II The Nitrogen Cycle A program of the Iowa Soybean Association August 2010, OnFarm Network, Ankeny, Iowa 50023. Permission to reproduce for educational and instructional purposes hereby granted. All other rights reserved.
The nitrogen cycle How nitrogen moves in the environment Illustration: Michael Pidwirny Used with permission Nitrogen cycle 1
Nitrogen in soil organic matter To understand the N in our soil, it is important to recognize that the soil is a huge reservoir of N. About 99% of the N in the soil is in the form of organic matter. Each percent of soil organic matter in the top 6 inches of the soil contains roughly 1,000 lbs. N per acre. However, only a small portion of this N will be available to the plant. Many Iowa soils are higher in organic matter than those in other states, making it important to understand this N source and the processes that occur in the soil that make it available for crop production. To understand how much N is in your soils, you can make a few simple calculations, based on the amount of organic matter in your soil. Use these assumptions to estimate total soil N: Six inches of soil over one acre weighs about 2,000,000 lbs. Soil organic matter is about 5% N. So, a soil containing 5% organic matter would have 5000 lbs. N per acre. (2,000,000 lbs. X 0.05 X 0.05 = 5,000 lbs.) While this sounds like a lot of N, it is important to realize that only a small portion of this will be available in a given year. The amount that will be released depends on several factors, primary of which are the soil biology and the effects of the weather. Soil organic matter is comprised primarily of plant residues, with a small percentage made up of soil microorganisms. Decomposition of plant residues is a biological process carried out by soil microorganisms, so the amount of N released from the organic matter into the soil is Figure 2.1 Field map of soil organic matter content Organic matter in Iowa soils contains high amounts of N. Understanding how it is released and how much might become available to the corn crop is important. a result of this biological process. In other words, the amount of N released depends on how much work the soil microorganisms perform. The microorganisms are more active when soils are warm and there are ample supplies of moisture and organic matter. The composition of the residue determines how easy it is for microbes to break down. A key relationship is the carbontonitrogen (C:N) ratio. These must be properly balanced for microorganisms to release N. When the C:N ratio is higher than about 30:1, more N is immobilized than released. Below that level, there is a net release of N. The normal ratio in most agricultural soils is about 10:1 C:N, which is conducive to N release. Microorganisms use carbon for energy and a small amount of N for cell wall growth and reproduction. This will be covered in more depth under mineralization and immobilization topics later in this chapter. The dark color of the soil is due to soil organic matter (SOM). Because the source of SOM is largely plant residues, the amount of SOM is indicative of the productivity of the soil. Other factors that affect the breakdown of SOM can have a major impact on its concentration in the soil. For example, decomposition goes on continuously in areas where soils are warm yearround, but in areas where soils freeze in the winter, biological activity is limited only to the warmer months. This explains why most soils in the upper Midwest have higher organic matter content and are darker in color than those in the in south or southwest. Organic matter differs within fi elds based on topography (Figure 2.1). At higher elevations (hillsides and tops) soil has less organic matter than lower ground because of erosion and, to a lesser extent, the differences in plant growth and in the rate of residue breakdown during wet conditions that may have occurred before many of the lower, wetter areas were drained. Nitrogen cycle 2
Mineralization in soils Nitrogen, as we have stated, is very dynamic and subject to many different processes. We have established that a key process for crop production is release of N from the organic matter to a form that crops can use. We call this process mineralization. In mineralization, N from organic matter in the soil is converted to ammonium (NH 4 ). The three major forms of biological N are proteins (amino acids), plant cell wall components (amino sugars, such as cellulose and chitin), and nucleic acids. Mineralization of organic matter is the degradation of proteins, amino sugars, and nucleic acids to the ammonium form. The mineralization process is done by heterotrophic (needing carbon) bacteria in two steps: ammonization, and ammonifi cation. The ammonium form of N is the fi rst form created that can be used by the plant. While plants can take up NH 4, it is typically transformed quickly into other forms such as nitrate (NO 3 ) or ammonia (NH 3 ). The ammonium form can also be consumed by microorganisms and become unavailable by a process called "immobilization," which is the opposite of mineralization. (This will be discussed later in this chapter.) NH 4 is readily available for biological transformation. It is a positively charged ion, which allows it to bind with negatively charged soil particles. As long as N remains in this form, it is not easily lost by leaching rainfall. While NH 4 is available to