Why is Rhizobium categorized as a 'symbiotic bacterium'? How does it act as a biofertiliser?

Explanation

Rhizobium lives as an endosymbiont in the root nodules of leguminous plants and is mutually beneficial to them. The bacterium derives food and shelter from the plant while fixes atmospheric nitrogen into nitrites and nitrates to imporve the growth of the plant. As the bacterium helps in `N_2` fixation so acts as a biofertillizer.