The Nitrogen Cycle

One of the most significant component present not only on our Earth but also in our body is the Nitrogen. Beginning from the body’s genetic constituents like nucleic acids and proteins to the other forms of nature it is considered to be essential ingredients. The 78 % of the volume is covered by the nitrogen only of all the other gases found in the atmosphere, however this abundant resources is still unused by numerous living beings. The part of nitrogen that is been consumed by the microbial organisms is then available to plants and then is passed to the other animal life.

Understanding Nitrogen Cycle

The movement of nitrogen within and between the layers such as atmosphere, biosphere, hydrosphere and geosphere is termed as nitrogen cycle. Nitrogen travels through the abiotic and biotic parts of the Earth’s system. It (nitrogen) enters the soils from the urea of urine, and dead and decayed material of the plants and animals. Further, this waste material ( urine and proteins) starts degrading and decomposing by the few bacteria present in the soil to form ammonia (NH3) and other products. This process is known as nitrogen fixation.

Gradually this ammonia is converted into nitrate (NO3-) and nitrite (NO2-) and certain other bacteria belonging to the Nitrobacter and Nitrosomonas genera. These products are used by the plants and other bacteria, like the nitrate is degraded by the microorganisms to release the nitrogen and thereby enters the atmosphere.

The available atmospheric nitrogen in the atmosphere is consumed by the nitrogen fixing bacteria (found in the roots of the leguminous plants) and is also used in the synthesis of biomolecules (like amino acids). So, ultimately the consumers that are predators and herbivores gain their nitrogen from the plants and animals they consume or eat. later on, this nitrogen is released back to the atmosphere with the animals and plants waste and then is again decomposed by the bacteria and fungi.

Finally, the energy is released and consumed and even the nitrogen is recycled numerous times in the atmosphere with the help of nitrogen and microorganisms by converting the nitrite and nitrate into nitrogen gas ( denitrification).

Elaborating the process, so there are five main steps involved in the nitrogen cycle like: nitrogen fixation, nitrogen uptake, nitrogen mineralization, nitrification and denitrification. These are been described below.

Nitrogen Fixation

In the process of nitrogen fixation, the nitrogen (N2) is converted to ammoniums (NH4+), the only way to fix or get the nitrogen from the atmosphere and the microorganisms involved in this are known as nitrogen-fixing organisms. These types of microorganisms belongs the genus Rhizobium, that has the potential to fix nitrogen and can it into ammonium by undergoing the metabolic processes.

These occurs often forms the symbiotic relationship with the host plants, this symbiosis is commonly observed in the legume family such as peas, beans and clover. In this relationship both the host plant and microorganisms are benefited. As the microorganism inhabit themselves on the roots nodules of the legume plants and receive carbohydrates and a safe environment to get the carbohydrates and in exchange they provide the nitrogen they fixed during the time.

However, there are free-living nitrogen fixing bacteria such as cyanobacteria. They do not need any host plant to fix the nitrogen from the atmosphere. In addition, there few high-energy natural events that are responsible for fixing nitrogen in a significant amount, these events are forest fires, lightning, and the volcanoes and hot lava flows. These events and their energy flow are capable of breaking the triple bonds of the N2 molecules and make the N atoms available to the atmosphere for further transformation.

Nitrogen Uptake (Assimilation)

NH4+→ Organic N

The nitrogen-fixing bacteria produce ammonium (NH4+), which is taken up either by the host plant or other organisms present in the soil, bacterial itself and is integrated in making proteins, amino acids and other organic nitrogen compounds.

Nitrogen Mineralization (Ammonification)

Organic N →NH4+

In this process, after the incorporation of the nitrogen into organic matter, it is once again converted into the inorganic nitrogen, which is also known as the ‘decay’. The waste of the dead pants and animals are consumed by the decomposers and are involved into the process of decomposing organic matter. Now again, the vital amount of nitrogen carried within the dead organisms is free to get converted into the ammonium. Now this free ammonium is ready to be consumed by the plants and then this will transformed into nitrate (NO3-) by the process of nitrification.


NH4+ → NO3-

Here, the ammonia is transformed into the nitrate with the help of bacteria present in the soil. Whereas, by the oxidation of ammonia, and in presence of Nitrosomonas bacteria, they produce nitrites. Sequentially, these nitirites are converted into the nitrates by the Nitrobacter. This break down process is very crucial one, as the ammonia gas is toxic to the plants.

Nitrification takes place in the presence of oxygen, and therefore the oxygen-rich environment such as flowing waters, top layers of the soil and sediments are the needed.

This is reaction:  2NH3 + 3O2→ 2NO2- +2H+ + 2H2O
2NO2- + O2 → 2NO3-


NO3- → N2 + N2O

Finally, when the nitrogen compounds makes it come back to the atmosphere is denitrification by the converting the nitrate (NO3-) into the gaseous nitrogen (N). The responsible bacterial species for the denitrification are the Pseudomonas and Clostridium denitrifying bacterial species, that are helpful in processing the bitrate into oxygen and releasing nitrogen gas as the byproduct.

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