Nitrogen cycle is a continuous cycle by which nitrogen circulates in the air, soil, water and organisms. Nitrogen cycle involves three main processes which are nitrogen fixation, nitrification and denitrification.
Nitrogen fixation
Nitrogen fixation is a process which combines free atmospheric nitrogen with hydrogen to form ammonium compounds by nitrogen-fixing bacteria such as Azotobacter and Rhizobium, in root nodules of leguminous plants.
Nitrification
Nitrification is a process which converts ammonium compounds into nitrites and nitrates by nitrifying bacteria such as Nitrosomonas and Nitrobacter. Nitrosomonas converts ammonia into nitrites whereas Nitrobacter converts nitrites into nitrates.
Denitrification
Denitrification is a process which converts nitrates into free atmospheric nitrogen by denitrifying bacteria such as Pseudomonas.