Which component is fixed by $Rhizobium$ bacteria?

Leghemoglobin is responsible for:

One of the free-living,anaerobic nitrogen-fixer is

How many $ATP$ molecules are required for the production of each $NH_3$ molecule during biological nitrogen fixation?

$A-$ 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.

Identify the process represented by the following reaction:
$N_2 + 8e^- + 8H^+ + 16ATP \rightarrow 2NH_3 + H_2 + 16ADP + 16P_i$