The conversion of molecular nitrogen into compounds of nitrogen especially ammonia is called nitrogen fixation. Nitrogen fixation, is a reductive process i.e., nitrogen fixation will stop if there is no reducing condition or if oxygen is present.
This nitrogen fixation may take place by two different methods - abiological and biological.
In abiological nitrogen fixation the nitrogen is reduced to ammonia without involving any living cell. Abiological fixation can be of two types: industrial and natural.
For example, in the Haber’s process, synthetic ammonia is produced by passing a mixture of nitrogen and hydrogen through a bed of catalyst (iron oxides) at a very high temperature and pressure.
N2 + 3H2 → 2NH3
This is industrial fixation wherein nitrogen gets reduced to ammonia.
In natural process nitrogen can be fixed especially during electrical discharges in the atmosphere. It may occur during lightning storms when nitrogen in the atmosphere can combine with oxygen to form oxides of nitrogen.
N2 + O2 → 2NO2
These oxides of nitrogen may be hydrated and trickle down to earth as combined nitrite and nitrate.
Chemically, this process is same as abiological. Biological nitrogen fixation is reduction of molecular nitrogen to ammonia by a living cell in the presence of enzymes called nitrogenases.