Bacteria found in root nodules of legumes are

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

The anaerobic condition for nitrogen fixation is maintained by ..........

Select the correct statement.

In which of the following angiosperms do the root nodules contain filamentous nitrogen-fixing microorganisms?

We find that $Rhizobium$ forms nodules on the roots of leguminous plants. Also,$Frankia$,another microbe,forms nitrogen-fixing nodules on the roots of the non-leguminous plant $Alnus$. What kind of relationship is observed between mycorrhiza and pine trees?