Nitrogen is found throughout the soils and atmosphere in many different, organic and inorganic, forms. The Nitrogen Cycle is the process by which atmospheric nitrogen enters the soil, is transformed by microbes, and re-enters the atmosphere (volatilization) and plants (assimilation).
How Does Nitrogen Enter the Soil?
Before nitrogen can be used by plants, it must enter the soil. Atmospheric nitrogen is forced to the ground by rainfall. Also, urine, solid and liquid waste from living organisms and living organisms that have died are deomposed by bacteria and fungi. The nitrogen from these sources then enter the soil. Commercial fertilizers are another source of nitrogen.
What Happens to Nitrogen in the Soil?
Plants cannot use organic nitrogen. Bacteria and fungi are needed to transform this unusable organic nitrogen into a usable form. Although most nitrogen fixation is completed by bacteria, some is accomplished through lightning strikes. Since ammonia is fatal to most plants, bacteria convert this ammonia (NH4) into nitrates (NO3) and nitrites (NO2). At this time, the nitrogen can be assimilated into the plant, leached into the ground water or be transformed into a gas and re-enter the air.
In very wet soils, the oxygen content is low. The bacteria in these soils take the oxygen out of the nitrates (NO3) and produce nitrogen gas. This process is call denitrification. Through a process called volatilization, the gas re-enters the atmosphere.