Introduction

Nitrogen is the most abundant gas present in our atmosphere. It makes more than 70% of the gases present in the atmosphere of our planet. Being a component of organic compounds like proteins and nucleic acids, nitrogen is an important nutrient required for the normal growth and development of living organisms.

The nitrogen cycle is a closed cycle during which nitrogen present in the atmosphere is converted to chemical compounds that are used by living organisms. Once the organic compounds are used by the living organisms, nitrogen present in them is returned to the atmosphere in gaseous form. This cyclic flow ensures that nitrogen in the atmosphere is never depleted and is recycled again and again.

In this article, we will discuss various aspects of the nitrogen cycle, its effect on life in an ecosystem, and modifications in the normal cycle because of human influences.

Components of Nitrogen Cycle

Like any other nutrient cycle, the nitrogen cycle is also divided into three components; reservoirs, sources, and sinks.

Reservoirs

Reservoirs are the major stores of any nutrient in the environment. The reservoirs of nitrogen are as follows:

• Atmosphere: The atmosphere is the most important reservoir of gaseous nitrogen. Nitrogen makes around 78% of the total gases present in the atmosphere.
• Biosphere: Nitrogen is present in the biosphere as a component of organic compounds found in living organisms. Nitrogen is a part of proteins and nucleic acids found in all the living cells. proteins make more than 50% of the dry mass of cells. They are the most abundant organic compounds present in the biosphere and contain nitrogen.
• Hydrosphere: Nitrates and nitrites present in oceans and seas constitute the nitrogen present in the hydrosphere.
• Lithosphere: Nitrogen is also present in the lithosphere as metallic salts of nitrates and nitrites.

Sources

Sources in a nutrient cycle are the means by which a nutrient gets added to the environment. The major sources of nitrogen are as follows;

• Ammonia produced during the metabolic processes is released into the environment.
• Urea also contains nitrogen that is excreted from the bodies of animals.
• Ammonia is also produced by some bacteria and added to the environment.
• Some special bacteria called denitrifying bacteria convert salts of nitrogen into gaseous nitrogen and release it into the atmosphere.

Sinks

Sinks are the ways by which a nutrient is consumed. Nitrogen is mainly consumed by living organisms to make proteins, nucleic acids, and other compounds of biological importance. It is required for the normal growth and development of organisms.

Read more about Nutrient Recycling

Process during Nitrogen Cycle

Various chemical processes are involved in the nitrogen cycle for the synthesis and conversion of different nitrogenous compounds. During the nitrogen cycle, nitrogen flows through several compounds before it is added back to the atmosphere. A brief detail of all these processes is discussed below.

Nitrogen Fixation

Nitrogen in gaseous form cannot be used by living organisms. It must be first converted into useable forms to begin the nitrogen cycle. During nitrogen fixation, nitrogen gas is converted to nitrates and nitrites that can be easily absorbed and used by living organisms.

Nitrogen fixation can take place in three ways; atmospheric, industrial, and biological.

• Atmospheric fixation: During this process, nitrogen and oxygen present in the atmosphere are converted to nitrogen oxides by lightning. These oxides then react with water to form nitrous or nitric acid. Acids upon disassociation form nitrates or nitrites.
• Industrial fixation: Industrial processes can reduce nitrogen to nitrates and nitrites by its reaction with various chemical compounds. There are various methods to carry out industrial fixation.
• Biological fixation: It is carried out by specific microorganisms called diazotrophs. Several bacteria are included in this group of microorganisms. They mainly harbor the root nodules of plants like legumes. These microorganisms have nitrogenase enzymes that combine hydrogen with nitrogen to form ammonia. Some other microorganisms can add this ammonia to organic matter.

Assimilation

During this process, nitrogen is made a component of organic compounds by using it in the synthesis of amino acids. It is done by plants.

Nitrates and ammonia are absorbed by plants using their roots. Plants reduce nitrate ions to nitrite ions that are then converted to ammonium ions. Ammonia is also converted to ammonium ions by plants. These ammonium ions are used to make amino acids, nucleic acids, and other nitrogen-containing compounds. 

It is the process of assimilation by which atmospheric nitrogen enters the food chain of an ecosystem.

Ammonification

Nitrogen is added back to the soil by animal wastes or by death and decay of the organic matter. It is present in organic form in the organic matter. Ammonification is the process by which nitrogen-containing organic compounds are converted to ammonium ions.

The ammonification process is carried out by some bacteria and fungi. It takes place during the decomposition of the organic matter present in the soil.