plants, it is readily transformed into other forms in the soil, some of which, though more stable than NH 4, have a neutral or negative charge, and so are not bound to the soil mixture. Examples are NO 2, N 2 O, and NO, which are easily lost to moving water or into the atmosphere. Because mineralization, the transformation of N by soil bacteria, is a biological process, it can be infl uenced by a number of environmental and management factors. As a practical matter, subtle differences in the environment can have a large effect on the release of N from organic matter. Mineralization is the mechanism by which N in organic matter is transformed by microbial processes in the soil into ammonium, a form that is available for plant growth and other biological processes. Factors such as rainfall, temperature, carbon (C), N availability and tillage can all have a big impact on the amount of N released through mineralization. Improving drainage in wet soils can encourage microbial activity and result in increased N availability. Typically, more N will be mineralized in warmer soils with optimal moisture and aeration than in cool, wet or arid soils. Microbial activity is restricted when soils contain too much or too little moisture. Figure 2.2 The mineralization process Nitrogen cycle 3
Factors affecting mineralization Anything that affects soil microorganisms will infl uence the rate at which N is mineralized from soil organic matter (SOM). The optimal soil environment for crop growth is very similar to that for optimal microbial activity. Soil temperature has a big impact on the mineralization rate. Different microorganisms have different optimal temperature ranges. From a practical point of view, the warmer the temperature, the better the conditions for mineralization. Very little microbial activity occurs when soil temperature drops below 40 degrees F. Remember, this process occurs in soil that is usually covered either by crop residue or the crop canopy, both of which serve to insulate the soil. This keeps soil temperatures lower than the daily high air temperature. Moisture level in the top 6 in. of soil is another key to mineralization rate, since this is where most of the organic matter is located. Soil microbes need the right balance of moisture and air (oxygen) to function. Too much or too little of either reduces their activity, and thus slows the rate of mineralization. Moisture and air share the pore space in soil, so when soil is saturated, air is limited. Usually, lowlying areas in a fi eld accumulate more water than higher areas of the fi eld. Over time, this can lead to large differences in SOM content between topographical areas in a fi eld. A shortterm dry period can signifi cantly reduce moisture in the top 6 in. of soil and slow microbial activity on SOM, even when there is plenty of subsoil moisture for crop growth. Installing tile drains to reduce moisture levels in wet soils can have increase the mineralization rate. Differences in SOM content and short term differences in soil moisture can result in big differences in N mineralization rates. The mineralization rate in lowlying areas can be very low in wet years, but quite high in drier years when soils in higher areas of the fi eld are too dry to produce optimal grain yields. Because soil compaction affects aeration and water infi ltration, it can also have an infl uence on mineralization. Tillage tends to improve aeration in the surface 6 in., where most SOM mineralization occurs. Incorporating plant residues into the soil with tillage also tends to increase the rate at which they decompose. Because the breakdown is a microbial process, more contact between the soil and the residue will result in faster decomposition. Additionally, cutting residue into smaller pieces (by tillage, chopping or shredding), also increases the decomposition rate since Mineralization is the biological process that releases N from organic matter in the soil. Factors that affect crop growth also affect microbial activity. Soil temperature and moisture have a big impact on the amount of organic N that will be available to plants. there is more surface area for bacteria to work on. Another key factor is the ratio of carbon (C) to N. Microorganisms need both C and N to form the various necessities of life. C is the energy source for most of the soil organisms that decompose the SOM. However, they also need a certain amount of N to continue to function and reproduce. The ratio of C to N (C:N) affects the amount of N that is released or tied up. When the C:N ratio is 15:1 or lower, N is typically released fairly rapidly. Decomposition rate varies with the type of organism, but usually when decomposing organic materials with a C:N ratio of 30:1 or higher, soil organisms will need more N than is contained in the organic matter. Decomposition will be slow unless an additional source of N is readily available. Corn stover has a C:N ratio somewhere around 60:1. This is why we often see corn residue remaining in the soil into the following season. Most soils have a large pool of organic matter, with an overall C:N ratio of about 10:1. N released from the breakdown of one type of SOM may be used to stimulate decomposition of corn residues, which are higher in C and are decomposed more slowly than residues that are higher in N, such as soybean biomass. A big difference between a cornbean and a corncorn rotation is that there is more rapid mineralization available after beans. This is because less N is tied up in breaking down soybean residue than in breaking down corn stalks. The socalled "soybean N credit" is usually considered to result from the symbiotic N fi xation by Rhizobia in soybean roots. While soybeans are legumes and can foster large populations of Rhizobia in their roots, they will do so only to the extent that they need N. If N is available in the soil, the soybean plant and Rhizobia will use that before fi xing additional N from the atmosphere. Nitrogen cycle 4
