Organic nitrogen is converted to inorganic nitrogen and made available for plant growth during the waste mineralization process. This conversion process is a two way reaction that not only releases nitrogen, but also consumes nitrogen.
Agricultural waste materials, especially livestock manure that has C:N ratios shown in chapter 4, increase the energy or food supplies available to the soil microbial population. This high energy stimulates soil microbial activity, which consumes more available nitrogen than the mineralization processes release. Thus, high microbial activity during initial waste mineralization can cause a reduction of available nitrogen below that needed for plant growth. Nitrogen deficiency also occurs if the waste mineralization cannot supply sufficient quantities of nitrogen to the plants during periods of rapid growth. This is most apparent in spring as the soil warms and crops exhibit a short period of nitrogen deficiency.
Ammonium nitrogen (NH4+) is the initial by-product of organic nitrogen mineralization. Ammonium is adsorbed to soil particles through the cation exchange. It can be used by plants or micro-organisms. Ammonium nitrogen is further oxidized by nitrifying bacteria to nitrate (NO3-). This form of nitrogen is not strongly adsorbed to soil particles nor easily exchanged by anion exchange.
Nitrate forms of soil nitrogen are susceptible to leaching and can leach out of the plant root zone before they can be used for plant growth. Nitrate can contaminate if leached below the soil root zone or transported off the field by runoff to surface water. Soils that have high permeability and intake rates, coarse texture, or shallow depth to a water table are the most susceptible to nitrate contamination of ground water. Those that have low permeability and intake rates, fine texture, or steep slopes have a high runoff potential and are the most susceptible to nitrogen runoff and erosional losses.
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