Nitrogen cycle is the cycle in which nitrogen gets fixed, used by plants and animals and later returned to the atmosphere. It involves processes such as nitrogen fixation, nitrogen assimilation , ammonification ,nitrification and denitrification. In nitrogen fixation, nitrogen gets fixed either by the lightning or by soil through a bacteria rhizobium. In nitrogen assimilation, the nitrogen compounds get converted into organic compounds In ammonification, these compounds get converted into ammonia. In the next process, nitrification, this ammonia gets converted to nitrates by nitrifying bacteria . Some of them are absorbed by the plants. In the last process i.e. denitrification, the rest of the nitrates are converted to free nitrogen gas by the denitrifying bacteria.