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

(N/A) $Rhizobium$ is a symbiotic bacteria present in the root nodules of leguminous plants.
The basic requirements for $Rhizobium$ to carry out nitrogen fixation are as follows:
$(a)$ Presence of the enzyme nitrogenase.
$(b)$ Presence of leghaemoglobin.
$(c)$ Non-haem iron protein,ferredoxin as the electron carrier.
$(d)$ Constant supply of $ATP$.
$(e)$ $Mg^{2+}$ ions as co-factors.
$Rhizobium$ contains the enzyme nitrogenase,a $Mo-Fe$ protein,which helps in the conversion of atmospheric free nitrogen into ammonia.
The reaction is as follows:
$N_2 + 8e^- + 8H^+ + 16ATP \rightarrow 2NH_3 + H_2 + 16ADP + 16Pi$
The $Rhizobium$ bacteria live as aerobes under free-living conditions,but require anaerobic conditions during nitrogen fixation. This is because the enzyme nitrogenase is highly sensitive to molecular oxygen. The nodules contain leghaemoglobin,which acts as an oxygen scavenger and protects nitrogenase from oxygen.