Nitrogen Cycle

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The nitrogen cycle represents one of the most important nutrients cycles found in terrestrial ecosystems. Nitrogen is used by living organisms to produce a number of complex organic molecules like amino acids, proteins, and nucleic acids. The store of nitrogen found in the atmosphere is about one million times larger than the total nitrogen contained in living organisms. Other major stores of nitrogen include Organic matter in soil and the oceans are other big storehouses of nitrogen. But in spite of its abundance in the atmosphere, nitrogen is often the most limiting nutrient for plant growth. This is mainly because most plants can only take up nitrogen in two solid forms: ammonium ion (NH4+ ) and the ion nitrate (NO3- ).


Why should I be aware of this?

Human activities are greatly increasing the amount of nitrogen cycling between the living world and the soil, water, and atmosphere. As a result of such activities, the rate of nitrogen entering the land-based nitrogen cycle has doubled. As nitrogen is essential to living organisms and plays a crucial role in the organization and functioning of the world’s ecosystems, such human activities are having serious impacts on ecosystems around the world.

The supply of nitrogen in many ecosystems on land and sea is a key factor in controlling the nature and diversity of plant life, the population dynamics of both grazing animals and their predators, and vital ecological processes such as plant productivity and the cycling of carbon and soil minerals. When there is excessive additions of nitrogen the ecosystem can be polluted and the ecological functioning and the living communities they support can get altered.

Most of the increases in global nitrogen are caused by human activities such as use of nitrogen fertilizers to the burning of fossil fuels in automobiles, power generation plants, and industries.

How does it affect me?

All organisms require Nitrogen (N) , which is an essential component of DNA, RNA, and [protein]]s, the building blocks of life. Although the majority of the air we breathe is N2, most of the nitrogen in the atmosphere is unavailable for use by organisms. This is because the strong triple bond between the N atoms in N2 molecules makes it relatively inert.

For N2 gas to become usable by plants and animals, it must first be converted to a more chemically available form such as ammonium (NH4+), nitrate (NO3-), or organic nitrogen (e.g. urea - (NH3)2CO).

All about Nitrogen cycle

Nitrogen is an essential component of proteins, genetic material, chlorophyll, and other key organic molecules and ranks fourth behind oxygen, carbon, and hydrogen as the most common chemical element in living tissues.

Nitrogen exists in both inorganic and organic forms as well as in many different oxidation states. The movement of nitrogen between the atmosphere, biosphere, and geosphere in different forms is described by the nitrogen cycle, one of the major biogeochemical cycles. The nitrogen cycle, just like the carbon cycle, consists of various storage pools of nitrogen and processes by which the pools exchange nitrogen.

Though the Earth’s atmosphere is 78 percent nitrogen gas, most plants and animals cannot use nitrogen gas directly from the air as they do carbon dioxide and oxygen. For their use nitrogen has to be pulled from the air and bonded to hydrogen or oxygen to form inorganic compounds, mainly ammonium (NH4) and nitrate (NO3).

Nitrogen fixation

The process of conversion of nitrogen for use by plants and animals is called nitrogen fixation. It comprises of:

  • Atmospheric fixation: High temperature due to lightning allows nitrogen to combine with oxygen to form oxides of nitrogen, which get fixed in the soil.
  • Biological Fixation: Nitrogen is fixed by bacteria and blue green algae. For example, root nodules of leguminous plants like peas and beans have nitrogen fixing bacteria which directly absorb nitrogen to form nitrogen compounds.
  • Industrial Fixation: This is artificial fixation brought about by science and technology. Ammonia is formed which is oxidized to form nitrates, that are used to synthesize nitrogenous fertilizers. Due to its increased and widespread use, it is estimated that nearly one third of all nitrogen in human body now comes from artificial sources.

Increased human activity

Increased human activities have led to a rise in the availability of nitrogen. Rather than being advantageous, this increase in activity and nitrogen availability is disrupting and helping to destroy parts of the Earths environment. The influence of added sources of nitrogen can cause cataclysmic changes in the environment and animal life in a biosphere.

Plant life that has been able to adapt to increased nitrogen levels has flourished, while the less adaptable species have dwindled away. Humans have contributed to the loss of this plant life by overuse of land and non-rotation of crops.

Rising population and increased food production saw an excessive use of chemicals which affected land and water security. Excessive use of nitrogen led to nitrate-leaching and caused water pollution. Nitrogen fertilizers are also based on inputs like petroleum, and are therefore ecologically and economically expensive.


  • About 78% of our atmosphere is nitrogen. Oxygen on the other hand is 21% of our atmosphere
  • When large amounts of nitrogen collect in a water body, eutrophication can result. This is an accumulation of excess nutrients which causes an algae bloom. The algae rapidly deplete all of the oxygen in the water, making it inhospitable for fish and other aquatic organisms.

See also


  • Of Microbes and Men
  • Human Alteration of the Global Nitrogen Cycle