What are the conditions necessary for fixation of atmospheric nitrogen by Rhizobium? What is their role in N_2- fixation?

The most important condition necessary for fixation of atmospheric nitrogen by Rhizobium is that it should occur in a symbiotic relationship with the roots of leguminous plants like alfalfa, sweet clover, sweet pea, lentils, peas, beans etc. Although Rhizobium also occurs as free living but can carry out fixation of atmospheric nitrogen as a symbiont.