Nitrification

As mentioned earlier, nitrogen-fixing bacteria convert atmospheric nitrogen to ammonia during the process of nitrogen fixation. This ammonia is highly toxic to plants. Moreover, it can be absorbed and used by plants for assimilation purposes. Nitrification is the biological process by which ammonia is oxidized to nitrate ions.

This process takes place in two steps;

1. In the first step, ammonia is converted to nitrite ions
2. The second step involves the oxidation of nitrite to nitrate ions

It is carried out by certain bacteria called nitrifying bacteria. Examples of such bacteria are Nitrosomonas and Nitrobacter species. These are the anaerobic bacteria commonly found in soil. Nitrification is an anaerobic process that takes place only in the absence of oxygen.

Denitrification

It is a process by which nitrates, and nitrites are reduced back to nitrogen gas. It is the process by which nitrogen is added back to the atmosphere.

It is carried out by certain bacteria called denitrifying bacteria.  An example of denitrifying bacteria is the members of Pseudomonas species.

The denitrification process takes place at the end of the nitrogen cycle. It serves to complete the cycle by returning nitrogen to the atmosphere.

The Cyclic Path

As we have understood the processes taking place during the nitrogen cycle. Let us now discuss the cyclic flow of nitrogen in a series.

• Nitrogen is present in the atmosphere in gaseous form.
• Nitrogen fixation by lightning, industrial or biological process converts this nitrogen into nitrates or ammonia.
• Ammonia is converted to nitrates by nitrifying bacteria.
• These nitrates are absorbed by plants from the soil.
• Plants use the nitrogen present in nitrates to make proteins, nucleic acids, and other nitrogen-containing compounds.
• Nitrogen enters the food web as plants are eaten by animals.
• Animals break down the nitrogenous organic compounds to simple molecules and use them for obtaining energy or for biosynthesis.
• Ammonia is produced in the animals during metabolic processes.
• Animals convert this ammonia to urea which is excreted in urine or feces.
• Urea present in animal dung is broken down to ammonium ions that are then oxidized to nitrates by nitrifying bacteria.
• These nitrates can again be consumed by plants and the cycle continues.
• As the animals of plants die, nitrogen present in their organic compounds is converted to ammonium ions during ammonification. These ammonium ions can also be oxidized to nitrates.  
• Nitrates present in the soil are acted on by denitrifying bacteria that reduced them to nitrogen gas.
• Nitrogen gas is released into the atmosphere and the cycle completes.

The cyclic process continues to operate in an ecosystem as long as it is not disturbed by external influences.

Role of Organisms

Under this heading, we will discuss the role of different organisms in the nitrogen cycle. Organisms at different levels in a food chain have the following significance during this cycle.

Producers

Producers are responsible for bringing the nitrogen into the food chain. Plants assimilate the nitrogen into organic compounds. These compounds become a part of the food chain as plants are eaten by animals.

Consumers

Animals present at this level eat plants and use nitrogen for various purposes. They convert excess nitrogen into ammonia gas and urea. Both these products are excreted from the bodies of animals. The excretory products of animals add nitrogen back to soil in the form of urea.

Decomposers

Bacteria and fungi present at this level play several roles during this cycle.

• Nitrogen-fixing bacteria are responsible for nitrogen fixation.
• Nitrifying bacteria convert ammonia to nitrates.
• Some bacteria and fungi decompose the dead plants and animals and convert organic nitrogen to ammonium ions.
• The denitrifying bacteria convert nitrates to nitrogen gas and release it into the atmosphere.

Human Influences and their Consequences

The nitrogen cycle continues to proceed in the normal way if left undisturbed. However, many activities of man cause considerable disturbances in the nitrogen cycle. The major human influences on the nitrogen cycle and its consequences are discussed under this heading.

Major Influences

• Human activities have doubled the annual transfer of nitrogen to biological forms like nitrates. It is due to increased legumes cultivation, excessive use of nitrogenous fertilizers, and air pollution.
• Reactive nitrogen is being generated due to human activities. It is being accumulated or trapped in the biological world as the rate of denitrification is much less than the rate of its production.
• Nitrous oxide is being added to the atmosphere due to excessive use of agriculture fertilizers, burning of biomass, and industrial processes.
• Ammonia is continuously being added to the atmosphere due to human activities.
• The increased combustion process has increased the production of nitrogen oxides by 6 to 7 folds.

Consequences

The above-mentioned disturbances in the nitrogen cycle have deleterious consequences on the biodiversity as well as atmospheric hygiene. The major impacts of human influences are mentioned below.

