Which bacterium fixes nitrogen in root nodules of legumes such as peas and clover?

Rhizobium leguminosarum fixes atmospheric nitrogen in root nodules of legumes such as peas and clover.

The bacterium forms a mutualistic relationship with its plant host. It enters young roots and stimulates the formation of nodules, specialized structures where the bacteria live. Inside the nodule, bacterial nitrogenase converts atmospheric nitrogen into ammonia-related compounds that the plant can use to build proteins and nucleic acids. In return, the plant supplies the bacteria with carbon compounds made by photosynthesis.

Nitrogen fixation is difficult because nitrogen gas is chemically stable, and the nitrogenase enzyme is damaged by oxygen. Legume nodules use the oxygen-binding protein leghemoglobin to help control oxygen levels. Rhizobium does not directly “fertilize” every plant in the same way, and not every nitrogen-fixing bacterium belongs to Rhizobium. The association is especially important in agriculture because it can reduce the need for synthetic nitrogen fertilizer.