There are four steps in the nitrogen cycle namely, nitrogen fixation, ammonification, nitrification and denitrification.
(i) Nitrogen fixation
(a) During this step, the atmospheric free nitrogen is converted into those nitrogen compounds which can be absorbed by plants.
(b) The fixation may be biological and carried out by free living bacteria, (e.g., Azotobacter, Clostridium) or cyanobacteria or bacterium in symbiotic association (e.g., Rhizobiun).
(c) The fixation can also occur by physical processes in the atmosphere.
(d) Ammonium compounds are formed as a result of this process.
(a) When the dead/waste parts of plants and animals are decomposed, ammonia is formed.
(b) It is carried out by certain fungi and certain bacteria.
(c) The process of formation of ammonia from organic nitrogenous wastes, is called ammonification.
(a) The process of conversion of ammonia into nitrites and then into nitrates is called nitrification.
(b) The first step, i.e., conversion of ammonia to nitrite is carried out by bacteria like Nitrosomonas and Nitrococcus.
(c) The second step, i.e., conversion of nitrites to nitrates is done by Nitrobacter.
2NH3 + 3O2 → 2NO2 - + 2H+ + 2H2O
2NO2- + O2 → 2NO3 -
(d) The nitrates are absorbed and metabolised by plants.
(a) The process by which the fixed forms of nitrogen, i.e., nitrites and nitrates, are converted back into free nitrogen, is called denitrification.
(b) It is carried out by Pseudomonas like bacterium.
(c) This nitrogen again enters the nitrogen cycle.