| Themes > Science > Earth Sciences > Geology > Soils > Soil Use > The Nitrogen Cycle |
Nitrogen is an essential component of proteins, genetic material, chlorophyll, and other key organic molecules. All organisms require nitrogen in order to live. It ranks fourth behind oxygen, carbon, and hydrogen as the most common chemical element in living tissues. Until human activities began to alter the natural cycle, however, nitrogen was only scantily available to much of the biological world. As a result, nitrogen served as one of the major limiting factors that controlled the dynamics, biodiversity, and functioning of many ecosystems. The Earth’s atmosphere is 78 percent nitrogen gas, but most plants and animals cannot use nitrogen gas directly from the air as they do carbon dioxide and oxygen. Instead, plants — and all organisms from the grazing animals to the predators to the decomposers that ultimately secure their nourishment from the organic materials synthesized by plants — must wait for nitrogen to be “fixed,” that is, pulled from the air and bonded to hydrogen or oxygen to form inorganic compounds, mainly ammonium (NH4) and nitrate (NO3), that they can use. The amount of gaseous nitrogen being fixed at any given time by natural processes represents only a small addition to the pool of previously fixed nitrogen that cycles among the living and nonliving components of the Earth’s ecosystems. Most of that nitrogen, too, is unavailable, locked up in soil organic matter — partially rotted plant and animal remains — that must be decomposed by soil microbes. These microbes release nitrogen as ammonium or nitrate, allowing it to be recycled through the food web. The two major natural sources of new nitrogen entering this cycle are nitrogen-fixing organisms and lightning. Nitrogen-fixing organisms include a relatively small number of algae and bacteria. Many of them live free in the soil, but the most important ones are bacteria that form close symbiotic relationships with higher plants. Symbiotic nitrogen-fixing bacteria such as the Rhizobia, for instance, live and work in nodules on the roots of peas, beans, alfalfa and other legumes. These bacteria manufacture an enzyme that enables them to convert gaseous nitrogen directly into plant-usable forms. Lightning may also indirectly transform atmospheric nitrogen into nitrates, which rain onto soil. Quantifying the rate of natural nitrogen fixation prior to human alterations of the cycle is difficult but necessary for evaluating the impacts of human-driven changes to the global cycling of nitrogen. The standard unit of measurement for analyzing the global nitrogen cycle is the teragram (abbreviated Tg), which is equal to a million metric tons of nitrogen. Worldwide, lightning, for instance, fixes less than 10 Tg of nitrogen per year — maybe even less than 5 Tg. Microbes are the major natural suppliers of new biologically available nitrogen. Before the widespread planting of legume crops, terrestrial organisms probably fixed between 90 and 140 Tg of nitrogen per year. A reasonable upper bound for the rate of natural nitrogen fixation on land is thus about 140 Tg of N per year. |
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