Nitrogen (N) is abundant in nature. Air is 78% N; rocks of the earth’s crust have 50 times more N than the atmosphere; and the surface layer of most cultivated soils contains 1,200 to 6,000 lb. N per acre, with more than 90% in organic forms. However, most of this N is not in a form that plants can take up and must be converted to plant-available ammonium (NH4) or nitrate (NO3), or supplied from atmospheric N2 fixation by plant-microbe symbiosis or industrial fertilizer manufacture.
Nitrogen is very reactive. It can change among many forms:
- Organic, such as amino acids, proteins and chlorophyll.
- Gases, such as ammonia (NH3), dinitrogen (N2) and nitrous oxide (N2O).
- Ions, such as NH4+, nitrite (NO2–) and NO3–.
Conversion from one form to another occurs by many chemical and biological processes, which are highly influenced by environmental conditions, especially temperature and moisture. The overall interaction between soil, air, microbes, plants, animals and humans is called the N cycle.
In soils, plants and microbes interact with all components of the cycle, with many processes occurring simultaneously and all having potential influence on the fate of N. A major factor complicating N management for crop production, also relating to the importance of climate, is that the soil is an open system, meaning that N can move out of the soil (“be lost”) to the atmosphere or to ground and surface waters. If such movement did not occur, then N management would be much less complicated.
Research continues to better our understanding of the intricacies of the soil N cycle and the influence of climate, with the goal of providing management options to enhance N use by crops and therefore improve agronomic efficiency, economic profitability and environmental quality.