Nitrogen Cycle

The term nitrogen cycle refers to biologists' attempts to locate and quantify the movement of the chemical element nitrogen, crucial (along with other elements in limited biological supply) to the survival of organisms, whose reproductive systems require nitrogen to function. Nitrogen, coming up through volcanoes and produced by life, and forming compounds with hydrogen and oxygen and other elements under the influence of light, is needed by cells to make proteins as well as the DNA and RNA molecules central to cell reproduction. But unlike, say, carbon, nitrogen is not readily assimilable by organisms. Although our atmosphere is mostly (79 percent) nitrogen, it cannot be directly accessed by most organisms. Intermediaries are thus necessary.

Some high-energy phenomena (ionizing cosmic radiation, lightning, and meteor trails) can combine nitrogen atoms with hydrogen or oxygen to make biologically assimilable molecules, but the lion's share of such work (collectively called nitrogen fixation) is done by marine organisms and soil bacteria, alone and as nodules in the roots of plants. The result is that fixed nitrogen is a valuable commodity, exploited by organisms that incorporate it into their bodies, releasing some (for example, in urea) as wastes that wend in complex bios-pheric cycles involving ammonium, nitrites, nitrates, nitrous oxides, and other compounds. Indeed, such a valuable commodity is nitrogen that human activity has significantly added to global nitrogen fixation—first by the planting of legumes (which contain nitrogen-fixing bacteria in their roots) and then by industrial methods. It is a rather astonishing statistic, but human, factory-based fixation of nitrogen means that, for a person living in Europe or the United States today, some 40 percent of the nitrogen atoms in your body have seen, at some point, the inside of a factory; if you live in China, an estimated 75 percent of the nitrogen atoms in your body come from a factory.

—Lynn Margulis and Dorion Sagan

See also: Agriculture and Biodiversity Loss: Genetic Engineering and the Second Agricultural Revolution; Agriculture and Biodiversity Loss: Industrial Agriculture; Atmosphere; Atmospheric Cycles; Carbon Cycle; Evolution; Five Kingdoms of Nature; Food Webs and Food Pyramids; Lichens; Microbiology; Nutrient/Energy Cycling; Pollution; Protoctists; Soil; Topsoil Formation

Bibliography

Rambler, Mitchell B., Lynn Margulis, and Rene Fester, eds. 1989. "Global Ecology: Towards a Science of the Biosphere." Boston: Academic; Sagan, Dorion, and Lynn Margulis. 1993. Garden of Microbial Delights: A Practical Guide to the Subvisible World. Dubuque, IA: Kendall/Hunt.

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