(i) Nitrogen fixation : It is the conversion of atmospheric nitrogen into water soluble compounds (nitrates and nitrite) either by the free living bacteria or Rhizobium bacteria which is found in the roots of legumes in special structures called root nodules.
Atmospheric nitrogen also gets converted into nitrates and nitrites during lightning.
(ii) Ammonification: Death and decay of plant bodies releases ammonia. Animals give out ammonia, urea and uric acid as excretory products. These are nitrogenous compounds which are converted to ammonia by putrefying bacteria and the process is known as ammonification.
(iii) Nitrification : Ammonia is converted first into nitrites and then into nitrates by the nitrifying bacteria and the process is called as nitrification. Plants generally take up nitrates and nitrites and convert them into amino acids which are used to make proteins.
(iv) Denitrification : Once the animal or the plant dies, other bacteria in the soil convert the various compounds of nitrogen back into nitrates and nitrites. A different type of bacteria (pseudomonas) converts die nitrates and nitrites into elemental nitrogen. This process is called denitrification. In this way this cycle repeats again and again.