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

• 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.

• 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

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

Nitrification 

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

Assimilation

• 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.

Ammonification

• 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.

De-nitrification

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