Nitrogen (N) is one of the most critical elements in all living organisms, and is often one of the most limiting nutrients in crop production. In addition, N is perhaps the most dynamic nutrient in the soil, and a number of factors can influence the amount, form and plant availability of N.
N can exist in a number of different forms in the soil. However, N is only available to plants as ammonium (NH4+) or as nitrate (NO3-). Both NH4+ and NO3- are inorganic forms of N, which simply means that they are not contained in an organic, or carbon based compound.
Organic materials, such as plant or animal remains or soil organic matter, contain N as an integral component of their structure. Much of this N is contained within organic compounds, and is therefore not plant available. However, when these materials are broken down by natural chemical and biological processes, much of the N is converted into an inorganic form and released into the soil solution.
The breakdown of organic N into inorganic N, a process called mineralization, is accomplished by soil microorganisms. These microorganisms consume the residues for energy and, in the process, release N as inorganic N compounds such as ammonium. Mineralization is affected by a number of different factors, including soil temperature and moisture content, that affect the rate at which these soil microorganisms function.
One of the most critical factors is the composition of the organic residue. The amount of N in a residue can vary widely from one material to another, and this has a strong bearing on not only the amount of N released but also on the rate at which it will be released from the residue. The amount of carbon (C) relative to the amount of N in a material is expressed as the Carbon to Nitrogen, or C:N ratio of the material. Materials with a high C:N ratio, such as wheat straw, will have a tendency to break down slower than materials with a low C:N ratio, such as soybean stubble. In addition, materials with a high C:N ratio may temporarily decrease the amount of inorganic N in the soil in order for the soil microorganisms to have adequate N to function, a process called N immobilization.
The release of N from organic residues is a critical process in natural systems. While we generally consider this to be a less important process in agricultural systems because of our use of fertilizer N, most agricultural crops can still receive a large portion of their total N needs through this process. Therefore, understanding the basics of N release from organic residues is critical for successful fertility management.