Question

what is the role of rhizobium on nitrogen cycle

Answer 1

Rhizobia turns the free Nitrogen in air into ammonia and nitrate for plant assimilation.

Answer 2

rhizobium is bacteria present under the soil which converts the nitrogen gas into a sloble substance by which plants can take in ... thus soil will be rich in nitrogen...
rhizobium bacteria live under the crops which need lots of nitrogen or lives under the plants which need nitrogen...
and its a symbiotic relationship