Nitrogen is very important constituents of all organisms life  because it forms the structural parth of proteins, nucleic acids and chlorophyll molecules.

              Earth 's atmosphere contains 79% of N₂   but it can not be used by living organisms.it has to be combined with other elements (C,H Or O) to make it into usable form. Thus, nitrogen cycle is the  biogeochemical cycle that describe the cycle flow of nitrogen and nitrogen containing compound in nature . it includes:

1.Fixation of Nitrogen in biosphere:

              Three processes are invole for nitrogen fixation:

(i)     Atmospheric fixation: combination of N₂ and O₂ into nitrogen oxide(NO,NO₂, NO₃)  in presence of thunder lightening. These nitrogen oxide dissolve in rain forming nitrate.

(ii)     Industrial fixation:  combination of N₂ and H₂ in presence of   cstalyst  and high pressure and  temperatures into NH₃, which  is used for making  urea and ammonium nitrate (fertilizer.)

(iii)    Biological fixation:  involves conversion of N₂ of air directly into organic nitrogen by  symbiotic (present in leguminous plants) bacteria or some by free living bacteria.

_{Rhizobium bacteria}

(a)   N₂ (air)              NO₂   and  NO₃     

                   (Root modulus of peas/beans)  

                              Azotobacters bacteria                                  

Plants utilize this NO₃ ions for their growth and multiplication.

2.       Nitrogen release into atmosphere:

      Nitrogen cycle is completed by release of N2 back into atmosphere. Certain bacteria(pseudomonas and clostridium) in soil carry out denitrification process and convert nitrate back into nitrogen gas.

  By anaerobic oxidation process, nitrate and ammonia are directly converted into gas.