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Nitrogen cycle - Fixation, Nitrification & Denitrification etc

news-details Image Source May 14, 2021 19:18 IST · 4 min read

Nitrogen cycle is the circulation and conversion of nitrogen into multiple chemical forms as it circulates from the atmosphere to the soil to organism and back into the atmosphere.

The conversion of nitrogen can be carried out through both biological and physical processes - fixation, ammonification, nitrification, and denitrification.

Although 78 percent by volume of the atmosphere is nitrogen gas, this abundant reservoir exists in a form unusable by most organisms.

The nitrogen gas exists in both organic and inorganic forms, organic nitrogen exists in living organisms, and they get passed through the food chain by the consumption of other living organisms.

Inorganic forms of nitrogen are found in abundance in the atmosphere, this nitrogen is made available to plants by symbiotic bacteria which can convert the inert nitrogen into a usable form - such as nitrites and nitrates.

Human activities such as fossil fuel combustion, use of artificial nitrogen fertilizers, and release of nitrogen in wastewater have dramatically altered the global nitrogen cycle.

Human modification of the global nitrogen cycle can negatively affect the natural environment system and also human health.

Process of Nitrogen Cycle consists of the following steps - Nitrogen fixation, Nitrification, Assimilation, Ammonification and Denitrification.

Nitrogen fixation

The conversion of nitrogen gas (N2) into nitrates (Ammonium nitrate, NH4-NO3) and nitrites (Ammonium nitrite, NH4-NO2) through atmospheric, industrial and biological processes is called nitrogen fixation.

Atmospheric nitrogen must be processed, or "fixed", into a usable form to be taken up by plants.

In Nitrogen fixation, nitrogen gas is converted into inorganic nitrogen compounds, it is mostly accomplished by certain bacteria (free-living or symbiotic bacteria known as diazotrophs) and blue-green algae.

A much smaller amount of free nitrogen is fixed by abiotic means (e.g., lightning, ultraviolet radiation, electrical equipment) and by conversion to ammonia (NH3) through the Haber-Bosch process.

Bacteria like "Azotobacter and Rhizobium" have the nitrogenase enzyme that combines gaseous nitrogen with hydrogen to produce ammonia (NH3), which is converted by the bacteria into other organic compounds.

Symbiotic nitrogen-fixing bacteria usually live in the root nodules of legumes (such as peas, alfalfa, and locust trees). Here they form a mutualistic relationship with the plant, producing ammonia (NH3) in exchange for carbohydrates.

Because of this relationship, legumes often increase the nitrogen content of nitrogen-poor soils.

Today, about 30 percent of the total fixed nitrogen is produced industrially using the Haber-Bosch process, which uses high temperatures and pressures to convert nitrogen gas and a hydrogen source (natural gas or petroleum) into ammonia(NH3).

The legume family consists of plants that produce a pod with seeds inside. Common edible legumes include pulses, lentils, peas, chickpeas, beans, soybeans, and peanuts.

Types of Nitrogen Fixation

1) Atmospheric fixation: It is A natural phenomenon where the energy of lightning breaks the nitrogen into nitrogen oxides and is then used by plants.

2) Industrial fixation: It is a man-made alternative that aids in nitrogen fixation by the use of ammonia. Ammonia is produced by the direct combination of nitrogen and hydrogen and later, it is converted into various fertilisers such as urea.

3) Biological fixation: Bacteria like Rhizobium and blue-green algae transform the unusable form of nitrogen into other compounds that are more readily usable. These nitrogen compounds get fixed in the soil by these microbes.


Plants absorbs (assimilate) nitrate or ammonium from the soil by their root hairs, it is first reduced to nitrite ions and then ammonium ions for incorporation into amino acids, nucleic acids, and chlorophyll.

Many animals, fungi, and other heterotrophic (cannot produce its own food) organisms obtain nitrogen by ingestion of amino acids, nucleotides, and other small organic molecules.


When a plant or animal dies or an animal expels waste, the initial form of nitrogen is organic. Bacteria or fungi convert the organic nitrogen within the remains back into ammonium (NH4+), a process called ammonification or mineralization.


Nitrification is the conversion of ammonium (NH4+) to nitrate (NO3-), performed primarily by soil-living bacteria and other nitrifying bacteria.

In the primary stage of nitrification, the oxidation of ammonium (NH4+) is performed by bacteria such as the Nitrosomonas species, which converts ammonia to nitrites (NO2-).

Other bacterial species such as Nitrobacter, are responsible for the oxidation of the nitrites (NO2-) into nitrates (NO3-).

It is important for the ammonia (NH3) to be converted to nitrates or nitrites because ammonia gas is toxic to plants.


Denitrification is the reduction of nitrates back into nitrogen gas (N2), completing the nitrogen cycle. This process is performed by bacterial species such as Pseudomonas and Paracoccus, under anaerobic conditions (absence of oxygen).

They use the nitrate as an electron acceptor in the place of oxygen during respiration.

Denitrification happens in anaerobic conditions e.g. waterlogged soils. The denitrifying bacteria use nitrates in the soil to carry out respiration and consequently produce nitrogen gas, which is inert and unavailable to plants.

Denitrification occurs in free-living microorganisms as well as obligate symbionts of anaerobic ciliates.

Related Questions
  1. Nitrogen fixing bacteria make combination with cell of the root of:

    A) Pulses

    B) Rice

    C) Wheat

    D) Sugarcane

    Show answer

    Nitrogen cycle

  2. Which of the following is associated with biological nitrogen fixation ?

    A) Red Algae

    B) Brown Algae

    C) Green Algae

    D) Blue-Green Algae

    Show answer

    UPPSC 2019

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