How are root nodules on the roots of leguminous plants useful for the crop?

Why are root nodules useful for the leguminous plants?

nitrogen fixing nodules on the roots of non leguminous plant Alnus harbours the microbe

Root nodules in leguminous plants:

We find that Rhizobium forms nodules on the roots of leguminous plants. Also Frankia another microbe forms nitrogen fixing nodules on the roots of non-leguminous plant Alnus. (a) Can we artificially induce the property of nitrogen-fixation in a plant, leguminous or non-leguminous ? (b) What kind of relationship is observed between mycorrhiza and pine trees ? (c) Is it necessary for a microbe to be in close association with a plant to provide mineral nutrition ? Explain with the help of one example.

Microbe producing nitrogen fixing nodule on the roots of non-leguminous plant is -

Root nodules are not formed in non-leguminous plants because they grow in

The fixation of free nitrogen from the air in the nodules of roots of leguminous plants is done by symbiotic bacteria