• In our atmosphere, nitrogen is present in a large amount that is 78%.
  • It is also a constituent of many molecules which are essential to our life like nucleic acids (DNA and RNA), proteins, and some vitamins.
  • It is also present in so many biologically important compounds like alkaloids and urea.
  • Thus, nitrogen is an important nutrient for all life forms and our life will be easy and simple if nitrogen is directly being used by all these life forms.
  • However, inert nitrogen molecules cannot be converted into forms that can be easily taken up and used like nitrates and nitrites, by other than a few forms of bacteria.
  • These nitrogen-fixing bacteria can be free-living or can be associated with some dicot plants.
  • Usually, the nitrogen-fixing bacteria are found in the roots of legumes in special structures called root nodules.
  • Other than these bacteria, the physical process is the only method by which nitrogen molecules can be converted into nitrates and nitrites.
  • During lightening, the high temperatures and pressure which are created in air covert the nitrogen molecules into oxides of nitrogen. These oxides then dissolve in the water and give nitric and nitrous acids and then fall on land while it is raining. These then can be utilized by various life forms.
  • Plants generally take up nitrates and nitrites and convert them into amino acids which are then used to make proteins.
  • Once the animal and plant die, other bacteria in the soil convert the various compounds of nitrogen back into nitrates and nitrites.
  • Thus, the nitrogen cycle is the cycle in which nitrogen passes from its elemental form of atmosphere into simple molecules in the water and soil, which gets converted to more complex molecules in living beings and back again to the simple nitrogen molecule in the atmosphere.

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