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Nitrogen Fixation and the Legume-Rhizobia Symbiosis

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Biological nitrogen fixation converts unusable atmospheric nitrogen gas into ammonia, carried out by soil bacteria called rhizobia living symbiotically in legume root nodules.

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A soybean's root nodules house rhizobia that fix nitrogen into ammonia in exchange for sugars the plant supplies from photosynthesis.

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Plants cannot fix nitrogen themselves — they lack the enzyme nitrogenase and depend on this symbiosis for usable nitrogen.