The Nitrogen Cycle

A program of the Iowa Soybean Association

ATESFAg Technology & Environmental Stewardship Foundation

August 2010, On-Farm Network, Ankeny, Iowa 50023. Permission to reproduce for educational and instructional purposes hereby granted. All other rights reserved.

The NitrogenThe Nitrogen Cycle Cycle

II

The nitrogen cycleHow nitrogen moves in the environment

Illustration: Michael PidwirnyUsed with permission

Nitrogen cycle - 1 ATESFAg Technology & Environmental Stewardship Foundation

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 inch-

es 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 de-pends 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 mi-croorganisms.

Decomposition of plant residues is a biological pro-cess carried out by soil microorganisms, so the amount of N released from the organic matter into the soil is

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 relation-ship is the carbon-to-nitrogen (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 agricul-tural 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 reproduc-tion. This will be covered in more depth under mineral-ization and immobilization topics later in this chapter.

The dark color of the soil is due to soil organic mat-ter (SOM). Because the source of SOM is largely plant residues, the amount of SOM is indicative of the pro-ductivity of the soil. Other factors that affect the break-down of SOM can have a major impact on its concen-tration in the soil.

For example, decomposition goes on continuously in areas where soils are warm year-round, 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 topog-raphy (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 differ-ences in plant growth and in the rate of residue break-down during wet conditions that may have occurred before many of the lower, wetter areas were drained.

Nitrogen in soil organic matter

Nitrogen cycle - 2

Figure 2.1 Field map of soil organic matter content


Nitrogen cycle - 3

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 mineraliza-tion, 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 compo-nents (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 am-monium 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 (NO3

-) or ammonia (NH3).

The ammonium form can also be consumed by mi-croorganisms and become unavailable by a process called "immobilization," which is the op-posite of mineralization. (This will be dis-cussed later in this chapter.)

NH4

+ 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 re-mains in this form, it is not easily lost by leaching rainfall. While NH

4+ is available

to plants, it is readily transformed into oth-er 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

2O,

and NO, which are easily lost to moving water or into the atmosphere.

Because mineralization, the transfor-mation of N by soil bacteria, is a biological process, it can be infl uenced by a num-ber of environmental and management factors. As a practical matter, subtle dif-ferences in the environment can have a large effect on the release of N from or-ganic 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.

Mineralization in soils

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 mi-crobial 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


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 vari-ous necessities of life. C is the energy source for most of the soil organisms that decompose the SOM. How-ever, 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. Decomposi-tion rate varies with the type of organism, but usually when decomposing organic materials with a C:N ra-tio 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 read-ily 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 stimu-late 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 corn-bean and a corn-corn 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 so-called "soybean N credit" is usually considered to result from the symbiotic N fi xa-tion 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.

Factors affecting mineralizationA

nything 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 mineraliza-tion rate. Different microorganisms have different opti-mal 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 usu-ally 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, low-lying 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 short-term dry period can signifi cantly reduce mois-ture 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 differ-ences in soil moisture can result in big differences in N mineralization rates. The mineralization rate in low-lying 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 mineraliza-tion. 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 decom-pose. Because the breakdown is a microbial process, more contact between the soil and the residue will re-sult in faster decomposition. Additionally, cutting resi-due into smaller pieces (by tillage, chopping or shred-ding), 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 mi-crobial activity. Soil temperature and moisture have a big impact on the amount of organic N that will be available to plants.

Nitrogen cycle - 4ATESFAg Technology & Environmental Stewardship Foundation

Nitrogen cycle - 5

The ammonium (NH

4+) form of N is the fi rst

form available from mineralization for a crop to take up. However, soil microorganisms

dont stop there. Nitrate (NO3

-) 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 nitrifi cation. (See Figure 2.3) The NO

3- form is the most important N form to un-

derstand from a crop production perspective for a vari-ety of reasons.

Because the amount of NO3

- 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, am-monium (NH

4+) is released. Further microbial activity

transforms this NH4

+ into NO3

-, making it available for use by crops.