Nitrification in soils The ammonium (NH4 ) form of N is the fi rst form available from mineralization for a crop to take up. However, soil microorganisms don t stop there. Nitrate (NO 3 ) is the most soluble form of N, and so it is the form that crops generally use the most. The process of converting the ammonium form to the nitrate form is called nitrification. (See Figure 2.3) The NO 3 form is the most important N form to understand from a crop production perspective for a variety of reasons. Because the amount of NO 3 is so critical to crop growth, an understanding of how it becomes available and how it transforms in the soil is essential if we hope to properly manage it. As microbes break down soil organic matter, ammonium (NH 4 ) is released. Further microbial activity transforms this NH 4 into NO 3, making it available for use by crops. Most of the N taken up by plants is in the NO 3 form. Unlike NH 4, NO 3 is very susceptible to loss by leaching. This is because NO 3 is negatively charged. Because soil also has a negative charge, NO 3 is not chemically bonded to the soil. In fact, the two actually repel each other. Figure 2.3 The nitrification process Of all the N forms, nitrate (NO 3 ) is probably the most important for crop production. It is created by a microbial process called nitrifi cation which is affected by soil conditions. This means that NO 3 remains free in the soil. Because it is highly soluble in water, it is easy for plant roots to take up. However, this high solubility also means NO 3 is easily lost to leaching as rainfall moves down through the soil profi le. High rainfall in tiledrained fi elds can lead to signifi cant loss. The process of nitrifi cation is actually more than one step, but, simply stated, it involves two different types of bacteria that oxidize the N ions to extract their energy. In the fi rst step, Nitrosomonas bacteria convert NH 4 into nitrite (NO 2 ). NO 2 can be toxic to plants, but this ion seldom accumulates to dangerous levels in the soil because it usually quickly acquires an oxygen atom from the air and is converted into NO 3. Through the oxidation process, NH 4 is converted into not just nitrate (NO 3 ) for plant growth, but also free hydrogen (H ). If this free H remains in the soil, it can combine with other elements to increase soil acidity (or lower soil ph). Generally, the more nitrifi cation that occurs in a soil, the more the soil ph level will drop. Mineralization Nitrogen cycle 5
Factors affecting nitrification The process of converting the ammonium (NH4 ) form of N to the nitrate (NO 3 ) form is called nitrifi cation. Because the conversion process is driven by microorganisms, understanding the environmental conditions that accelerate or delay the formation of NO 3 is critical. This is important not only for knowing what can be available to a crop, but also for understanding what can be lost. NO 3 is much more easily lost from the soil than is NH 4. Unless added to the soil as commercial fertilizer that is in the NO 3 form, the NO 3 used by plants comes from the nitrifi cation of NH 4. It doesn t matter whether the NH 4 comes from mineralization of SOM or indirectly from added fertilizer. Keep in mind that roughly half of the N a plant takes up comes from mineralization, and the rest comes from fertilizer sources. The fi rst form of N released by mineralization that the crop can use is NH 4. However, soil microorganisms don t stop at mineralization of organic matter into NH 4. In warm soils, NH 4 is quickly converted into NO 3 through nitrification. This process is really quite complex, but simply stated, two types of bacteria oxidize NH 4 ions for energy. Nitrification depends on the presence of NH 4, the right bacteria, and adequate moisture and oxygen. Soil temperature and soil ph also influence nitrifi cation. Soil temperature For nitrifi cation to occur at a rate suffi cient to supply NO 3 for plant growth, soil must be warm enough to encourage a high degree of microbial respiration and reproduction. While some nitrifi cation does occur at soil temperatures below 40 degrees F, the process slows considerably when soil temperature drops below 50 degrees F. Figure 2.4 Temperature effects on soil nitrification rate Nitrifi cation is a biological process. Factors that affect microbial growth have a big impact on how much nitrate will be available to plants. For this reason, many agronomists advise growers to follow the 50degree rule when applying anhydrous ammonia in the fall. That is, when soil temperatures drop below 50 degrees, the nitrification process slows, so there is less risk of losing N. Figure 2.4 shows that the warmer the temperature, the higher the rate of nitrification. The Q10 rule says that the rate of nitrification doubles for every 10degree C temperature increase when temperatures