• The reactive nitrogen is harmful to live organisms. Its consumption in the biological processes can cause some serious health effects.
• Nitrous oxide acts as a catalyst in the breakdown of ozone. It also acts as a greenhouse gas and is responsible for increasing the temperature and global warming.
• Ammonia being added to the atmosphere can be converted to nitric acid in the presence of water. The nitric acid thus produced is responsible for acid rain.
• The presence of ammonia in the atmosphere can cause several respiratory problems.
• The increased availability of nitrogen can cause a decrease in biodiversity by increasing nitrogen-demanding species and degrading nitrogen-poor species of plants.

Summary

The nitrogen cycle is the cyclic flow of nitrogen through an ecosystem.

The atmosphere is the major reservoir of nitrogen gas. Nitrogen in other forms is present in the biosphere, hydrosphere, and lithosphere.

Nitrogen is added to the atmosphere by denitrifying bacteria or by the use of nitrogen fertilizers.

Nitrogen is used by living organisms to make organic compounds.

The major processes in the nitrogen cycle are as follows.

• Nitrogen fixation (conversion of nitrogen gas to ammonia or nitrates)
• Assimilation (using nitrates to make nitrogen organic compounds)
• Ammonification (converting organic nitrogen to ammonia)
• Nitrification (converting ammonia to nitrates)
• Denitrification ( conversion of nitrates and nitrites to nitrogen gas)

During the nitrogen cycle, it gets converted to various chemical compounds by the above-mentioned process, moves through the food chain, and is finally returned to the atmosphere by denitrifying bacteria.

Producers are responsible for the assimilation of nitrogen.

Consumers get nitrogen by eating plants and excrete it in the form of ammonia and urea in their metabolic wastes.

Decomposers are involved in nitrogen fixation, ammonification, nitrification, and denitrification.

Human activities due to industrialization, urbanization, and over-population are becoming a cause of adding nitrogen to the nitrogen cycle. More nitrogen is converted to the biological process. Increased nitrogen availability has disturbed the atmosphere as well as the biodiversity.

Human influences of the nitrogen cycle are responsible for harmful health effects, acid rains, global warming, ozone depletion, acid rain, and loss of biodiversity.

Frequently Asked Questions

What is the nitrogen cycle?

The nitrogen cycle is a cyclical process by which nitrogen present in the atmosphere is converted to compounds that are used by living organisms and nitrogen is finally returned to the atmosphere.

What are the 5 stages of the nitrogen cycle?

The 5 stages of the nitrogen cycle include nitrogen fixation, mineralization, nitrification, immobilization, and denitrification. 

Why nitrogen cycle is important?

Nitrogen is an essential element to sustain life on earth. It is a component of amino acids and nucleic acids that are essential for living organisms. It is the nitrogen cycle through which organisms get nitrogen from the atmosphere for making these compounds.

What is the role of nitrogen in living organisms?

Nitrogen is a component of amino acids and nucleotides. These are used to make proteins and nucleic acids. Thus life is not possible without nitrogen.

References

1. Fowler, David; Coyle, Mhairi; Skiba, Ute; Sutton, Mark A.; Cape, J. Neil; Reis, Stefan; Sheppard, Lucy J.; Jenkins, Alan; Grizzetti, Bruna; Galloway, JN; Vitousek, P; Leach, A; Bouwman, AF; Butterbach-Bahl, K; Dentener, F; Stevenson, D; Amann, M; Voss, M (5 July 2013). “The global nitrogen cycle in the twenty-first century”. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 368 (1621): 20130164. doi:10.1098/rstb.2013.0164. PMC 3682748. PMID 23713126.
2. Vuuren, Detlef P van; Bouwman, Lex F; Smith, Steven J; Dentener, Frank (2011). “Global projections for anthropogenic reactive nitrogen emissions to the atmosphere: an assessment of scenarios in the scientific literature”. Current Opinion in Environmental Sustainability. 3 (5): 359–369. doi:10.1016/j.cosust.2011.08.014. hdl:1874/314192. ISSN 1877-3435.
3. Britto, Dev T.; Kronzucker, Herbert J. (2002). “NH4+ toxicity in higher plants: a critical review”. Journal of Plant Physiology. 159 (6): 567–584. doi:10.1078/0176-1617-0774.
4. Camargoa, Julio A.; Alonso, Álvaro (2006). “Ecological and toxicological effects of inorganic nitrogen pollution in aquatic ecosystems: A global assessment”. Environment International. 32 (6): 831–849. doi:10.1016/j.envint.2006.05.002. PMID 16781774.