Most of the N taken up by plants is in the NO3

- form. Unlike NH

4+, NO

3- is very susceptible to loss by leach-

ing. This is because NO3

- is negatively charged. Be-cause soil also has a negative charge, NO

3- is not

chemically bonded to the soil. In fact, the two actually repel each other.

Of all the N forms, nitrate (NO3-) is probably

the most important for crop production. It is created by a microbial process called nitrifi -cation which is affected by soil conditions.

Nitrifi cation in soils

This means that NO3

- remains free in the soil. Be-cause 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 tile-drained 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 en-ergy. 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, NH4

+ 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 oc-

curs in a soil, the more the soil pH level will drop.

Figure 2.3 The nitrifi cation processMineralization

M


Factors affecting nitrifi cation

The process of converting the ammonium (NH

4+)

form of N to the nitrate (NO3

-) form is called nitri-fi cation. Because the conversion process is driv-

en 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 NH4

+. Unless added to the soil as commercial fertilizer that

is in the NO3

- form, the NO3

- used by plants comes from the nitrifi cation of NH

4+. It doesnt matter whether the NH

4+

comes from mineralization of SOM or indirectly from add-ed 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 dont

stop at mineralization of organic matter into NH4

+. In warm soils, NH

4+ is quickly converted into NO

3- through nitrifi ca-

tion.This process is really quite complex, but simply stated,

two types of bacteria oxidize NH4

+ ions for energy. Nitrifi -cation depends on the presence of NH

4+, the right bacte-

ria, and adequate moisture and oxygen. Soil temperature and soil pH also infl uence nitrifi cation.

Soil temperatureFor 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.

Nitrifi cation is a biological process. Factors that affect microbial growth have a big im-pact on how much nitrate will be available to plants.

For this reason, many agronomists advise growers to follow the 50-degree rule when applying anhydrous am-monia in the fall. That is, when soil temperatures drop be-low 50 degrees, the nitrifi cation process slows, so there is less risk of losing N.

Figure 2.4 shows that the warmer the temperature, the higher the rate of nitrifi cation. The Q10 rule says that the rate of nitrifi cation doubles for every 10-degree C tem-perature increase when temperatures are in the range of 5-35 degrees C (40-95 degrees F). Under the right condi-tions particularly warm, moist soils the nitrifi cation rate can be very high and much of the fertilizer and soil-de-rived NH

4+ can be converted to NO

3- in a matter of days. If

NO3

- is available at the time of the highest risk for leaching and denitrifi cation, major losses can occur.

Soil pHFigure 2.5 shows the difference in the amount of N con-verted from NH

4+ to NO

3- in Iowa at different soil pH lev-

els. 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 infl uence on the rate at which microorganisms convert NH

4+ to NO

3-.

Generally speaking, the higher the soil pH, the higher the nitrifi cation rate.

In Iowa, calcareous soils containing free calcium car-bonate can have soil pH values as high 8.2. These soils can have a much higher rate of nitrifi cation than soils with a pH of 6.0 or less.

Nitrogen cycle - 6

Figure 2.5 Effect of soil pH on nitrifi cation of fall applied anhydrous ammonia

Figure 2.4 Temperature effects on soil nitrifi cation rate


Nitrogen cycle - 7

Denitrifi cation, depicted in Figure 2.6, is a mi-crobial process that reduces N-containing 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 af-fected.

A soil does not have to be completely saturated for denitrifi cation to occur. When it does occur, nitrous ox-ide gas (N

2O) and other N gases can be produced and

lost into the atmosphere.

Saturated soil conditions restrict oxygen movement from the air above the ground into the soil. Some oxygen is present in the water and micro-pockets in the soil. A single rain event seldom creates an oxygen-limiting condi-tion 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 oc-curs during the fi rst day or two of soil satu-ration, 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.

