organic

5 matter cannot be predicted. There is less tillage and subsequent organic matter decomposition in conservation tillage planting systems. +N Nitrification is a two step process. First NH 4 is converted to nitrite N (NO 2 N). The NO 2 N then combines with oxygen to form nitrate N (NO 3 N). Separate bacterial organisms run each reaction. The NH + 4 can, in addition to organic sources, originate from: 1) anhydrous ammonia; 2) urea; 3)ammonium nitrate; 4) ammonium phosphate; 5) ammonium sulfate; and, 6) ureaammonium nitrate (see Figure 2). From the plant s perspective, the initial source of N doesn t make a difference. Whether organic (manure) or inorganic (8200 etc.), the NH + 4 is the same. Plant processes that involve NH + 4 are the same regardless of the initial source of NH + 4. So, if rate of N supplied is nearly equal there should be an equal effect when organic and inorganic sources of N are compared for production and crop quality. As stated previously, nitrification is an acid forming reaction. The H + is produced in the first step of the twostep reaction. Again, source of NH + 4 (organic or inorganic) is not important. A reduction in soil ph caused by the use of various sources of fertilizer is shown in Table 3. The study was conducted in Kansas and rate of N applied from all sources was 200 lb. per acre. The N was applied annually for 10 years. Table 3. Soil ph after 10 years of repeated application of various sources of fertilizer N. N Source ph at 4 inches None 6.4 anhydrous ammonia (8200) 5.7 ammonium nitrate (3300) 5.7 urea (4600) 5.7 UAN solution (2800) 5.7 In Minnesota, there are many fields having soils formed on calcareous glacial till where soil ph at a depth of 0 to 6 inches is in the range of 5.8 to 6.2. A routine soil test for these fields shows a requirement for lime. However, the ph increases with depth and, therefore, there is some question about the benefit of lime applied to these soils. Results of research with alfalfa, soybeans, and corn conducted at the Southern Research and Outreach Center, Waseca show that liming does not increase yields in these situations. Soil bacteria are involved in both mineralization and nitrification. Therefore, factors that affect biological activity affect these reactions. Soil temperature is important especially for nitrification. Nitrification begins slowly at or slightly above freezing and rate continues to increase as soil temperatures increase to about 85 0 F. Above 85 0, the rate decreases. The management suggestion to delay fall applications of fertilizer N until soil temperature at 6 inches is 50 0 F is designed to minimize the nitrification reaction.

6 Both mineralization and nitrification require oxygen. Therefore, practices that increase soil compaction and reduce soil aeration reduce the rate at which these reactions proceed in soils. Bacteria remain active in dry soil conditions, but not in waterlogged soils. Soils with sufficient moisture to grow crops will have optimum moisture for mineralization and nitrification. Environmental conditions (temperature, moisture, etc.) that are conducive to high crop yields are also very favorable for both mineralization and nitrification. In these years, high amounts of N0 3 N are released from soil organic matter and used by crops. Both mineralization and nitrification can occur in the ph range of 4.5 to 10.0, but a ph of 6.0 to 8.5 is optimum. Mineralization also interacts closely with the reaction of immobilization. Immobilization is the reaction in which N03 N is used for growth and development of soil bacteria. The N0 3 remains in the soil system, but is not available for plant growth. There is, therefore an equilibrium as shown below, between the mineralization and immobilization reactions. mineralization immobilization If the rate of mineralization exceeds the rate of immobilization, N0 3 N is added to the soil system. If immobilization exceeds the rate of mineralization, N0 3 N is removed from the soil system and less is available for plant growth. Frequently, there will be a small drop in N0 3 N following the application of fertilizer N. When this occurs, soil bacteria are using the N0 3 in fertilizer for growth and development at an accelerated rate. The slight reduction is followed by a rapid increase in mineralization and subsequent production of NO 3 N produced by the rapidly developing soil bacteria. Considering this equilibrium, the amount of NO 3 N added to the soil system is affected by the type of residue that is being mineralized. The ratio of carbon (C) to nitrogen (N) in the residue is important. This ratio is determined by dividing the percentage of carbon by the percentage of nitrogen. Residue with a narrow (low) C/N ratio is easily decomposed or mineralized and NO 3 N is usually added to the soil system. Alfalfa is one example of a crop with a narrow (low) C/N ratio. Corn stover and wheat straw, on the other hand, have high or wide C/N ratios. When microorganisms break down these residues, N0 3 N from the soil may be needed for their growth and development. Thus the decomposition of these residues may result in less availability of NO3 N in the soil system. In general, if the C/N ratio is less than 20:1, mineralization will dominate the equilibrium and NO 3 N will be added to the soil system. If the C/N ratio is higher than 40:1, immobilization dominates and NO 3 N is taken from the soil N pool. Since the C/N ratio of corn stover is higher than 40:1, there is some thinking that an application of N in the fluid form (2800) after harvest might enhance decomposition

