Nitrogen in the soil: Additions, losses, and the impact on crops

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Whether trying to determine the amount of nitrogen (N) trapping or additions from cover crops, the N credit from a legume crop, or how to make an N recommendation based on soil test results for a given crop and yield goal, there are certain principles of soil N that must always be considered. The first thing to understand is that there are three primary pools of nitrogen (N) present in soils:

1. An inorganic N pool comprised primarily of ammonium and nitrate. The inorganic N pool of ammonium and nitrate is readily available to crops. This pool is primarily the result of mineralization of organic residues and soil organic matter, along with residual or “left-over” N from fertilization. It also will contain N added through atmospheric deposition, and fixed N falling to earth with rain as the result of lightning passing through the atmosphere.

2. A dynamic pool of organic materials, including living organisms, plant roots and crop residues which are actively undergoing decomposition and transformation. The end products of these transformations include CO2, NH4, (and other nutrients such as phosphate and sulfur) and true SOM. The dynamic N pool breaks down fairly slowly, taking from as little as a few weeks to 4 or 5 years to fully decompose, depending on the relative carbon and nitrogen content of the materials added to that pool each year. As this material decomposes, N can be mineralized quickly, if the residues have a low C:N ratio. This is the source of the N accounted for in the “previous crop credit” which is included in the N recommendation from soil test laboratories. 

When plant residue is high in N, such as with alfalfa roots and crowns, the residue breaks down quickly and N begins to be released almost immediately. This release may be sizeable and can continue for two or three years, giving a second or even third year credit for the alfalfa. Soybeans are intermediate, with most of the N released in the first year and little additional release in subsequent years. 

However when the residue is very low in N, with a high C:N ratio -- such as corn, sorghum or sunflower stalks, or wheat straw -- the limiting factor to decomposition is N, or protein, to expand the microbial population so they can take advantage of all the carbon. Any N released will be used to produce more microbial biomass as long as large amounts of carbon “fuel” is present. Over time the C will be used as energy for the microbes and will eventually be depleted. At that point N will be released as the microbial population dies back.

3. A stable, recalcitrant pool of organic materials produced from the decomposition of plant and animal residue is referred to as soil organic matter (SOM). The stable or recalcitrant pool, true soil organic matter, is only slowly available, with perhaps 1 to 2% of the N present becoming available each year. This amounts to about 20 pounds of available N added to the inorganic pool per 1% SOM per year. This 20 pound credit for each one percent of SOM is used by K-State when determining N recommendations for summer crops. But only half that much, or 10 pounds N for each one percent of SOM, is given as a credit for fall or winter crops such as wheat or canola. This is due to the slower rate of SOM decomposition in fall, winter, and early spring, when wheat is actively taking up N.

Additions of N to the soil system

There are several ways N is added naturally to soil systems. These include N fixation by lightning, and biological N fixation by both free living organisms and legumes. In our native prairie ecosystems this can account for 20 to 40 pounds of N per acre per year. In modern managed systems we also gain N through acid deposition from burning fossil fuels, adding N fertilizers, growing leguminous crops such as alfalfa and soybeans, and applying waste materials, especially animal manure on farm lands. The total additions of N to intensively managed farmland can exceed 250 pounds N per acre per year when manure is involved.

Losses of N from the soil system

N is also lost from cropland soils each year, even in Kansas. Key loss mechanisms include:

* Leaching of nitrate through the soil, a key issue on sandy soils, especially with irrigation.

* Denitrification, the biological conversion of nitrate under anaerobic conditions to various forms of gaseous nitrogen oxide products. This process can ultimately result in nitrous oxide and nitrogen gas. This is the primary nitrogen loss mechanism on poorly drained, heavy textured or claypan soils. 

* Ammonia volatilization from surface applied urea or manure, a concern at high soil pH or in high residue no-till systems.

* Soil erosion

We also can’t forget crop removal, which is generally the largest N loss annually from an agricultural system, especially when we harvest forage crops or corn grain.

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