The nitrogen cycle is the series of biogeochemical transformations in which the element nitrogen is transferred among organisms and nonliving reservoirs such as the soil, the oceans, and the atmosphere. Nitrogen is an essential element for all living things because it is a principal component of proteins and nucleic acids. Whereas animals generally have access to abundant nitrogen, it is often in short supply for plants.
Most of the world's nitrogen is in the atmosphere, in the form of nitrogen gas (N 2 ), which is extremely unreactive. Atmospheric nitrogen is ammonified, or converted to ammonia (NH 3 ) or ammonium ion (NH 4 + ), in several ways. It occurs through enzymatic nitrogen fixation, which is carried out by either free-living or symbiotic bacteria; through lightning, volcanic eruptions, and other high-energy events in the atmosphere; and finally by industrial processes. Industrial ammonification, which requires large amounts of energy, is used to create ammonia and nitrate for use as agricultural fertilizer. Industrial processes convert approximately 80 million metric tons of nitrogen per year, whereas bacterial nitrogen fixation converts slightly more, with about half of that carried out by crop plants. Ammonia is also produced through the action of fungus and bacteria breaking down organic compounds in the soil.
Nitrification, Denitrification, and Assimilation
Aerobic soil bacteria convert ammonia and ammonium to nitrate (NO 3 - ), which can be absorbed by plants. This process, called nitrification, is counterbalanced by denitrification, which forms N 2 and N 2 O, carried out by anaerobic bacteria. Nitrate is assimilated, or absorbed by plants, through their roots. Within the plant, nitrate is reconverted to ammonium for use in building organic compounds. Nitrogen moves through the food chain in these compounds and is eventually returned to the environment through urine, feces, or the decomposition of the organism.
Human nitrogen use has had a major impact on the nitrogen cycle. The agricultural use and overuse of nitrogen fertilizer has caused pollution of water bodies both near farms and more distantly. Nitrate in the soil is easily washed out and can become a pollutant of both groundwater and surface water. Nitrogen is usually a limiting nutrient in aquatic ecosystems , and therefore its runoff often produces overgrowth, or "eutrophication." Chronic eutrophication can change species composition of lakes, streams, and rivers.
Berner, Elizabeth Kay, and Robert A. Berner. Global Environment: Water, Air, and Geochemical Cycles. Upper Saddle River, NJ: Prentice Hall, 1996.
Human Alterations of the Global Nitrogen Cycle. <http://esa.sdsc.edu/tilman.htm> .