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close this bookBasic Concepts in Environment, Agriculture and Natural Resources Management: An Information Kit (IIRR, 1993, 151 p.)
close this folderEcological basics
View the documentEcosystem degradation
View the documentHabitat and niche
View the documentThe food chain
View the documentBiological magnification
View the documentNitrogen cycle
View the documentSociety and the carbon-oxygen cycle
View the documentHealth consequences of environmental degradation
View the documentPopulation and the environment

Nitrogen cycle

Nitrogen cycle


· Living things need nitrogen (N) to manufacture proteins. Plant growth can be limited by a lack of nitrogen available from the soil. Too little nitrogen can also cause malnutrition in humans because many of the body's essential functions require nitrogen-containing molecules, such as proteins.

· The nitrogen cycle outlines the process in which nitrogen is converted into various forms and transported through the biosphere.

· The nitrogen gas which accounts for 78 percent of the volume of the earth's atmosphere is not usable by most plants and animals. Fortunately, the process of nitrogen fixation allows for the conversion of atmospheric nitrogen gas into forms useful to plants. This process is accomplished by soil bacteria; rhizobium bacteria living in root nodules of leguminous plants such as beans, peas and ipil-ipil; blue-green algae such as azolla found in water and soil; lightning; and, industrial manufacture of fertilizers.

· Plants convert nitrates obtained from soil water into large, nitrogen-containing molecules necessary for life and good health. Animals get most of the nitrogen-containing molecules they need by eating plants or other animals that have eaten plants. When plants and animals die, decomposers break down the nitrogencontaining molecules into ammonia gas and other compounds. Other specialized bacteria convert these into soil nitrates and nitrogen gas which is released to the atmosphere to begin the cycle again.

· Humans intervene in the nitrogen cycle in several important ways:

· Large quantities of Nitrous oxide (NO) and Nitrous dioxide (NO2) are added to the atmosphere when fossil fuels are burned in power plants and vehicles. These nitrogen compounds react with other chemicals in the atmosphere to form smog and acid rain, endangering the health of humans.

· Nitrogen gas and hydrogen gas are converted by an industrial process into ammonia gas and then ammonia compounds used as inorganic fertilizers.

· Mineral deposits of compounds containing nitrates are mined and used as inorganic fertilizers.

· Excess nitrates from different sources (e.g., runoff of animal wastes from livestock feedlots, runoff of, inorganic fertilizers from croplands and discharge of treated and untreated sewage) enter aquatic ecosystems, causing rapid growth of algae, depleting the water of dissolved oxygen gas and causing fish kills. This is known as cultural eutrophication, a process that speeds up the natural ageing of lakes.