Conditions for $\mathbf{N}_2$ Fixation. (i) Presence of enzyme nitrogenase (= dinitrogenase) (ii) Occurrence of anaerobic conditions in the area of nitrogenase activity. (iii) ATP. (iv) Source of hydrogen, NADPH or $\mathrm{FMNH}_2$. (v) Source of electron donor, ferredoxin. (vi) Deficient occurrence of nitrate in the soil. (vii) Source of organic carbon in the form of ketoacids.
Role in Nitrogen Fixation. ATP provides energy to enzyme nitrogenase while ferredoxin supplies electrons from respiratory substrate.
Atmospheric nitrogen attaches to Fe-Mo component of dinitrogenase. Bonds between its two atoms are weakened by this attachment. In the presence of energy and electrons, hydrogen combines with nitrogen atoms to form $\mathrm{N}_2 \mathrm{H}_2$ (dimide), $\mathrm{N}_2 \mathrm{H}_4$ (hydrazine) and $2 \mathrm{NH}_3$ (ammonia). A reducing environment is essential for this. It is provided by leghaemoglobin which stores excess oxygen and sends only small limited quantities of oxygen inside the bunch of bacteriods. $\mathrm{NH}_3$ is immediately assimilated with the help of organic acids.