What is the Nitrogen Cycle?

Nitrogen Cycle Image

Most of us are familiar with the phenomenon of the Oxygen Cycle and the Carbon Cycle but have you heard of ‘The Nitrogen Cycle? 

Nitrogen is an important element required by all living organisms to accomplish the fundamental processes of life. To understand the nitrogen cycle better, it’s a necessity to comprehend that the nitrogen cycle is explained and the role of nitrogen in the lives of organisms. 

The Function of Nitrogen and Nitrogen Cycle for Living Beings

How is the Nitrogen Cycle important to humans? Nitrogen is a constituent of the cells and the main building block of DNA. It enables the determination of genetics. It is found almost everywhere but in different forms. 

The plants utilize nitrogen from the soil and water for their growth and the development of food that we cultivate. Just like balance and stability in life are needed, an equilibrium in levels of nitrogen in the environment is necessary too. 

The lack of nitrogen can lead to stunted growth of plants and yellowing. The excess of nitrogen can result in Eutrophication, polluting aquatic life and harming marine animals. That’s where the phenomenon of the Nitrogen Cycle steps in. 

Now you would ask, What is the Nitrogen Cycle and how does it work? The Nitrogen Cycle is the process by which nitrogen is modified between various chemical forms. This conversion can be carried out through both biological and physical methods.   

Forms in which Nitrogen can occur in the environment : 

  1. Inorganic Nitrogen as Nitrogen gas (N2)
  2. Organic Nitrogen as – 
  • Ammonium (NH4+)
  • Nitric oxide (NO)
  • Nitrite (NO2)
  • Nitrate (NO3)
  • Nitrous Oxide (N2O)

What are the stages of the Nitrogen Cycle? 

To understand how the nitrogen cycle works, we must learn about the steps involved. The nitrogen cycle comprised of the following steps 

  1. Nitrogen fixation
  2. Nitrogen Assimilation
  3. Ammonification
  4. Nitrification
  5. Denitrification and 
  6. Sedimentation

These steps take place in two crucial stages which are as follows: 


In this stage of the nitrogen cycle, the microbes restore the elusive organic form of nitrogen to a simpler assimilable form of nitrogen. 


In this stage of the nitrogen cycle, the accessible form of nitrogen, i.e. ammonia and nitrate are utilised by the living organisms, basically, plants and microbes, who transform it into bound organic form. 

Nitrogen Fixation

The conversion of available nitrogen in the atmosphere into the biologically favourable form of the nitrogenous compound is called nitrogen fixation. 

A few ways to convert N2 into more chemically reactive forms are:

  • Industrial Nitrogen Fixation
  • Biological Nitrogen Fixation
  • Physicochemical Nitrogen Fixation 

Industrial Nitrogen Fixation 

In the nitrogen cycle, the nitrogen is fixed industrially under great pressure, at a temperature of 600°C and with the aid of an iron catalyst, hydrogen and atmospheric nitrogen. 

This process is called the Haber-Bosch process. This process results in the production of ammonia by humane methods. 

Biological Nitrogen Fixation

Biological Nitrogen Fixation

Biological nitrogen fixation is the process that occurs when the atmospheric nitrogen is recycled to ammonia by an enzyme called nitrogenase. 

Some symbiotic bacteria, blue-green algae and some free-living bacteria can fix nitrogen as organic nitrogen. 

The chemical reaction taking place can be written as: 

N2 + 8 H+ + 8 e− → 2 NH3 + H2

Physicochemical Nitrogen Fixation 

In this process, the atmospheric nitrogen mixes with oxygen (as ozone) during lightning or electrical explosions in the clouds and generates different nitrogen oxides.  

Nitrogen Assimilation

As we move on to the next step in our nitrogen cycle, the formation of organic nitrogenous compounds like amino acids from inorganic compounds with the help of plants takes place.

Nitrates are first converted into ammonia which is absorbed by green plants via roots and later, integrated with organic acids to develop amino acids. Amino acids are consumed in the synthesis of proteins, enzymes, chlorophyll, nucleic acid, etc. 

Some plants, species of bacteria and fungi depend on ammonia and nitrate to meet their Nitrogen demands as they cannot assimilate Nitrogen gas (N2) otherwise. Other organisms, like animals, rely entirely on organic nitrogen for their sustenance.


The process of discharging ammonia by certain microbes manipulating organic compounds developed from dead organic remains of plants and animals is called Ammonification. It includes all the chemical reactions implicated in the production of Ammonia or ammonium. 

Nitrogen in the form of ammonia is consumed by living organisms. The bound organic nitrogen present in organic matter as proteins, amines, amides, urea etc is transformed to ammonia by the microbial community. 

Ammonia is an unstable blend. Hence it either gets scattered or maybe an edge to the negatively charged clay particles in the soil as it is positively charged. Thus ammonia may get deteriorated easily directing to Nitrification which is the next step of the Nitrogen cycle.


What is Nitrification in the nitrogen cycle? Nitrification in the nitrogen cycle is the process of biological oxidation of Ammonia to Nitrite to Nitrate? The conversion of ammonia is mainly the rate-limiting step of nitrification. 

It is of two types: 
  • Heterotrophic Nitrification
  • Chemoautotrophic Nitrification 

Both of these types are aerobic and hence, result in the release of energy. The microbes carrying out the process are known as chemoautotrophs. 


The process of reduction of nitrates or nitrites, i.e. the conversion of nitrate to nitrogen gas is called denitrification. It is an anaerobic process, unlike nitrification. 

Some denitrifying bacteria are:
  • Pseudomonas aeruginosa
  • Thiobacillus Denitrificans
  • Micrococcus denitrificans this 



The process of discovering the settled down nitrates from the rocks that are washed down by the sea or being deposited as sediment. The process is called sedimentation.

Frequently Asked Questions

  1. What is the terrestrial nitrogen cycle’s largest inorganic pool?

Nitrate is the extensively abundant form of inorganic nitrogen in the earth, being the ultimate product of the breakdown of organic materials under aerobic circumstances. The soil is the final suite for all the organic nitrogen of the terrestrial nitrogen cycle. 

  1. Is the nitrogen cycle good and what is the nitrogen cycle driven by?

Yes, the nitrogen cycle is a crucial phenomenon required to maintain a balance in the ecosystem. Without the nitrogen cycle, either the lack or excess of nitrogen in the atmosphere will take place which will lead to various crises for humankind.

The nitrogen cycle is moved by physical and biological processes that take place depending on an assortment of environmental aspects such as solar power, precipitation, temperature, soil composition and moisture, the existence of other nutrients, and atmospheric CO2 concentration. 

  1. What is lightning in the Nitrogen Cycle? 

Lightning acts as an important process involved in the fixation of atmospheric nitrogen. The increased climatic temperature of a lightning bolt can halt the bonds of atmospheric nitrogen molecules. 

Available nitrogen atoms in the air bond with oxygen in the air to generate nitrogen oxides, which decompose in moisture to give rise to nitrates that are held up to Earth’s surface by precipitation. 

  1. What is the nitrogen cycle bacteria? 

The bacterias and microbes involved in the biological fixation of nitrogen during the terrestrial nitrogen cycle are called the fixators. 

  • Nitrogen fixation – Nitrogen-fixing bacteria.
  • Ammonification – Decomposers
  • Nitrification – Nitrifying bacteria
  • Denitrification – Denitrifying bacteria

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