NCERT Notes: The Nitrogen Cycle [Geography Notes For UPSC]

NCERT notes on important topics for the UPSC civil services exam. These notes will also be useful for other competitive exams like banking PO, SSC, state civil services exams and so on. This article talks about the Nitrogen Cycle.

Nitrogen Cycle (UPSC Notes):- Download PDF Here

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  • Nitrogen is a chief constituent of the bodies of living organisms as the Nitrogen atoms are found in all proteins and DNA
  • It is a common limiting nutrient in nature and agriculture.
  • It exists in the atmosphere as N2
  • Usually, nitrogen is usable only after it is fixed.
  • Nitrogen fixation is a process where bacteria convert N2 into ammonia, a form of nitrogen usable by plants.
  • Only a few types of organisms like and blue-green algae and certain species of soil bacteria are skilful of consuming nitrogen directly in its gaseous form.
  • When animals eat the plants, they obtain usable nitrogen compounds.
  • A limiting nutrient is a nutrient that’s in shortest supply and limits growth.
  • When fertilizers comprising phosphorous and nitrogen are transported in a runoff to rivers and lakes, they can result in blooms of algae. This is called eutrophication.[/su_box]

 [su_box title=”The Nitrogen Cycle” box_color=”#7960a0″]

  • The nitrogen cycle is the biogeochemical cycle.
  • Nitrogen is a main constituent of the atmosphere encompassing about 75% of the atmospheric gases.
  • It is also a vital constituent of different organic compounds such as the vitamins, nucleic acids, pigments, amino acids, and proteins.
  • The major source of free nitrogen is the action of soil micro-organisms and associated plant roots on atmospheric nitrogen found in pore spaces of the soil.


  • Fixation is the primary step in the process of converting nitrogen, usable by plants.
  • Normally, bacteria change nitrogen into ammonium.


  • This is the process by which ammonium converted into nitrates by bacteria.
  • The plants absorb these Nitrates.


  • Through assimilation only plants get nitrogen.
  • They absorb nitrates from the soil into their roots.
  • Then nitrogen gets used in chlorophyll, nucleic acids, and amino acids.


  • This is part of the decaying process.
  • When a plant or animal expires, decomposers such that bacteria and fungi turn the nitrogen back into ammonium so it can go back into the nitrogen cycle.


  • Surplus nitrogen in the soil gets put back out into the air.
  • There are special bacteria that execute this job as well.[/su_box]

Nitrogen Cycle (UPSC Notes):- Download PDF Here

Frequently Asked Questions about Nitrogen Cycle

What are the three stages of a Nitrogen Cycle?

The nitrogen cycle involves three major steps: nitrogen fixation, nitrification, and denitrification. It is a cycle within the biosphere which involves the atmosphere, hydrosphere, and lithosphere.

Why is nitrogen cycle important?

Nitrogen cycle helps bring in the inert nitrogen from the air into the biochemical process in plants and then to animals. Plants need nitrogen to synthesize chlorophyll and so the nitrogen cycle is absolutely essential for them.

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