Which element plays an important role in nitrogen fixation?

$A$ farmer adds Azotobacter culture to soil before sowing maize. Which mineral element is being replenished?

$A-$ The microbe $Frankia$ produces nitrogen-fixing nodules on the roots of non-leguminous plants.
$R-$ Both $Rhizobium$ and $Frankia$ are free-living in soil,but as symbionts,can fix atmospheric nitrogen.

Which of the following red pigments is present in the root nodules of leguminous plants?

Choose the incorrect pair$(s)$:
$(I)$ Free-living nitrogen-fixing aerobic microbes - Beijerinckia
$(II)$ Rhodospirillum - Aerobic microbes
$(III)$ Bacillus - Free-living
$(IV)$ Cyanobacteria - Anabaena and Azotobacter

$nif$ genes occur in