Nitrogen Cycle

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. Before further discussing the nitrogen cycle, we must know some facts about nitrogen. Nitrogen is an essential constituent of all organisms. Nitrogen atoms can be detected in all proteins and nucleic acids. Nitrogen generally exists as nitrogen gas (N2) in the environment. Nitrogen fixation is the process by which bacteria transform gaseous nitrogen into ammonia, a type of nitrogen that plants may use. Plants provide usable nitrogen molecules to animals when they eat them. In both nature and agriculture, nitrogen is a typical limiting nutrient. A limiting nutrient is one that is in limited quantity and hence restricts development. It is thus clear that nitrogen is an essential constituent of nature. The article focuses on the details of the nitrogen cycle.

What is 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

There are several steps of the nitrogen cycle as mentioned above, the complete process can be classified into nitrogen fixation, assimilation, ammonification, nitrification, and denitrification steps. Each of the steps is described below in the article. 

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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 their nitrogen fixation in nature, e.g., rhizobium.

There are three ways nitrogen fixation can take place.

• Atmospheric Nitrogen Fixation: The inert nitrogen present in the atmosphere is converted to nitrous oxide with 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.

• 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: For example, Azotobacter, and Clostridium.

• Symbiotic Bacteria: For example, Rhizobium that is present in root nodules of individual leguminous plants like Nostoc and Anabaena.

• 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.

Ammonification 

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.

Nitrification

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 chemical equation to represent the reaction is mentioned below.

2NH4+ + 3O2 → 2NO2– + 4H+ + 2H2O

2. Conversion of Nitrites to Nitrates: This takes place by the action of Nitrobacter species, which convert the nitrates in the soil into nitrates. The chemical equation of the reaction is mentioned below.

2NO2– + O2 → 2NO3– 

Assimilation

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 enter the food web. Assimilation is the absorption of nitrates and other nitrogen compounds. The nitrogen compounds are essential for the formation of crucial biomolecules.

Denitrification

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 make their way back to the atmospheric nitrogen. 

The Nitrogen Cycle in the Marine Ecosystem

The marine ecosystem also has a similar manner to the 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 ammonia. Some bacteria can consume 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.

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Importance of the Nitrogen Cycle

There are various important uses of the nitrogen cycle, some of which are summarised below. 

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

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

• 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.

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

• 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.

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

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

Important Points on the Nitrogen Cycle

Some of the interesting facts about the nitrogen cycle are summarised below. 

• 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 are 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 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.

• The 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.

To conclude the article, it can be said that nitrogen is an essential part of the environment and the nitrogen cycle plays an important role in the sustenance of the ecosystem. In this article, we have learnt about the nitrogen cycle and its importance.