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

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.