The circulation of nitrogen in environUKment Iis termed as nitrogen cyclel
Here nitrogen is fixed in form of n2 compounds by nitrogen fixing bacteria such as azobacter, rhizobium etc.
Then the plant uses these compounds of nitrogen for growth, animals feed on plants and at last when these animals and plants die get converted into ammonia and bacteria named ceamoautotrophic bacteria converts back Nh4 into No2 and No3.
Then the No2 and No3 is converted back to nitrogen by destroying bacteria
Example -psudomonas, thyobasillus