nitrogen cycle

nitrogen cycle

the natural circulation of nitrogen by living organisms. Nitrates in the soil, derived from dead organic matter by bacterial action (see nitrification, nitrogen fixation), are absorbed and synthesized into complex organic compounds by plants and reduced to nitrates again when the plants and the animals feeding on them die and decay

Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005

Nitrogen cycle

The collective term given to the natural biological and chemical processes through which inorganic and organic nitrogen are interconverted. It includes the process of ammonification, ammonia assimilation, nitrification, nitrate assimilation, nitrogen fixation, and denitrification.

Diagram of the nitrogen cycle

Nitrogen exists in nature in several inorganic compounds, namely N₂, N₂O, NH₃, NO₂^-, and NO₃^-, and in several organic compounds such as amino acids, nucleotides, amino sugars, and vitamins. In the biosphere, biological and chemical reactions continually occur in which these nitrogenous compounds are converted from one form to another. These interconversions are of great importance in maintaining soil fertility and in preventing pollution of soil and water.

An outline showing the general interconversions of nitrogenous compounds in the soil-water pool is presented in the illustration. There are three primary reasons why organisms metabolize nitrogen compounds: (1) to use them as a nitrogen source, which means first converting them to NH₃, (2) to use certain nitrogen compounds as an energy source such as in the oxidation of NH₃ to NO₂^- and of NO₂^- to NO₃^-, and (3) to use certain nitrogen compounds (NO₃^-) as terminal electron acceptors under conditions where oxygen is either absent or in limited supply. The reactions and products involved in these three metabolically different pathways collectively make up the nitrogen cycle.

There are two ways in which organisms obtain ammonia. One is to use nitrogen already in a form easily metabolized to ammonia. Thus, nonviable plant, animal, and microbial residues in soil are enzymatically decomposed by a series of hydrolytic and other reactions to yield biosynthetic monomers such as amino acids and other small-molecular-weight nitrogenous compounds. These amino acids, purines, and pyrimidines are decomposed further to produce NH₃ which is then used by plants and bacteria for biosynthesis, or these biosynthetic monomers can be used directly by some microorganisms. The decomposition process is called ammonification.

The second way in which inorganic nitrogen is made available to biological agents is by nitrogen fixation (this term is maintained even though N₂ is now called dinitrogen), a process in which N₂ is reduced to NH₃. Since the vast majority of nitrogen is in the form of N₂, nitrogen fixation obviously is essential to life. The N₂-fixing process is confined to prokaryotes (certain photosynthetic and nonphotosynthetic bacteria). The major nitrogen fixers (called diazotrophs) are members of the genus Rhizobium, bacteria that are found in root nodules of leguminous plants, and of the cyanobacteria (originally called blue-green algae).

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.

nitrogen cycle

[′nī·trə·jən ‚sī·kəl]

(nuclear physics)

carbon-nitrogen cycle

McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.