Nitrogen Cycle

Nitrogen Cycle Definition: The nitrogen cycle can be defined as one of the biogeochemical cycles that converts the unusable inert nitrogen existing in the atmosphere into a more usable form of nitrogen for living organisms. 

What is the Nitrogen cycle?

The nitrogen cycle is a biogeochemical process in which nitrogen, in various forms, is circulated from the atmosphere to the living organisms and later back to the atmosphere. Living organisms require nitrogen for the synthesis of nucleic acid and proteins. The atmosphere contains almost 78% of nitrogen present in an inert form (N2). This nitrogen cannot be used by living organisms unless it is converted to ammonia, nitrates, and other usable compounds of nitrogen.

The nitrogen cycle is a cyclic process where the nitrogen travels from inorganic form in the atmosphere and to the organic way in the living organisms. The nitrogen cycle contains several steps, such as Nitrogen fixation, assimilation, ammonification, nitrification, and denitrification. This cycle is essential in maintaining a proper ecological balance and is present in both marine and terrestrial ecosystems.

Nitrogen Cycle Steps

Nitrogen Fixation

The first step involves the fixation (conversion) of atmospheric inert nitrogen into a usable form of nitrogen. Here, the N2 form of nitrogen is converted into NH3 (Ammonia). This process is carried out by symbiotic bacteria present in the soil called Diazotrophs. These are bacteria, primarily known for its nitrogen fixation in nature. E.g., Rhizobium. There are three ways nitrogen fixation can take place.

  1. Atmospheric Nitrogen Fixation: The inert nitrogen present in the atmosphere is converted to nitrous oxide by the help of lightning due to the high-temperature present during lightning. The nitrogen is broken down into nitrogen atoms which react with oxygen to form nitrous oxide, nitrogen peroxide, and nitric oxide. These compounds later dissolve in the rain to form dilute nitric acid. When the dilute nitric acid reaches the earth's surface, it reacts with the alkalies present to form nitrates that plants can easily absorb.

  2. Biological Nitrogen Fixation: There exist nitrogen-fixing bacteria and blue-green algae that convert nitrogen present in the atmosphere into nitrates. There are two types of nitrogen-fixing bacteria:

  • Free-living bacteria: E.g., Azotobacter, and Clostridium.

  • Symbiotic bacteria: E.g., Rhizobium that is present in root nodules of individual leguminous plants like nostoc and Anabaena.

  1. Industrial Nitrogen Fixation: It is a human-made alternative where the atmospheric nitrogen is converted into ammonia by Haber's process and later into nitrates in various fertilizers.


The dead remains of plants, and animals are buried in the soil. They decay and create ammonia, carbon dioxide, and water, with the help of fungi like actinomyces. This process of formation of ammonia is called ammonification. Already ammonia exists in the soil with the help of nitrogen-fixing bacteria. Ammonification increases the concentration of ammonia in the ground.


The process in which the ammonia is converted into nitrites and later into nitrates is called Nitrification. This process takes place in two steps:

  1. Conversion of ammonia into nitrites: This takes place by the action of Nitrosomonas bacteria. They oxidize the ammonia present in the soil and convert them to nitrites.

The reaction is as follows:

  1. Conversion of nitrites to nitrates: This takes place by the action of Nitrobacter species, which convert the nitrates in the soil into nitrates.

The reaction is as follows:


In this process, the formed nitrates in the soil get absorbed by the plants through their root system. The plants contain nitrates that are consumed by the consumers and then later process through the food chain and enters the food web. Assimilation is the absorption of nitrates and other nitrogen compounds. The nitrogen compounds are essential for the formation of crucial biomolecules.


The plants do not absorb some Nitrates. They are converted into atmospheric nitrogen with the help of pseudomonas and clostridium. This process is the last step where the nitrogen compounds present in the soil makes its way back to the atmospheric nitrogen. 

The Nitrogen Cycle in the Marine Ecosystem


The marine ecosystem also has a similar manner of nitrogen cycle. The nitrogen from the atmosphere gets absorbed in the water, and nitrogen-containing compounds sediment as rocks on the ocean floor. Many species cannot break the strong bond between the nitrogen. But few bacteria can oxidize the nitrogen molecule and convert it into ammonia. The phytoplankton plants can absorb the ammonia. Some bacteria can consume the ammonia and release nitrites. The nitrites are then converted to nitrates that can later be used by another microorganism in the marine ecosystem. This process of converting ammonia into nitrates is called Nitrification. Larger organisms like the whale, fish, etc. get their supply of nitrogen by consuming phytoplankton. When the fish die eventually, they sediment to the ocean floor. They are decomposed by the bacteria present and release ammonia which is again converted to nitrates by Nitrification, and the cycle continues.

Importance of the Nitrogen Cycle

  1. Chlorophyll is an essential pigment for the process of photosynthesis. The nitrogen cycle helps the plants to manufacture chlorophyll from the compound of nitrogen. 

  2. It is essential for the survival of plants as plants need nitrates to survive and grow. 

  3. During the process of formation of ammonia, the dead and decayed organic matter is decomposed by bacteria. This process helps the environment to be cleaned up from organic matter and also provides essential nutrients required by the soil.

  4. Nitrogen compounds enrich the soil and make it fertile and suitable for growing plants.

  5. Nitrogen is a necessary element in the cells and tissues of living organisms. It forms proteins and nucleic acid, which form the essential elements of life. Without nitrogen compounds, life could not exist.

  6. Combustion of fuels and fertilizers also contains nitrogen that increases the percentage of nitrogen in the atmosphere.

  7. Eutrophication is the accumulation of nitrogen in water bodies when the nitrogen from the fertilizers in the soil is washed away.

Important Points on Nitrogen Cycle

  • Nitrogen is present in the atmosphere in abundance, but cannot be used by the plants and other organisms directly from the atmosphere.

  • Nitrogen is fixed in three ways which are atmospheric, industrial, and biological means. The atmospheric nitrogen is converted into ammonia.

  • Nitrogen-fixing bacteria like Azotobacter and Rhizobium play a vital role in the formation of nitrogen compounds. 

  • Dead and rotten plants are decomposed by fungi like actinomyces and ammonia, carbon dioxide and water is released. This process is called ammonification.

  • Nitrosomonas convert ammonia into nitrites and later to nitrates by Nitrobacter bacteria by the process of Nitrification.

  • Plants absorb the nitrates, and nitrogen is used to form important cell organelles and biomolecules. The process of absorption of nitrogen compounds from the plants is called assimilation.

  • The nitrates present in the soil are converted into free nitrogen by pseudomonas bacteria. This process is called denitrification.

  • The cycle repeats, and the nitrogen percentage in the atmosphere remains stable.

  • Nitrogen cycle also exists in the marine ecosystem where the phytoplankton plants and other bacteria convert the nitrogen into nitrogen compounds. 

  • This cycle is a critical biogeochemical cycle in nature that is necessary for life processes.