Dave Franzen, Fargo, ND

Nitrogen in soils

The atmosphere contains about 4,250 quadrillion (a million millions) tons of nitrogen (N). The earth surface (not including material below the rooting zone) contains about 300 billion tons of nitrogen. The N content of North American soils contains about 50 billion tons of nitrogen. Nitrogen is a component of all soils and is relatively high in organic materials, including coal, oil, and all organic materials, both animal and vegetable (Stevenson, 1982).

Most ecologists and agronomists refer to a nitrogen cycle. The nitrogen cycle includes the following additions, subtractions and transformations:

Additions-

Biological N fixation

Atmospheric addition through lightning

Subtractions-

Fixation of N by soil clay minerals

Leaching out of the root zone

Volatilization of N gases

Transformations

Decomposition of organic materials

Denitrification

Nitrification

Although a term like nitrogen cycle implies that all N is conserved, this is not true for the N that is in the soil and especially for available N for plants. N is conserved globally, but if it ends up in the atmosphere, or in ground or surface water, its usefulness for crop production is over and its potential to adversely affect the environment is increased.

Nitrogen, unlike carbon, tends to be conserved in soils during residue and organic matter decomposition. The carbon to nitrogen (C/N) ratio of organic matter is about 10:1, with a range from 9:1 and 12:1. Crop residues vary in C/N ratio from a high in wheat residues of between 60/1 to 100/1 to a low in alfalfa, potato and sugarbeet tops of between 9:1 to 12:1. When residues decompose, only the carbon is released as a gas normally. Carbon is respired by microorganisms and other decomposing organisms as carbon dioxide. As a rule of thumb, about one-third of the carbon in organic residues eventually becomes organic matter. The other two-thirds of the carbon is released as carbon dioxide. Residues with a C/N ratio below 30/1 tend to release N as they decompose. Residues with C/N ratio above 30/1 tend to tie up N as they decompose.

Wheat residues therefore tend to tie up N over time, while green manures and residues like alfalfa and sugarbeet tops release N to the soil.

Soil organic matter consists of many different materials. These can be categorized into fresh residue, somewhat decomposed residue that still looks like what it used to be, very decomposed residue that is not recognizable, and various stages of organic matter that is increasingly more resistant to additional decomposition. We think about organic matter is being something identifiable, but the closer one looks, the more nebulous it becomes.

Mineralization is the process of decomposition of residue that releases N from the residue in a form that is useable by plants. This form is either free ammonia or nitrate. Residues that contain free nitrate may release this nitrate very quickly if the cell walls and cell vacuole membranes are compromised early in the decomposition process. Otherwise, nitrate is produced only after ammonia is released by the decomposition process.

The mineralization process is not a one-step operation. It has been described as a "cascade", with many steps, each instituted by another organism or group of organisms. Residues are first broken down in size and structure by a variety of organisms. Macroorganisms are important in breaking down larger structures. Insects and earthworms take larger pieces, tear them, digest them and leave behind a first step in decomposition. Larger structure decomposition is also conducted by bacteria, fungi and other small organisms, as well as sunlight, freezing and thawing, chemical and physical processes.

In a prairie system, N is conserved. Residues are left on the surface and beneath the surface as a perennial root and microorganism system. Carbon decomposes slowly due to limited oxygen entering the soil and limited soil organism contact with surface residue. In a tilled system, soil biology changes. Oxygen, soil and residue is mixed together and residue decomposition is rapid.

Prior to the introduction of seeding tools and other tools that made one-pass seeding possible, one of the goals of tillage was to eliminate residue as an obstacle to a good seedbed. Tillage is very successful in accomplishing this goal. A tillage system that has been in place for many years reaches a steady-state of organic matter levels, but it is very difficult to build organic matter levels. Building organic matter levels is only possible with large yields of high residue containing crops, accompanied by large N inputs.

In a one-pass seeding system, residue eventually is decomposed, but it takes many years to complete the process. Each year more residue is added. If decomposition of the first year of residue is 40% in year one, 20% in year two and 5% the following two years, the cascade of years follows:

Remaining residue, %

Year 1 Year 2 Year 3 Year 4

60 40 35 30

60 40 35

Total 60 100 135 165

Whereas in a tilled system, each year the residue is essentially decomposed during a twelve month period, the one-pass seeding system conserves residue over a number of years. The total amount of residue remaining increases with each year of the system compared to a tilled system of similar rotation. The slower residue decomposition, the different biology under the system and the restricted oxygen in the system results in a system that can increase organic matter over time.

Nitrogen is released due to organic matter mineralization all through the growing season. Growers that fallow see evidence of these phenomena every year. It is more difficult to see mineralization and N release from organic matter in one-pass seeding. Mineralization took place in prairie soils before they were ever plowed and take place regardless of the level of organic matter in the soil. Mineralization is a natural process. With one-pass seeding, a system that tends to build organic matter, one should not think of it as "mining", but consider it as a beneficial effect of grower stewardship.