are in the range of 535 degrees C (4095 degrees F). Under the right conditions particularly warm, moist soils the nitrification rate can be very high and much of the fertilizer and soilderived NH 4 can be converted to NO 3 in a matter of days. If NO 3 is available at the time of the highest risk for leaching and denitrification, major losses can occur. Soil ph Figure 2.5 shows the difference in the amount of N converted from NH 4 to NO 3 in Iowa at different soil ph levels. The study, conducted in late April, shows that the soil bacteria involved in this transformation are very sensitive to soil ph. Therefore, soil ph has a strong influence on the rate at which microorganisms convert NH 4 to NO 3. Generally speaking, the higher the soil ph, the higher the nitrification rate. In Iowa, calcareous soils containing free calcium carbonate can have soil ph values as high 8.2. These soils can have a much higher rate of nitrification than soils with a ph of 6.0 or less. Figure 2.5 Effect of soil ph on nitrification of fall applied anhydrous ammonia Nitrogen cycle 6
Denitrification in soils Denitrifi cation, depicted in Figure 2.6, is a microbial process that reduces Ncontaining compounds in the soil to their simplest forms. Nitrifi cation makes N available to the plant, but it is also one of the primary ways in which N is lost. Denitrifi cation occurs when soil microorganisms use the oxygen from the nitrate ion (NO 3 ) rather than from air in the soil for respiration. This happens most often when soils are saturated and temperatures are warm enough to encourage a high level of microbial activity. Both fertilizer and organic sources of nitrogen are affected. A soil does not have to be completely saturated for denitrifi cation to occur. When it does occur, nitrous oxide gas (N 2 O) and other N gases can be produced and lost into the atmosphere. Warm wet conditions can result in large losses of NO 3 as a gas that is lost from the soil. While NO 3 is an important form to the plant, signifi cant losses can occur under wet conditions. el. The result is that multiple, varying microenvironments occur within the same soil. In a wet soil, there are pores that have fi lms of water around the edges and air in the middle. In this case, it s possible to have both mineralization and denitrifi cation occurring at the same time. Saturated soil conditions restrict oxygen movement from the air above the ground into the soil. Some oxygen is present in the water and Figure 2.6 The denitrification process micropockets in the soil. A single rain event seldom creates an oxygenlimiting condition in the soil. However, in low areas where water ponds after a rain, soils may become saturated. The longer the soil is saturated, the more severe the oxygen depletion will be. While relatively little denitrifi cation occurs during the fi rst day or two of soil saturation, the denitrifi cation rate increases as the saturated soil condition continues. And the longer a soil with NO 3 sources present is saturated, the higher the denitrifi cation rate will be. Major losses can occur after four or fi ve days of saturated conditions if a signifi cant amount of N is in the NO 3 form. While mineralization is restricted in saturated soil conditions, these same conditions also tend to increase denitrification. Because mineralization and denitrifi cation are caused by microorganisms, they occur at a microscopic lev Mineralization Nitrifi cation Denitrifi cation Nitrogen cycle 7
Table 2.1 Factors affecting denitrification in the soil Factors Conditions Effects and Implications Nitrate availability Soil moisture Oxygen availability Soil carbon Soil temperature Soil ph Nitrate must be present for denitrification. There must be adequate moisture for soil microorganisms to live and reproduce. Reduced oxygen availability increases the rate of denitrification. This is because the microorganisms use oxygen from nitrate ions to get their energy when adequate oxygen is not available elsewhere. In addition to nitrate, carbon is also needed for the bacteria to function. There is an optimal soil temperature for the bacteria to function. Warmer soils result in higher denitrification rates if other conditions are also favorable. Because the process is biological, there is an optimal soil ph range that limits bacterial action on N. Higher levels of nitrate will increase the amount of denitrification. Preventing or reducing the formation of nitrate is a key management strategy for reducing loss by denitrification. The indirect effect of high soil moisture is usually a decrease in soil oxygen concentration. Because of the large range of pore sizes and water pockets in a soil, a lack of oxygen can occur when a soil is not completely saturated. Standing water does not mean there is a lack of oxygen in the soil. It does mean that oxygen is is likely to become limited. The rate of denitrification will be lower in the first two days that a soil is saturated than on the third day or after. When the oxygen dissolved in the water or trapped in pores has been used, microorganisms use oxygen from nitrate ions in the soil. Added carbon increases the demand for oxygen by soil microorganisms, so carbon additions (manures) can induce denitrification where it would not otherwise occur. There is little microbial activity while the ground is frozen. As the soil warms, the denitrification rate will increase. Denitrification rates will be much higher in a June timeframe than a March timeframe with similar wet conditions because of the difference in soil temperature. In most Iowa soil conditions, higher ph will increase the rate of denitrification, but ph is less of a factor than oxygen availability and soil temperature. Nitrogen cycle 7