Whi le minera l i za t ion i s res t r i c t -ed in saturated soil conditions, these same conditions also tend to increase denitrification. Because mineralization and denitrifi cation are caused by microor-ganisms, they occur at a microscopic lev-

Denitrifi cation in soilsWarm wet conditions can result in large loss-es of NO3

- as a gas that is lost from the soil. While NO3

- is an important form to the plant, signifi cant losses can occur under wet condi-tions.

el. The result is that multiple, varying microenviron-ments 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, its possible to have both mineralization and denitrifi cation occurring at the same time.

Figure 2.6 The denitrifi cation process

Mineralization

M

Nitrifi cation

Den

itrifi

catio

n


Table 2.1 Factors affecting denitrifi cation in the soil

Nitrogen cycle - 7

Factors Conditions Effects and Implications

Nitrate availability Nitrate must be present for denitrification.

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.

Soil moisture There must be adequate moisture for soil micro-organisms to live and reproduce.

The indirect effect of high soil moisture is usually a decrease in soil oxygen concentration.

Oxygen availability Reduced oxygen availability increases the rate of deni-trification. This is because the microorganisms use oxygen from nitrate ions to get their energy when adequate oxygen is not available elsewhere.

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, micro-organisms use oxygen from nitrate ions in the soil.

Soil carbon In addition to nitrate, carbon is also needed for the bacteria to function.

Added carbon increases the demand for oxygen by soil microorganisms, so carbon additions (manures) can induce denitrification where it would not otherwise occur.

Soil temperature There is an optimal soil temperature for the bacteria to function. Warmer soils result in higher denitrification rates if other conditions are also favorable.

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 time-frame than a March time-frame with similar wet conditions because of the difference in soil temperature.

Soil pH Because the process is biological, there is an optimal soil pH range that limits bacterial action on N.

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 - 8

Nitrate leachingL

eaching is the term used to describe the loss of water soluble nutrients from the soil. As ni-trogen is transformed into nitrate (NO

3-) in the

soil, it becomes highly susceptible to loss with any wa-ter movement through the soil profi le.

While some losses (i.e., denitrifi cation) are depen-dent on biological processes, leaching (shown in Fig-ure 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 down-ward 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.

NO3

- 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 bacte-rial activity. This NO

3- can be leached in the late fall,

winter and early spring when conditions are right for water movement.

The reason NO3

- is more easily leached than ammo-

Figure 2.7 Nitrate movement with water through the soil

NO3-

NO3-

NH4+

NO3-

NH4+

NH4+

NH4+

NH4+

NO3-

Soil

nium (NH4

+) is because NO3

- and soil both have a neg-ative 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

NH4

+ 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 dur-ing the year, is more than suffi cient to allow leaching, and there are extensive tile drainage networks to re-move excess water from fi elds.

Mineralization

M

Nitrifi cation

Den

itrifi

catio

n

Leaching

Figure 2.8 Nitrate leaching from a soil


Figure 2.9 The nitrifi cation process in soil

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 immobi-lization.

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 non-crop plants, and so is no longer available for crop production.

Because immobilization is dependent on biologi-cal processes, primarily microbial growth, the condi-tions for microbial growth heavily infl uence the immo-bilization 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 lim-ited 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.

Immobilization is the opposite of mineraliza-tion. Adding high amounts of carbon from manure or crop residue with little N can de-lay the breakdown of the organic matter and actually decrease the amount of available N initially.

Carbon content and immobilization

Nitrogen cycle - 9

Although it varies somewhat with the type of organ-isms present in the soil, organisms decomposing resi-dues 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 im-mobilization 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 immobi-lization. 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 prac-tical 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 uni-form and that areas of both immobilization and miner-alization typically can occur at the same time within the soil matrix.

Mineralization

Nitrifi cation

Immo

bilizat

ion

nnn

ImmobilizationMI


The Nitrogen Cycle

Documents

total soil n

soil microorganisms

percent of soil organic

inches of soil

soil biology

n ratio

n release

n source