Which of the following bacteria can fix atmospheric nitrogen in free state?

Both II and III

a-  Rhizobium

b- Azospirillum

c- Azotobacter

d- Both II and III are correct answer.

Bacteria can fix atmospheric nitrogen while free-living in the soil (examples Azospirillum and Azotobacter), thus enriching the nitrogen content of the soil.

Rhizobium, on the other hand, is a bacteria that forms a symbiotic relationship with leguminous plants, forming nodules on their roots and fixing nitrogen in the root nodules. It does not fix atmospheric nitrogen in a free-living state.