1)atmospheric nitrogen cannot be absorbed directly by plant and they can only used nitrogen in the form of ammonium,nitrate or nitrite ions which they absorbed from the soil
2)nitrogen fixing bacteria such as nostoc sp. can use atmospheric nitrogen to make ammonium compound through a process:nitrogen fixation
3)nitrates are absorbed by plant roots and converted into plant protein 
4)when animal eats plant the organic nitrogen is transferred into the body of the animals and becomes animal protein
5)waste materials,dead plants and animals are decomposed by decaying bacteria and fungi into ammonium compounds
6)ammonium compounds are converted into nitrites and nitrates by nitrifying bacteria through process of nitrification 
-ammonia is oxidised into nitrites by nitrosomonas sp.
-nitrites are oxidised into nitrates by nitrobacter sp.
7)the cycle is balanced by continuous return of nitrogen to the atmosphere by denitrifying bacteria through denitrification .these bacteria break down nitrates into gaseous nitrogen and oxygen.the oxygen is used by the bacteria while the nitrogen is returned to the atmosphere