What is the nitrogen cycle?

We explain what the nitrogen cycle is, what its stages are and their importance. In addition, the nitrogen cycle in water.

1. What is the nitrogen cycle?

The nitrogen cycle is the biogeochemical circuit that supplies nitrogen to living things and keeps it circulating in the biosphere . It is composed of biotic and abiotic processes . Ammonium (NH ₄ ⁺ ) and nitrate (NO ₃ ^– ) form some of the most important presentations of this element, as well as biatomic nitrogen in a gaseous state (N ₂ ).

It is one of the most important biogeochemical cycles for the balance of life, since nitrogen (N) is an extremely abundant chemical element in the composition of organic matter and in the Earth’s atmosphere (78% of its volume).

In this cycle the different levels of living beings , autotrophs and heterotrophs , the tiny decomposing organisms of organic matter, and the immense volume of nitrogen in the atmosphere are interrelated .

The nitrogen cycle can be summarized as follows:

• Nitrogen fixation . This gaseous element is fixed by bacteria and other prokaryotes by various metabolic processes , turning it into different usable organic compounds, such as ammonia (NH ₃ ). These microorganisms can be found in soil and water, or as plant symbiotes . These nitrogen molecules are used by plants, which make up various organic molecules with them.
• Transmission to animals . Following the order of the food chain , the nitrogen in the plants passes to the herbivorous animals and then to the carnivores , spreading between the different links of the food pyramid. Excess nitrogen is expelled from their bodies through urine, rich in ammonia, thus returning to the ground to continue the cycle.
• Nitrificante decomposition . The soil ammonia , coming from the urine of the animals or the action of the fixing bacteria, serves as food for another type of microorganisms of nitrifying action, that is, that break down the ammonia and oxidize it into nitrites (NO ₂ 
  ^– ) and nitrates (NO ₃ ^– ).
• Denitrifying decomposition . The latter compounds, in turn, serve as food for other types of prokaryotes, this time of denitrifying metabolism, that is, that break down the nitrite and nitrate molecules, obtaining energy to live and releasing the nitrogen into the atmosphere gas, so that the cycle can restart.

2. Importance of the nitrogen cycle

The nitrogen cycle is a vital circuit for the existence of life as we know it, since life forms as animals, plants and even humans are unable to fix nitrogen from its gaseous form (N ₂ ), although we need it tremendously for our tissues .

For that reason, we depend on the manipulation of gas by other life forms, which are not less important by microscopic. This is how nitrogen comes to us through a long chain of transmission.

3. Nitrogen cycle in water

The nitrogen cycle does not vary much when it occurs in water, that is, on the surface of lakes, seas and rivers. Nitrogen can reach the water by runoff , the result of human or natural fertilizers. In other cases, it is transmitted by marine trophic chains , in which many land animals are incorporated into the sea.

In any case, this entry of nitrogenous organic substances is distributed among the different predators , leaving a residue of nitrogen material in the ocean floor , where it is decomposed by various types of microorganisms. Thus, the microscopic cycle between nitrifiers and denitrifiers is repeated, and the gaseous nitrogen is released back into the atmosphere.