Biochemistry and Molecular Biology - Belyasova N.A. 2002

Metabolism. Processes Requiring Energy Input
Metabolism of Nitrogen-Containing Compounds
Nitrogen Cycle in Nature

In nature, inorganic forms of nitrogen exhibit considerable diversity, including molecular nitrogen—which accounts for the vast majority of the biosphere's nitrogen reserves—oxidized forms (such as nitrates, nitrites, and various nitrogen oxides), and reduced forms (ammonia and ammonium salts). In Organic compounds, nitrogen is predominantly found in the amino groups of Amino Acids. Figure 16.1 illustrates the Main Pathways of nitrogen transformation across its various forms.

It is readily apparent that animals utilize only organic nitrogen, which enters the body through food. The reserves of organic nitrogen are replenished by plants and microorganisms capable of reducing oxidized forms of nitrogen and incorporating them into amino groups. However, oxidized forms of nitrogen are easily leached from the soil into seas and oceans by precipitation and are additionally reduced by denitrifiers into nitrogen oxides and N₂, which escape into the atmosphere. Consequently, the planet's reserves of fixed nitrogen would soon be depleted were it not for the continuous processes of MOLECULAR Nitrogen Fixation. This vital process is carried out partly by human industrial production of nitrogen fertilizers—a highly expensive and energy-intensive Procedure—and partly by nitrogen-fixing Bacteria, which include both free-living forms and those that establish symbiotic relationships with plants. A small quantity of oxidized nitrogen forms is also produced in the atmosphere by lightning discharges and cosmic radiation. Thus, enzymatic fixation of molecular nitrogen serves as the primary natural mechanism for replenishing fixed nitrogen reserves, thereby sustaining life on Earth.

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Fig. 16.1. The Nitrogen Cycle in nature: 1 — industrial nitrogen fixation; 2 — activity of free-living and symbiotic nitrogen fixers; 3 — physical forces of nature