Biological nitrogen fixation (nitrogen fixation that is performed by living organisms) is carried out by

Bacteria
Nitrogen fixing bacteria are those that convert atmospheric nitrogen to different nitrogenous ions and add them to the soil. These bacteria may either be free living in the soil, or they may develop a mutualistic relationship with some organisms, such as the nitrogen fixing bacteria that live in the root nodules of leguminous plants. An example of a genus of nitrogen fixing bacteria is the genus Azotobacter.

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