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Nitrogen Cycle: Definition, Steps, and Its Importance

Jul 9, 2022

Nitrogen is the most abundant gas in the atmosphere. Around 78% is only Nitrogen in the environment.

Nitrogen and its compounds are essential for maintaining life processes in the biosphere. Both animals and plants require Nitrogen for their development and growth. It is an essential component of proteins that make up the bodies of animals and plants.


Nitrogen is also present in chlorophyll, vitamins, and nucleic acids. In nature, it is circulated again and again through living and non-living things. This circulation process of Nitrogen in nature occurs with the help of a cycle.

This article aims to let you understand this Nitrogen cycle, its process, and the importance of the Nitrogen cycle in nature.


What is Nitrogen Cycle?

Nitrogen is an odourless, colourless, most important gas for all organisms on earth. It is present in DNA, RNA, and other proteins, which are the building blocks for the human body and other living organisms.

Nitrogen present in the air is in usable form. Certain bacteria can convert atmospheric nitrogen into nitrogen compounds. All living beings can use these nitrogen compounds through the nitrogen cycle.


This process begins with nitrogen absorption from the soil and plants. Then animals consumed these plants. After which, when the animals become dead, various bacteria decompose them. As a result, nitrogen returns to the soil. This continuous exchange of Nitrogen within the ecosystem is known as the nitrogen cycle.

Simply put, you can explain the nitrogen cycle as the circulation of nitrogen elements through living things and non-living environments. It is a biogeochemical cycle whereby Nitrogen is transformed into various forms as it circulates among the terrestrial, atmosphere, and ecosystem.


Explain Nitrogen Cycle

Five steps to convert the atmospheric Nitrogen into usable Nitrogen explain the nitrogen cycle.

Step 1:- Nitrogen fixation

Nitrogen existing in the air is in the stable form, N2. It cannot be absorbed by an organism until the help of Nitrogen-fixing bacteria fixes it.


Nitrogen enters the soil and surface waters through the process of precipitation. When it settles down in the water or soil, it breaks the bond between N2by undergoing some changes. As a result, hydrogen from the water gets attached to the free electrons of N2 and forms NH2+. The resulting compound is known as ammonium. This conversion is completed by the microorganisms present in the soil.

The nitrogen-fixing bacteria, lightning in the sky, and blue-green algae fix nitrogen gas from the atmosphere as organic nitrogen. They convert it into nitrogen compounds that go into the soil. The nitrogen-fixing bacteria are present in the soil and the root nodules of leguminous plants.


This process is completed by many bacteria, mainly Rhizobium bacteria. The chemical reaction that takes place is

3[CH2O] + 2N2 + 3H2O + 4H+ → 3CO2 + 4NH4+


Step 2:- Nitrification

The process of oxidising ammonia into nitrites and nitrates is known as nitrification. Some bacteria, especially Nitrosomonas and Nitrobacter, are involved in this process which occurs naturally in the environment.

In the first step, the bacteria Nitrosomonas and Nitrococcus convert the ammonia into nitrite. Then, as a second step, Nitrobacter bacteria convert these nitrites into nitrates.

All these bacteria reside in the soil and are known as nitrifying bacteria. The bacteria gain energy from this process by converting nitrogen into a usable form. They need oxygen to carry out the process.

The chemical reaction that takes place in this step is

NH3+ (3/2)O2→ H++ NO2 H2O; (nitrite formation)

NO2 + (½)O2 → NO3; (nitrate formation)

Step 3:- Assimilation

In this step, the plants take nitrogen compounds formed in the above two steps, from the soil, through their roots and utilise them for their growth. They convert these compounds into plant proteins and the other organic compounds which make up their body.

Then, animals eat plants as their food and convert plant proteins into animal proteins and other compounds to make up their body. This way, herbivores get nitrogen compounds from plants, and carnivores get them from eating herbivores.

This way, the NH3, NO3, and NO3 get assimilated into the biological tissues of plants and animals, and they use them in their cellular processes.

Step 4:- Ammonification

When animals and plants die, the complex nitrogen compounds in their body start to decompose. Then, these compounds are converted into simple organic compounds by certain bacteria and fungi in the soil. Animal excretion also converts complex nitrogen compounds into simple organic compounds.

