Nitrogen Cycle

Nitrogen is primarily present in the atmosphere freely as dinitrogen or nitrogen gas. Molecular Nitrogen or diatomic nitrogen (N2) is highly stable as it is triple bonded (N≡N). Because of this stability, molecular nitrogen as such is not very reactive in the atmosphere under normal conditions.

Nitrogen is an essential constituent of living beings. Nitrogenous bases are part of nucleic acids and proteins are made up of amino acids of which Nitrogen is an important constituent. 

Air has 78% N2 but most of the living beings cannot utilize this atmospheric Nitrogen. Nitrogen cycle converts this nitrogen into a usable form. Lightning fixes Nitrogen to NH3, and nitrogen fixing bacteria like Rhizobium (which live in roots of leguminous plants like pea, rajma, beans, pulses) also convert N2 into NH3.

Most plants absorb nitrates from soil and reduce it to NH3 in the cells for further metabolic reactions. Dead organisms and their excreta like urea are decomposed by bacteria into NH3 and by a different set of bacteria into nitrates. These are left in the soil for use by plants.