The mineralization potential of soil is difficult to quantify in a given year. Mineralization is related to moisture, aeration, temperature and the quantity and "quality" of organic matter. One percent organic matter in the top six inches of soil weighs about 20,000 lbs/acre. In a three-percent organic matter soil, there is therefore about 60,000 lb/acre of organic matter. With a 10/1 C/N ratio, that means there is about 6,000 lb N/acre in this soil. Within a small region, organic matter differences may explain or predict more closely differences in mineralization during a growing season. However, over larger regions, equating organic matter with mineralization potential is probably not valid. In the Red River Valley, a three-percent organic matter soil is probably an eroded, possibly sandy soil with low mineralization potential and higher seasonal N demand. In Dickinson, however, a three-percent organic matter soil is probably a very productive, well managed soil with high mineralization potential.

Soil tests have been developed and are being tested, with varying degrees of success, that attempt to predict the level of easily mineralizable N in soils. There are soils that for whatever reason do not respond to N fertilizer applications. Often, these are soils with a history of manure application. Sometimes these manure applications were made over one hundred years ago, but were so heavy that the effects linger until today. I suppose we can think of it as being locked in a semi-load full of cheeseburgers for a very long time. Eating even two pounds of burgers a day, it would take 20,000 days to eat through the trailer to the cab.

So how much N will be released from your soil? It will depend on your crop, moisture and temperature. The drier the soil, the less mineralization. If the moisture is good for crop production, and not too wet, mineralization will be high. If temperature is cold, mineralization will be low. If temperature is good for crop production, mineralization will be higher, but only if moisture is adequate. Finally it depends not only on the crop that is being grown, but also on the previous crops.

Nitrogen available to a crop depends on the previous residue. In explaining the source of the previous crop N credit from soybeans, Blackmer and Green (1995) showed that the source of the N release was not a release from the soybeans themselves, but the difference in N tie-up between corn residue and soybean residue. Corn residue, at yields realized in Iowa can be as great as 10,000 lb/acre. Soybean residues from a 50 bushel crop would be closer to 3,000 lb/acre. The C/N ratio of corn residue and soybean residue is surprisingly close. However, 7,000 lb/acre more corn residue than soybean residue means that N tie-up after corn is higher than following soybeans. The result is about 40 lb N/acre expressed after soybeans, not directly from the soybeans, but from organic matter mineralization. Moraghan (2002) showed in North Dakota that addition of 1,500 lb wheat residue tied up 20 lb N/acre compared to soil with no residue applied. Or looking at it a different way, 1,500 lb less residue would have resulted in a 20 lb N/acre apparent release. The difference is that one should look on many residues as modifying the effect of the organic matter mineralization, not releasing N itself.

Vanotti and Bundy (1995) studied a long-term rotation, comparing different rotations containing soybeans and found not only a nitrogen release following soybeans, but lower N mineralization occurring two years following soybeans. Their explanation was that soil biology was accelerated during and directly following soybeans, which resulted in the most easily mineralizable fraction of soil organic matter being depleted during the soybean year, reducing mineralization two years afterwards.

The result of these studies is an understanding of why the organic matter in the Morrill Plot rotational study in Illinois decreased suddenly compared to continuous corn in a corn and soybean rotation. The common misconception is that just growing an annual legume is soil building. Growing an annual legume is soil building if the grain is not harvested and the crop is used as a green manure. Growing an annual legume for grain results in lower residues than some grain crops, accelerated mineralization of organic matter and a tendency to reach a lower steady-state organic matter level than a continuous high residue crop.

However, this different view of how we used to think about annual legume crops should not be a deterrent towards growing them. It is very useful to grow a crop with a no supplemental N requirement. It is also useful because N rates to the subsequent crop is lower. Finally, the rotational benefits of lowering disease, insects and varying soil biology is also very positive.

Nitrogen will be released from soil within an individual farm based on temperature, moisture, organic matter, present and previous crops. Years ideal for crop production will be high in organic matter mineralization. Crops seeded after high residue, high C/N ratio crops will generally require higher N rates due to greater tie-up of N than crops seeded after low residues or after low C/N ratio crops. Soil tests are being evaluated that may aid in locating areas of high mineralization potential. However, the actual mineralization potential of any year will still be dependent on daily environmental conditions.

Blackmer and Green. 1995. Agronomy Journal.

Moraghan, J.T. 2002. Sugarbeet Research and Extension Reports. www.sbreb.org

Stevenson, F.J. 1982. Nitrogen in Agricultural Soils.

Vanotti and Bundy. 1995. Agronomy Journal.