In this way, the nitrogen compounds absorbed by the plants from the soil return to the soil. Then, new plants absorb these compounds and continue this process endlessly.

The conversion of organic Nitrogen into ammonium in the presence of bacteria and fungi is called ammonification. The ammonium produced can be used in other biological processes of plants.

Step 5:- Denitrification

The denitrification process decomposes some nitrogen compounds. Bacteria, like Paracoccus Denitrificians, Pseudomonas, etc., convert the nitrates from nitrogen compounds into nitrites and N2.

This process transpires in the absence of oxygen, i.e., anaerobic conditions. In this way, the nitrogen gas removed from the atmosphere during the first stage of the cycle is put back into the atmosphere.

The reduction of NO3and NO2  into N2 followed by the recycling of N2  into the atmosphere is known as denitrification. This process is also known as the reverse fixation of Nitrogen. Places like deep soils and deep water are the places without oxygen, where the anaerobic process occurs.

The chemical reaction that takes place in this step is

4NO3 + 5[CH2O] + 4H+→ 2N2↑ + 5CO2 ↑ + 7H2O

Nitrogen Cycle Diagram

It is easy to understand the above explanation with the help of a nitrogen cycle diagram.

Nitrogen cycle

Why is the Nitrogen Cycle Important?

The nitrogen cycle is important for biological processes. But, the question is: why is the nitrogen cycle important? It is because –

  1. It helps with the stable optimum growth of the plants.
  2. A large amount of nitrogen is obtained from the atmosphere and used to produce NH3. It is a raw material for the manufacture of Nitric acid. Hence, there would be some changes in the manufacture of Nitrogen-based products.
  3. It helps in the synthesis process of natural and artificial fertiliser, e.g., ammonium sulphate.
  4. It helps the plants to manufacture chlorophyll from the nitrogen compound.
  5. It is essential for the survival of plants because plants need nitrates for their survival and growth.
  6. During ammonia formation, the dead and decayed organic matter is decomposed by bacteria. This process helps clean the environment and provides essential nutrients to the soil.
  7. It provides the nitrogen compounds that enrich the soil and make it fertile and suitable for growing plants.
  8. Nitrogen is a crucial element for living organisms. It forms proteins and nucleic acid, which are essential elements of life. Without nitrogen compounds, life could not exist. The nitrogen cycle maintains the amount of Nitrogen in the atmosphere.
  9. Fertilisers also contain Nitrogen that is necessary for the plants.
  10. Eutrophication is the process of accumulating Nitrogen in water bodies when the Nitrogen from the fertilisers in the soil is washed away.


Nitrogen plays a vital role in the atmosphere. It is important for both plants and animals’ life. The nitrogen cycle in nature helps maintain the percentage of Nitrogen in the atmosphere and keep it constant. That is why the nitrogen cycle is important for everyone.

Frequently Asked Questions

1. What is the most important advantage of a nitrogen cycle?

A. The nitrogen cycle helps in converting the inert Nitrogen from the air for the use of plants and animals. Plants need Nitrogen to synthesise chlorophyll, and animals need plants. Therefore, the nitrogen cycle is essential for them.

2. How are nitrogen cycles one of the causes that change the water in the fish tank?

A. The nitrogen cycle also takes place in the marine ecosystem. When a fish creates biological waste, it produces ammonia. Ammonia is harmful to the fish, and a large amount can kill them.

Therefore, there is a need for bacteria to sort out the ammonia from the disposal of fish. This bacteria converts ammonia into nitrite. Then another bacteria convert nitrite to nitrate. A small amount of nitrate is not as harmful to fish. But, not the case when there will be a large amount of nitrate present.

That is why removing the nitrate accumulated in the tank requires changing the water in the fish tank. 

3. How can I be connected to the nitrogen cycle?

A. When you eat food, it contains proteins and amino acids. They contain at least one nitrogen atom. Nitrogen has originated from the nitrogen compounds in the soil in which the plants grow. Some animals eat other animals and indirectly consume these nitrogen compounds.

When you release the waste products from your body, these compounds get converted into Nitrogen, eventually returning to the atmosphere. Therefore, it would not be wrong to say that you can connect with the nitrogen cycle by being alive. That is why the nitrogen cycle is important.




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