Nitrate leaching Leaching is the term used to describe the loss of water soluble nutrients from the soil. As nitrogen is transformed into nitrate (NO 3 ) in the soil, it becomes highly susceptible to loss with any water movement through the soil profi le. While some losses (i.e., denitrifi cation) are dependent on biological processes, leaching (shown in Figure 2.7) depends on chemical and physical processes. It occurs when water fi lls the much of the pore space in the soil. When that happens, water either moves downward into the subsoil or fl ows laterally (often through tile lines)into surface water. NO 3 in the soil is readily absorbed into water, so it moves along with the excess water in the soil. NO 3 leaching can occur when soils are too cold for biological activity, but still permit water movement. NO 3 can be mineralized throughout the fall and winter when temperatures are warm enough to permit bacterial activity. This NO 3 can be leached in the late fall, winter and early spring when conditions are right for water movement. The reason NO 3 is more easily leached than ammo nium (NH 4 ) is because NO 3 and soil both have a negative chemical charge and so repel each other. On the other hand, NH 4, with its positive charge, is attracted to soil particles. The result is a much higher movement of NO 3 than NH 4 as water moves through the soil. Like NH 4, other elements such as phosphate and potash are also generally chemically bound to the soil. While they do not leach as easily as NO 3, they can be lost if soil particles are moved off the fi eld, through erosion. If water moving through the soil is intercepted by a drainage tile, as shown in Figure 2.8, the NO 3 it carries may end up in streams which feed into lakes or rivers and eventually into the ocean. From Iowa, the last stop is in the Gulf of Mexico. Leaching is a major source of N loss in much of the Midwest, where rainfall, especially at some times during the year, is more than suffi cient to allow leaching, and there are extensive tile drainage networks to remove excess water from fi elds. Figure 2.7 Nitrate movement with water through the soil Figure 2.8 Nitrate leaching from a soil NH4 Soil NO3 NH4 NO3 NH4 Denitrifi cation Nitrifi cation NH4 NO3 Mineralization Leaching NH4 NO3 Nitrogen cycle 8
Carbon content and immobilization Previously we discussed how N is released from organic matter and converted into a form that plants can take up. The opposite process, converting inorganic N to organic N, is called immobilization. Immobilization (see Figure 2.9) occurs when N that is available for plant growth, either mineralized from soil organic matter or applied as fertilizer, is used by microorganisms or noncrop plants, and so is no longer available for crop production. Because immobilization is dependent on biological processes, primarily microbial growth, the conditions for microbial growth heavily infl uence the immobilization rate. While temperature and moisture affect the rate of immobilization, the carbon to nitrogen ratio (C:N) is also important. For microorganisms to grow and break down organic matter, they need a food source that contains both C and N in a specifi c ratio. When there is more C available in relationship to N in the soil, the microorganisms will use available N from the soil to help break down the high C residue. This means that adding crop residue to soil where N is limited is likely to initially reduce N availability because the microorganisms working to decompose the organic matter require N to break down the C. Eventually, most of the N from both the soil and the plant residues will be released and available to the plant. Figure 2.9 The nitrification process in soil Immobilization is the opposite of mineralization. Adding high amounts of carbon from manure or crop residue with little N can delay the breakdown of the organic matter and actually decrease the amount of available N initially. Although it varies somewhat with the type of organisms present in the soil, organisms decomposing residues with a C:N ratio of 30:1 will need other sources of N to stimulate decomposition. When the C:N ratio is 15:1 or lower, decomposition will typically result in a rapid release of N without tying up additional N. Corn stalks have a C:N ratio high enough that N immobilization will occur initially in the decomposition process. Wheat and oat straw have a higher C:N ratio than cornstalks, resulting in a longer period of immobilization. The C:N ratio of a crop residue can be a major factor in crop rotation. As microorganisms consume the carbon from plant residue, the C:N ratio will become increasingly more favorable for net mineralization to occur. From a practical point of view, the C:N ratio in organic material affects how fast mineralization will occur, rather than whether it will occur. Finally, it is important to realize that soil is not uniform and that areas of both immobilization and mineralization typically can occur at the same time within the soil matrix. Immobilization Mineralization n Nitrifi cation Immobilization Nitrogen cycle 9