What process takes place in these nodules

Nitrogen fixation

Explanation:

Biological nitrogen fixation is carried out by two types of microorganisms namely, symbiotic and free living microorganisms. Symbiotic microorganisms are bacteria f the genus Rhizobium. These are found in root nodules of leguminous plants. They convert atmospheric nitrogen into ammonia which is used directly by leguminous plants to form nitrogen containing organic compounds.