Simple Explanation of the Nitrogen Cycle

Nitrogen Cycle
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Nitrogen Cycle, natural cyclic process in the course of which atmospheric nitrogen enters the soil and becomes part of living organisms, before returning to the atmosphere. Nitrogen, an essential part of the amino acids, is a basic element of life. It also makes up 78 percent of the Earth’s atmosphere, but gaseous nitrogen must be converted to a chemically usable form before it can be used by living organisms. This is accomplished through the nitrogen cycle, in which gaseous nitrogen is converted to ammonia or nitrates. The high energies provided by lightning and cosmic radiation serve to combine atmospheric nitrogen and oxygen into nitrates, which are carried to the Earth’s surface in precipitation. Biological fixation (see Nitrogen Fixation), which accounts for the bulk of the nitrogen-conversion process, is accomplished by free-living, nitrogen-fixing bacteria; symbiotic bacteria living on the roots of plants (mostly legumes and alders); cyanobacteria (formerly known as blue-green algae); archaebacteria (also known as archaea) in deep-sea hydrothermal vents and other geothermal environments; certain lichens; and epiphytes in tropical forests.

Nitrogen “fixed” as ammonia and nitrates is taken up directly by plants and incorporated in their tissues as plant proteins. The nitrogen then passes through the food chain from plants to herbivores to carnivores (see Food Web). When plants and animals die, the nitrogenous compounds are broken down by decomposing into ammonia, a process called ammonification. Some of this ammonia is taken up by plants; the rest is dissolved in water or held in the soil, where microorganisms convert it into nitrates and nitrites in a process called nitrification. Nitrates may be stored in decomposing humus or leached from the soil and carried to streams and lakes. They may also be converted to free nitrogen through denitrification and returned to the atmosphere.

In natural systems, nitrogen lost by denitrification, leaching, erosion, and similar processes is replaced by fixation and other nitrogen sources. Human intrusion in the nitrogen cycle, however, can result in less nitrogen being cycled, or in an overload of the system. For example, the cultivation of croplands, harvesting of crops, and cutting of forests all have caused a steady decline of nitrogen in the soil. (Some of the losses on agricultural lands are replaced only by applying energy-expensive nitrogenous fertilizers manufactured by artificial fixation.) On the other hand, the leaching of nitrogen from overfertilized croplands, cutover forestland, and animal wastes and sewage has added too much nitrogen to aquatic ecosystems, resulting in reduced water quality and the stimulation of excessive algal growth. In addition, nitrogen dioxide poured into the atmosphere from automobile exhausts and power plants breaks down to form ozone and reacts with other atmospheric pollutants to form photochemical smog.