Nitrogen Cycle

Atmospheric N₂ is very abundant, but this molecule is highly unreactive and cannot be assimilated by most organisms and used in their nutrition. To be useful to plants, dinitrogen must be "fixed" into inorganic forms that can be taken up by roots or leaves. Dinitrogen fixation can occur through non-biological processes. For example, during a lightning strike there is a brief occurrence of a very high temperature and high pressure, conditions that favor the combination of atmospheric N₂ and O₂ to form NO.

The nitrogen cycle. Illustration by Hans & Cassidy. Courtesy of Gale Group.

However, biological fixation of dinitrogen is typically more important. A few species of microorganisms can synthesize an enzyme, called nitrogenase, which is capable of catalytically breaking the triple bond of N₂, generating two molecules of ammonia for each molecule of dinitrogen that is reacted. Because the activation energy for this chemical reaction is rather high, it takes quite a lot of biological energy to fix atmospheric dinitrogen in this way, about 12-15 moles of adenosine triphosphate (ATP) per mole of dinitrogen that is converted. Despite the high energy costs, the dinitrogen fixation reaction is still very favorable ecologically, because access is gained to ammonia and ammonium, chemical forms of nitrogen that organisms can utilize for their nutrition. Because the nitrogenase enzyme is denatured by oxygen (O₂), the fixation reaction can only occur under anaerobic conditions, where oxygen is not present.

There are numerous species of free-living microorganisms that can fix atmospheric dinitrogen, including species of true bacteria, blue-green algae or bacteria, and actinomycetes. These microorganisms are most abundant in wet or moist environments, especially in situations where nutrients other than nitrate or ammonium are relatively abundant, for example, in rotting logs or other dead biomass, or in lakes that are well fertilized with phosphate from sewage. Such conditions are typically relatively deficient in available forms of nitrogen, and are commonly anoxic, creating obviously favorable opportunities for species of microorganisms that can utilize dinitrogen.

Some species of plants live in an intimate and mutually beneficial symbiosis with microorganisms that have the capability of fixing dinitrogen. The plants benefit from the symbiosis by having access to a dependable source of fixed nitrogen, while the microorganisms benefit from energy and habitat provided by the plant. The best known symbioses involve many species in the legume family (Fabaceae) and strains of a bacterium known as Rhizobium japonicum. Some plants in other families also have dinitrogen-fixing symbioses, for example, red alder (Alnus rubra) and certain actinomycetes, a type of microorganism. Many species of lichens, which consist of a symbiotic relationship between a fungus and a blue-green bacterium, can also fix dinitrogen.

Biological dinitrogen fixation is an ecologically important process, being ultimately responsible for most of the fixed nitrogen that occurs in the biomass of organisms and ecosystems. The only other significant sources of fixed nitrogen to ecosystems are atmospheric depositions of nitrate and ammonium with precipitation and dustfall, and the direct uptake of NO_x gases by plants.