Myrold, David D.; Ruess, Roger W.; Klug, Michael J. 1999. Dinitrogen fixation. In: Robertson, G. Philip; Coleman, David C.; Bledsoe, Caroline S.; Sollins, Phillip, eds. Standard soil methods for long-term ecological research. New York, NY: Oxford University Press: 241-257.
Biological N2 fixation is the conversion of atmospheric N2 to NH3 by bacteria. The reduction of the triple bond of N2 to form NH3 is performed under anaerobic conditions by the nitrogenase enzyme complex at the expense of 12-16 ATP with H2 as a by-product. The NH3 produced by dinitrogen fixation is subsequently as similated into organic forms of nitrogen by standard metabolic pathways. This input of organic nitrogen from N2 fixation is often the largest input to the nitrogen cycle of nonfertilized soil ecosystems.
The bacteria that fix N2 are taxonomically diverse. They differ in their source of energy (some are phototrophs, but most are chemotrophs), carbon (some are autotrophs, but most are heterotrophs), their tolerance for oxygen, and the degree of their association with other organisms. This latter characteristic is the most important with respect to the measurement of N2 fixation because it largely determines the magnitude of fixation. Free-living N2-fixing bacteria, such as Azotobacter and the cyanobacteria, typically fix
Nitrogen fixation is often measured as part of an ecosystem's nitrogen cycle, inwhich case an annual rate of N2 fixation is desired. Estimates of N2 fixation are also made over shorter periods to assess the effects of environmental factors or treatment manipulations. These short-term measurements are often focused on understanding the controls and regulation of N2 fixation.