J.E. Carnahan in i960 first reported the conversion of atmospheric nitrogen into ammonia by the free living nitrogen fixing bacteria, Clostridium pasteurianum.

The fixation of nitrogen takes place as follows. Phosphoroclastic breakdown of pyruvate results in the production of electrons, acetyl phosphate and hydrogen. Ferrodoxin accepts electrons coming from pyruvate and itself get reduced. Acetyl-phosphate reacts with ADP to generate ATP.

In the presence of reduced ferrodoxin -and ATP, molecular nitrogen is adsorbed on the surface of the enzyme nitrogenase. Electrons get transferred to X2 resulting in reduction of nitrogen.The enzyme is set free only when nitrogen has been completely reduced to ammonia (XH3).

Ammonium ions are not readily absorbed by the roots of plants. They are adapted to absorb nitrate ions. Many soil bacteria such as Nitrosomonas and Nitrococcus convert ammonia into nitrite ions. Nitrobacter present in the soil oxidise these nitrites into nitrates. The steps of nitrification are as follows.

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Nitrate assimilation in plants:

The root systems of higher plants absorb nitrates. It is first reduced in cells to nitrite by enzyme nitrite reductase. Ferrodoxin act as the electron donor for reduction of nitrite. Ammonia so formed does not accumulate and is utilised for the synthesis of amino acids and other nitrogenous compounds.