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Unformatted text preview: Nitrogen fixation. The principal reservoir of nitrogen on earth is the atmosphere, which contains about 80 percent nitrogen. In the process of nitrogen fixation , nitrogen gas from the atmosphere is used to form ammonia by the chemical process of reduction. Nitrogen fixation is performed by free- living bacteria as well as by bacteria growing in symbiosis with leguminous plants (plants that bear their seeds in pods, such as peas, beans, alfalfa, clover, and soybeans). Nitrogen fixation is accomplished by species of Rhizobium inhabiting the roots of leguminous plants in a mutually beneficial (symbiotic) relationship. These Gram-negative bacteria penetrate the root hairs and form an infection thread that becomes a root nodule. Here the bacteria fix atmospheric nitrogen, while deriving nutrients from the plant. There are many genera of free-living bacteria that exist apart from legumes and fix nitrogen in the...
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- Fall '08