Nitrogen within the soil: The Nitrogen Cycle

Figure 1: The various forms of nitrogen and processes
within the nitrogen cycle. Sourced from wikipedia.org.

Written by Dr. Larry Oldham (Mississippi State Extension Service) and David Doll.

Nitrogen is in organic and inorganic forms in soils. Over 90 percent of soil N is associated with soil organic matter. Nitrogen is in compounds identifiable as part of the original organic material such as proteins, amino acids, or amino sugars, or in very complex unidentified substances in advanced stages of decomposition.

Plants may use either ammonium (NH4+), or nitrate (NO3-) which behave quite differently in soils. Positively charged NH4+ is attracted to negatively charged sites on soil particles as are other cations. It is available to plants, but the electrostatic attraction protects it from leaching. Conversely, negatively charged NO3- does not react with the predominately negatively charged soil particles, so it remains in the soil solution, moves with the soil water, and is susceptible to leaching.

Nitrogen transformations in soils/Nitrogen Cycle:
Nitrogen conversions depend on soil moisture conditions, soil acidity, temperature, and microbial activity. Ammonium is absorbed on the cation exchange complex or taken up by plants without transformation, but most likely it is converted to NH4+ soon after its formation or addition as fertilizer. This nitrification is a two step process involving two different groups of soil bacteria. First Nitrosomas bacteria produce nitrite (NO2-). Nitrobacter species then convert NO2- to NO3- soon after its formation. The carbon used by these bacteria is derived solely from atmospheric CO2.

a) 2NH4+ + 3O2 = 2NO2- + 2H2O + 4H+ + energy
b) 2NO2- + O2 = 2NO3- + energy

Two things to note: 1) NH4+ has a short residence time in soils before conversion to the more mobile NO3- form; and 2) hydrogen ions are produced which lower the soil pH.

Mineralization is the process of converting organic N to plant available inorganic forms. It is a gradual breaking down of large molecules to smaller molecules by a succession of soil microorganisms. After these microbes complete their relatively brief life cycle, they are decomposed by other microbes. Energy for this process is obtained from carbon in the material being used, so introduction of fresh plant materials stimulates breakdown activity.

Immobilization is the process of incorporating inorganic into organic form by microbes or plants. Because it is largely dependent on microbes, the availability of carbon and other nutrients determine the rate of immobilization. When residues with high carbon:N ratios are being decomposed, all readily available N within the soil system may be tied up by the microbes and therefore unavailable for plant uptake. This effect eventually fades because, without external N, the microbial population dies off and decomposes, releasing N which is available to plants. The risk of immobilization is avoided by mixing plant residues into the soil well before the next cropping cycle.

Loss of nitrogen from soils:
Nitrogen can be permanently removed from soil by crop harvest, erosion/loss of organic matter, leaching, denitrification, and/or volatilization. With the exception of crop harvest, nitrogen losses are detrimental to the environment and practices should be employed to reduce these losses. These practices will be covered in a later entry.

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