Since the time of life's origin, the atmosphere and oceans have contained nitrogen. This component of all proteins and nucleic acids moves in an atmospheric cycle called the nitrogen cycle. Gaseous nitrogen (N2) makes up about 80% of the atmosphere --- the largest nitrogen reservoir. Triple covalent bonds hold its two atoms together, and few organisms can break the bonds. Only certain bacteria, volcanic action, and lightning can convert N2 into forms that can enter food webs.

Nitrogen is often the scarcest of all nutrients required for plant growth. Today, nearly all nitrogen in soils has been put there by nitrogen-fixing organisms. Ecosystem lose it through the activities of bacteria that 'Unfix" the fixed nitrogen. Land ecosystems lose more through leaching of soils, although this is the basis of nitrogen inputs to aquatic ecosystems such as streams, lakes, and the oceans.

source: (Biology Fundamentals, John Wiley & Sons, Inc. 1995)