Nitrogen Cycle - A Biogeochemical Cycle

Nitrogen plays a vital role in the survival of all living organisms. It is one of the primary nutrients and component of biomolecules such as chlorophyll, DNA, and proteins. Nitrogen is the most abundant element present in the atmosphere. Even though it is widely available, it acts as a scarce resource. Only when used in ammonia, it can be used by primary producers such as plants.

The simple diagram of the nitrogen cycle is as shown below.

Nitrogen Cycle

Nitrogen gas (N2) exists in both organic and inorganic forms. Therefore it undergoes various types of transformation in order to maintain a balance in the ecosystem. The important transformations that nitrogen supports are nitrogen fixation, assimilation, ammonification, nitrification, and denitrification process. The conversion of this gas into its oxidation states plays an important role in maintaining the ecological balance.

Here are the different types of processes combined with the nitrogen cycle.

Nitrogen fixation

Atmospheric nitrogen is converted into the usable form by lightning strikes or symbiotic bacteria which are known as Diazotrophs. These bacteria consist of a nitrogenase enzyme which has the capability to combine gaseous nitrogen with hydrogen to form ammonia.


When plants or animal die organic nitrogen is again released back into the soil. Bacteria or fungi present in the soil convert them back into ammonium. This process is also called as mineralization.


Primary producers take in this gas from the soil with the help of their roots in the form of amino acids, nitrite ions, nitrate ions or ammonium ions. This way it enters the food cycle when the consumers eat the plants.


In this process, the ammonia is converted into nitrate by the presence of bacteria in the soil. Nitrites are formed by the oxidation of Ammonia with the help of Nitrosomonas bacterium species. Later, the produced nitrites are converted into nitrates by Nitrobacter. This conversion is very important as ammonia gas is toxic for plants.



In order to complete the nitrogen cycle, the nitrites are reduced back to inert nitrogen gas in this process. This is done by bacterial species such as Clostridium and Pseudomonas in the absence of oxygen.

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Practise This Question

Identify the correct sequence of denitrification