Which one of the following elements plays an important role in biological nitrogen fixation?

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 $Alnus$. Can we artificially induce the property of nitrogen fixation in a plant,whether leguminous or non-leguminous?

What is the composition of the enzyme nitrogenase?

Which component from the atmosphere is fixed in the soil by Azospirillum?

The important contribution of molybdenum is in:

What will happen if $Rhizobium$ bacteria are introduced into a wheat field?