7 thereby enhancing the yield of the subsequent crop. Wile this practice might appear to be practical, there is no research to suggest that it would increase yield over and above application of the 2800 at the same rate at another time or in another way. 4. Loss of N from Soil 4a. Denitrification 4b. Volatilization The processes of mineralization, nitrification, and immobilization involve the conversion of nitrogen from one form to another. For example, mineralization is the conversion of N in organic compounds to inorganic N. The loss of N from soils, of course, is a serious consideration when management decisions that involve N applications are considered. Plant uptake is an obvious avenue for removal and will not be discussed in detail in this lesson. The loss mechanisms of leaching, denitrification, and volatilization are major considerations in N management decisions. Leaching losses are widely discussed and have been the focus of numerous research activities. Losses by this mechanism are fairly well understood by both farmers and ag professionals. Leaching, of course, involves the loss of N as NO 3 in the presence of excess moisture in the root zone. The end result, if leaching is excessive, is an elevated concentration of NO 3 N in the groundwater and perceived health problems. Since there has been so much discussion of this mechanism of N loss in the past, this lesson will restate the obvious. 4a. Denitrification Denitrification is not as well understood. This is the process by which bacteria growing without oxygen (anaerobic) change NO 3 N (available) to unavailable gaseous N forms in the soil atmosphere. A general picture of this process is provided in Figure 3. The chemical reaction is as follows: NO 3 nitrate N 2 or N 2 O nitrate nitrogen nitrous gas oxide NO 2 For this reaction to proceed, there must be: 1) a source of soluble organic matter (energy source for microorganisms); 2) waterlogged soils (no oxygen); and, 3) favorable temperature. Denitrification rates are highest when soil temperatures are between 75 0 F and 85 0 F. Even though the soil may be saturated, denitrification does not occur when soils are frozen.

8 Figure 3. Denitrification takes place very rapidly. If water stands on the soil surface for only 2 to 3 days during the growing season, most of the NO 3 N will be lost by denitrification. Because of low soil temperatures, loss of N by denitrification is not a concern during the spring thaw. Saturated soils after planting and throughout June are very conducive to denitrification. Although there have been several attempts to develop a means which by denitrification could be predicted, none have been successful. There is no practical way to measure loss of NO 3 N by this process. In Minnesota, loss from denitrification can be minimized by placement of fall applied N as deep as possible. This is usually not a problem when anhydrous ammonia is the N source. Fallapplied urea should be incorporated to a depth of 4 inches when incorporation with shallow tillage is not adequate. 4b. Volatilization + Volatilization is a process of N loss in which useable N (NH 4 or NO 3 ) is converted to ammonia gas (NH 3 ). Volatilization can also be the direct loss of NH 3 during the application of This NH 3 is lost in the vapor that escapes before the soil seal behind the applicator. Growers and Ag professionals have frequently asked about the magnitude of N lost in this process. Although there have been many attempts at measurement, there is no acceptable procedure to measure this N loss. The presence of soil water is a key to preventing loss by volatilization. When anhydrous ammonia is added to the soil, it combines with soil moisture according to the following chemical reaction: NH 3 + H 2 O NH H 2 O + OH The ammonium ion (NH + 4 ) is then attached to soil particles and held in the soil system. The presence of OH in the soil solution means that this is not an acid forming reaction. In fact, the soil ph in all but the most calcareous soils increases slightly when 8200 is + applied; then decreases as NH 4 N is nitrified to NO 3 N.

9 The loss of N by NH 3 volatilization does not have to be immediate. If soils are very dry at the time of application, NH 3 volatilization can be very slow. The free NH 3 can diffuse slowly back to the soil surface and be lost into the atmosphere. This slow loss cannot be detected by the human nose. Loss of N by volatilization can also take place when the popular N product, urea, is applied to soils. This is especially true for calcareous soils. The loss follows the following chemical reaction: CO(NH 2 ) H 2 O 2 NH3 + H 2 CO 3 The H 2 CO 3 is carbonic acid. An enzyme called urease makes this reaction go. There are products on the market classified as urease inhibitors that delay this reaction. The NH 3, of course, is not lost if the urea is incorporated into the soil. Incorporation can be by tillage, rainfall, or irrigation water. In the case of tillage, only a shallow pass is needed to prevent volatilization loss. Research has shown that 0.25 inches of rain is needed for incorporation. Potential for loss explains the Best Management Practice (BMP) of incorporating urea N following application. As with the application of anhydrous ammonia, volatilization loss of N from urea has been difficult to measure. Discussion in this lesson emphasizes that N in the soil system is very transient; constantly changing from one form to another. Unfortunately, it is difficult to measure the speed of the reactions at any one point in time and under all conditions. Therefore, more precise guidelines are not possible and N management, in many ways is still an enigma.

10 References 1. Blanchet, K. and M.A. Schmitt. 2007(rev.). Manure Management in Minnesota. University of Minnesota Extension publication WW Available at (verified August 19, 2008). 2. Lamb, J., G. Randall, G. Rehm, and C. Rosen. 2008(rev.). Best Management Practices for Nitrogen Use in Minnesota. University of Minnesota Extension publication DC8560. Available at (verified August 13, 2008).