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Nitrogen Cycle

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The nitrogen cycle is a complex biological process that describes the transformation of nitrogen and its compounds in the environment. Nitrogen is an essential element that helps in the growth and survival of the living organism. The nitrogen cycle is a continuous process that involves several key steps, including nitrogen fixation, nitrification, assimilation, ammonification, and denitrification. The nitrogen cycle is a vital component of global biogeochemical cycles and plays a critical role in maintaining the balance of nitrogen in the environment. The nitrogen cycle is important for agriculture, environmental management, and ecosystem conservation.

Nitrogen Cycle

Nitrogen, which is found in proteins and nucleic acids, is necessary for life on Earth to exist. Nitrogen is made available to plants through a sequence of microbial changes, which in turn support all animal life.

The nitrogen cycle is a biogeochemical process in which inert nitrogen in the atmosphere is converted into a form that may be used by living organisms.

Organic nitrogen is found in live creatures and is transferred down the food chain through other living organisms’ intake. There is a massive amount of inorganic nitrogen in the atmosphere. This nitrogen is made available to plants by symbiotic bacteria that convert inert nitrogen into helpful forms such as nitrites and nitrates. Nitrogen undergoes numerous changes to maintain ecosystem balance. The marine nitrogen cycle is one of the most complex biogeochemical cycles, affecting a wide variety of biomes.

Nitrogen Cycle


Stages of Nitrogen Cycle

Nitrogen fixation, nitrification, assimilation, ammonification, and denitrification are the steps in the nitrogen cycle process. Via this nitrogen circulates in the ecosystem.

Nitrogen Fixation

Nitrogen fixation is a chemical process that turns nitrogen gas into nitrates and nitrite in the atmosphere, which has a strong triple covalent bond, into ammonia or related nitrogenous compounds, primarily in the soil or aquatic environments but also in industry.

The symbiotic nitrogen-fixing bacteria infiltrate host plant root hairs, multiplying and stimulating the production of root nodules, plant cell enlargements, and bacteria in close proximity. The bacteria within the nodules convert free nitrogen to ammonia, which the host plant uses for growth. Seeds are frequently injected with commercial cultures of appropriate Rhizobium species to promote sufficient nodule formation and optimum legume growth.

Types of Nitrogen Fixation

  1. Atmospheric Fixation: It is a natural phenomenon in which the lightning energy breaks the nitrogen into nitrogen oxide, which circulates in the ecosystem. Nitrogen enters the ecosystem with the help of plants.
  2. Industrial Nitrogen Fixation: Human power make the alternative of nitrogen fixation, by ammonia usage. Ammonia is formed from the direct combination of nitrogen and hydrogen, which is further converted into different fertilizers. Example- Urea.
  3. Biological Nitrogen Fixation: Nitrogen in the air is not directly taken by the plants. Plats take the nitrogen from the soil with the help of roots and bacteria (rhizobium and blue-green algae), which make the symbiosis interaction between the plant and animal. Bacteria convert the unusable nitrogen into usable compounds taken by the plant.


Plants use their roots to absorb nitrogen molecules from the soil, which are available in the forms of ammonia, nitrite ions, nitrate ions, or ammonium ions and are used to make plant and animal proteins. Plants enter the food web in this way when they are eaten by primary consumers. It converts first into the nitrite ion and then to the ammonium ion which adds in amino acids, nucleic acids, chlorophyll, etc. Plants have a symbiotic association with rhizobia, plants provide nutrients to the bacteria and bacteria provide nitrogen to the plant.


When plants or animals die, the nitrogen in organic matter is released back into the soil. Decomposers, present in the soil, turn organic waste back into ammonium, this process is known as ammonification or mineralization. During the decomposition process, ammonia is created, which is then used in other biological activities.

Nitrogen in the ecosystem should be available in a form that living organisms can use. In this condition, ammonification is critical because it gives nitrogen to the soil in a way that plants can use and pass along the food chain. Ammonification is the greatest technique for obtaining nitrogen for many types of plants that live in acidic soils. When fertilizers are used to increase ammonia levels in the soil, algae can overgrow, causing soil toxicity and ecosystem imbalance. Enzymes involved in the ammonification are:

  • Gln Synthetase
  • Glu-2 oxoglutarate aminotransferase.
  • Glu dehydrogenase


The presence of microorganisms in the soil converts ammonia to nitrate in this process. When Nitrosomonas bacteria species oxidize ammonia, they produce nitrates. Nitrobacter then transforms the generated nitrites into nitrates. This response is crucial because ammonia gas is toxic to plants.

2NH4+ + 3O2 → 2NO2 + 4H+ + 2H2O

2NO2 + O2 → 2NO3


This biogeochemical process is one of the primary responses to changes in oxygen content in the environment. Denitrification is a natural process that occurs at extreme concentrations in both terrestrial and aquatic ecosystems in regulated environments such as marine and freshwater settings, tropical and temperate soils, wastewater treatment plants, aquifers, manure storage, and so on. Denitrification did by Pseudomonas and Paracoccus during anaerobic condition example Waterlogged soil. Bacteria use the nitrate of the earth to carry out the respiration process.

Marine Nitrogen Cycle

Marine Nitrogen Cycle


The marine nitrogen cycle follows the same pattern as the terrestrial nitrogen cycle. There is only one difference the marine cycle process is performed by marine bacteria and archaea.

Sediment rocks are the main source of nitrogen in the marine ecosystem. Due to weathering of the sediment rock, nitrogen is added to the marine ecosystem which is later consumed by the plant.

Importance of Nitrogen Cycle

Following are the importance of nitrogen cycle are:

  1. Nitrogen is an important part of the cell. Nitrogen is used from the important protein and biomolecules of the cell.
  2. Nitrogen is used in plants to make chlorophyll,
  3. Used in various biochemical processes of plants.
  4. Nitrogen compounds like nitrites and nitrates maintain the richness of the soil, making the soil good for cropping.
  5. The bacteria help in the decomposition of plants and animals during the process of ammonification which indirectly helps to clean the environment.
  6. Nitrogen is an important component in fertilizers.

Consequences of Human Modification of Nitrogen Cycle

  1. Impacts of Natural Resources- Higher concentrations of nitrogen negatively impact both terrestrial and marine cycles. It can be toxic to some plant species.
  2. Impact on Humans- Excess secretion of Nr (reactive nitrogen) because of human activities in the ocean leads to nitrate accumulation, which is harmful to human health.
  3. Impact on Human Health- Due to human activities nitrogen gases and their products are released into the atmosphere called nitrogen pollution which changes the nitrogen cycle as well. When nitrogen and its components are released into the atmosphere it makes smog, aerosols, and particulate matter which affect human health by affecting the AQI of air.

FAQs on Nitrogen Cycle

Q1: What is the nitrogen cycle?


The nitrogen cycle is the biogeochemical cycle through which nitrogen is converted into different forms.

Q2: Why do plants need nitrogen?


Nitrogen is the basic building agent which is used for the formation of chlorophyll. Chlorophyll is the important green pigment that is important for the photosynthesis process. A deficiency of nitrogen directly affects the growth of plants and other irregularities.

Q3: What are the different steps of the nitrogen Cycle?


Following are the different steps of nitrogen cycle are:

  1. Nitrogen Fixation
  2. Assimilation
  3. Ammonification
  4. Nitrification
  5. Denitrification

Q4: Who discovered the Nitrogen Cycle?


Jules Reiset in 1856 discovered the nitrogen cycle.

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Last Updated : 02 May, 2023
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