The nitrogen cycle is a biogeochemical cycle. It describes the transformations of nitrogen and nitrogen containing compounds in nature.
The major source of nitrogen is air, of which it forms about 78 percent by volume. Nitrogen is essential for many biogical processes, for example it is included in all amino acids (the stem amin derives from ammonia) incorporated in to proteins and is present in the four bases that make up DNA. Processes are necessary to convert gaseous nitrogen in to forms usable by living organisms.
All nitrogen is obtained by animals through eating plants at some stage down the food chain. Plants get nitrogen from the soil by absorbition at their roots in the form of either nitrate ions (NO2-) or ammonia (NH3). Ammonia is produced in the soil by nitrogen fixing organisms such as Azobacter vinelandii which produces the enzyme nitrogenase. Nitrogenase combines gaseous nitrogen with hydrogen to produce ammonia. Some nitrogen fixing bacteria, such as Rhizobium, live in root nodules of leguminous plantss (such as peas or beans). Here they form a symbiotic relationship with the plant, producing ammonia in exchange for supplies of carbohydrate. Low nutrient containing soils can be planted with leguminous plants to enrich them with nitrogen. Rotation of crops with leguminous plants in order to fertilize soil was pioneered by George Washington Carver.
Another source of ammonia is the decomposition of dead organic matter by saprophytic bacteria called decomposers, which produce ammonium ions (NH4+). In well oxygenated soil, these are then oxygenated first by bacteria such as Nitrosomonas in to nitrites (NO2-) and then by Nitrobacter in to nitrates. This conversion of ammonia in to nitrates is called nitrification.
Ammonium ions can bind to soils and clays, however nitrates, due to their negative charge, cannot. They are also very soluble in water and after heavy rain leaching (draining) in to rivers can occur. Leaching of large amounts of nitrates results in eutrophication, a process leading to the death of river life, can occur. Abuse of nitrate and phosphate containing fertilizers has caused severe problems in some rivers and in Britain their use has been restricted. In the presence of anaerobic (low oxygen) conditions in soils, denitrification by bacteria such as Thiobacillus denitrificans can happen. This is the reverse of nitrification and results in nitrates being converted to nitrogen gas and lost to the atmosphere.