Ammonification

Humans have a major influence on the nitrogen cycle, especially through the use of fertilizers in agriculture. Under nutrient-limited conditions, farmers commonly attempt to increase the availability of soil nitrogen, particularly as nitrate, and to a lesser degree, as ammonium. Rates of fertilization in intensive agricultural systems can exceed 446.2 lb/ac (500 kg/ha) of nitrogen per year. The nitrogen in the fertilizer may be added as ammonium nitrate (NO₄NH₄) or as urea. The latter compound must be ammonified before inorganic forms of nitrogen are present, that is, the ammonium and nitrate that can be taken up by plants. In some agricultural systems, compost or other organic materials may be added to soils as a conditioner and fertilizer. In such cases, the organic nitrogen is converted to available ammonium through microbial ammonification, and nitrate may subsequently be generated through nitrification.

In situations where the rates of fertilization are excessive, the ability of the ecosystem to assimilate the nitrogen input becomes satiated. Although the ammonium produced by ammonification does not leach readily, the nitrate does, and this can lead to the pollution of groundwater and surface waters, such as streams and rivers. Pollution of groundwater with nitrate poses risks for human health, while surface waters may experience an increased productivity through eutrophication.