Nitrogen Cycle - An Elemental Cycle

78 percent of the air we breathe contains nitrogen. All living organisms use nitrogen for their life processes. And also, it makes up the amino acids, proteins and even the nucleic acids in our DNAs. Even then, the concentration of nitrogen never goes below 78 percent. Ever wondered why?

Nitrogen cycle is the key to all these questions. Read on to find out more about the nitrogen cycle.

Nitrogen Cycle


Nitrogen is a crucial element for our survival because it is an integral part of many cellular components such as nucleic acids, proteins, vitamins, etc. Nitrogen is also an important constituent of chlorophyll in plants. Plants cannot obtain their nitrogen directly from the air, instead, they rely on bacteria and archaea present in the soil to break down the nitrogen into a usable form. Hence, a complex, interconnected network systems helps in maintaining the concentration of nitrogen in the atmosphere and also to ensure the survival of dependent lifeforms.

Basically, nitrogen cycle is the re-circulation and re-use of nitrogen in various forms to meet the demand for various environmental and biological activities. Nitrogen in the atmosphere is present in a diatomic form i.e., N2. Therefore, plants cannot use the nitrogen in this form. Therefore, they need nitrogen fixing bacteria, lightning, root nodules, forest fire etc to convert the nitrogen into nitrate compounds like NO, NO2, NO3.


Nitrogen fixation

Ammonification: When decomposition takes place, organic nitrogen (present in animals and plants) is converted into inorganic components like ammonia or ammonium ions. As a result, the ecosystem gets enriched with nutrients. This process is called ammonification.

Nitrification: In this step, ammonia obtained is first converted to nitrite (NO2) by bacteria like Nitrosomonas, Nitrococcus, etc. and then to nitrate (NO3) by Nitro Bacterium. Bacteria involved in nitrification are called chemoautotrophs. Here is the reaction involved in the process of Nitrification.

2 NH3 + 3O2 ―→ 2NO2– + 2H+ + 2H2O

2NO2– + O2 ―→ 2NO3


Once the nitrate is utilized by plants, the excess nitrate in the soil is reduced back to nitrogen by Pseudomonas and Thiobacillus bacteria. This process is known as denitrification.

Nitrogen Fixation

The concentration of usable form of nitrogen in the atmosphere is less. But certain bacteria called N2- fixers help to fix this problem. Nitrogen fixation is the process in which diatomic nitrogen is converted into ammonia by bacteria like Rhizobium, Azotobacter, etc. The conversion is carried out by an enzyme called nitrogenase. Nitrogenase is an oxygen-sensitive enzyme which requires a strict anaerobic environment. A compound called leghaemoglobin acts as an oxygen scavenger and fulfills the demand of the enzyme.

The process of nitrogen fixation is initiated within the nodule of the roots in certain plants like legumes. A symbiotic relationship exists between the plants and the bacteria Rhizobium. These bacteria divide and forms colonies around the root hairs where they aid in nitrogen-fixation for the plant. The nitrogenase enzyme in the root nodule catalyzes the formation of ammonia. The whole process is carried out at the expense of ATP which is produced during plant respiration.


Atmospheric fixation: Is a natural phenomenon where the energy of lightning breaks the nitrogen into nitrogen oxides and is then used plants.

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

Biological nitrogen fixation: We already know that nitrogen is not usable directly from the air for plants and animals. 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 soil by these microbes.


Nitrogen is present abundantly in the atmosphere, but it is unusable to plants or animals unless it is converted into nitrogen compounds. Nitrogen fixing bacteria play a crucial role in fixing the atmospheric nitrogen into nitrogen compounds that can be used by the plants. The plants absorb the usable nitrogen compounds from the soil through their roots. Then, these nitrogen compounds are used for the production of proteins and other compounds in the cell. Animals assimilate nitrogen by consuming these plants or other animals that contain nitrogen. Humans consume proteins from these plants and animals and then, the nitrogen assimilates into our system.

During the final stages of the nitrogen cycle, bacteria and fungi help decompose organic matter, where the nitrogenous compounds get dissolved into the soil which is again used by the plants. Some bacteria then convert these nitrogenous compounds in the soil and turn it into nitrogen gas. Eventually, it goes back to the atmosphere. These set of processes repeat continuously and thus maintain the percentage of nitrogen in the atmosphere.

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