The cycle of nitrogen it is the process of nitrogen movement between the atmosphere and the biosphere. It is one of the most relevant biogeochemical cycles. Nitrogen (N) is an element of great importance, since it is required by all organisms for their growth. It is part of the chemical composition of nucleic acids (DNA and RNA) and proteins.
The largest amount of nitrogen on the planet is found in the atmosphere. Atmospheric nitrogen (Ntwo) cannot be used directly by most living things. There are bacteria capable of fixing it and incorporating it into soil or water in ways that can be used by other organisms..
Subsequently, nitrogen is assimilated by autotrophic organisms. Most heterotrophic organisms acquire it through food. Then they release the excesses in the form of urine (mammals) or excrement (birds).
In another phase of the process there are bacteria that participate in the transformation of ammonia into nitrites and nitrates that are incorporated into the soil. And at the end of the cycle, another group of microorganisms uses the oxygen available in nitrogenous compounds in respiration. In this process they release nitrogen back into the atmosphere.
Currently, the largest amount of nitrogen used in agriculture is produced by humans. This has resulted in an excess of this element in soils and water sources, causing an imbalance in this biogeochemical cycle..
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Nitrogen is considered to have originated by nucleosynthesis (creation of new atomic nuclei). Stars with large masses of helium reached the pressure and temperature necessary for nitrogen to form.
When the Earth originated, nitrogen was in a solid state. Later, with volcanic activity, this element became a gaseous state and was incorporated into the planet's atmosphere..
Nitrogen was in the form of Ntwo. Probably the chemical forms used by living things (ammonia NH3) appeared by nitrogen cycles between the sea and volcanoes. In this way, the NH3 it would have been incorporated into the atmosphere and together with other elements gave rise to organic molecules.
Nitrogen occurs in various chemical forms, referring to different oxidation states (loss of electrons) of this element. These different forms vary both in their characteristics and in their behavior. Nitrogen gas (Ntwo) is not rusty.
Oxidized forms are classified into organic and inorganic. The organic forms appear mainly in amino acids and proteins. The inorganic states are ammonia (NH3), the ammonium ion (NH4), nitrites (NOtwo) and nitrates (NO3), among others.
Nitrogen was discovered in 1770 by three scientists independently (Scheele, Rutherford, and Lavosier). In 1790 the French Chaptal named the gas as nitrogen.
In the second half of the 19th century, it was found to be an essential component of the tissues of living organisms and in the growth of plants. Likewise, the existence of a constant flow between organic and inorganic forms was evidenced..
Nitrogen sources were initially considered to be lightning and atmospheric deposition. In 1838, Boussingault determined the biological fixation of this element in legumes. Then, in 1888, it was discovered that the microorganisms associated with the roots of legumes were responsible for the fixation of Ntwo.
Another important discovery was the existence of bacteria that were capable of oxidizing ammonia to nitrites. As well as other groups that transformed nitrites into nitrates.
As early as 1885, Gayon determined that another group of microorganisms had the ability to transform nitrates into Ntwo. In such a way, that the nitrogen cycle on the planet could be understood.
All living things require nitrogen for their vital processes, but not all use it in the same way. Some bacteria are able to use atmospheric nitrogen directly. Others use nitrogen compounds as a source of oxygen.
Autotrophic organisms require a supply in the form of nitrates. On the other hand, many heterotrophs can only use it in the form of amino groups that they obtain from their food.
The largest natural source of nitrogen is the atmosphere, where 78% of this element is found in gaseous form (Ntwo), with some traces of nitrous oxide and nitrogen monoxide.
Sedimentary rocks contain approximately 21% that are released very slowly. The remaining 1% is contained in organic matter and the oceans in the form of organic nitrogen, nitrates and ammonia.
There are three types of microorganisms that participate in the nitrogen cycle. These are fixatives, nitrifiers, and denitrifiers.
They encode a complex of nitrogenase enzymes that are involved in the fixation process. Most of these microorganisms colonize the rhizosphere of plants and develop within their tissues..
The most common genus of fixing bacteria is Rhizobium, which is associated with legume roots. There are other genres like Frankia, Nostoc Y Raisin that make symbiosis with roots of other groups of plants.
Cyanobacteria in free form can fix atmospheric nitrogen in aquatic environments
There are three types of microorganisms involved in the nitrification process. These bacteria are capable of oxidizing the ammonia or ammonium ion present in the soil. They are chemolythrophic organisms (capable of oxidizing inorganic materials as a source of energy).
Bacteria of various genera intervene in the process sequentially. Nitrosoma and Nitrocystis they oxidize NH3 and NH4 to nitrites. Later Nitrobacter Y Nitrosococcus oxidize this compound to nitrates.
In 2015, another group of bacteria that intervene in this process was discovered. They are capable of directly oxidizing ammonia to nitrates and are located in the genus Nitrospira. Some fungi are also capable of nitrifying ammonia.
It has been suggested that more than 50 different genera of bacteria can reduce nitrates to Ntwo. This occurs under anaerobic conditions (absence of oxygen).
The most common denitrifying genera are Alcaligenes, Paracoccus, Pseudomonas, Rhizobium, Thiobacillus Y Thiosphaera. Most of these groups are heterotrophs.
In 2006 a bacterium (Methylomirabilis oxyfera) which is aerobic. It is methanotrophic (it obtains carbon and energy from methane) and is capable of obtaining oxygen from the denitrification process..
The nitrogen cycle goes through several stages in its mobilization across the planet. These phases are:
It is the conversion of atmospheric nitrogen to forms considered reactive (which can be used by living beings). The breaking of the three bonds that the N molecule containstwo requires a large amount of energy and can occur in two ways: abiotic or biotic.
Nitrates are obtained by high-energy fixation in the atmosphere. It is from the electrical energy of lightning and cosmic radiation.
The ntwo combines with oxygen to form oxidized forms of nitrogen such as NO (nitrogen dioxide) and NOtwo (nitrous oxide). Subsequently, these compounds are carried to the earth's surface by rain as nitric acid (HNO3).
High energy fixation incorporates approximately 10% of the nitrates present in the nitrogen cycle.
It is carried out by microorganisms in the soil. These bacteria are generally associated with the roots of plants. Annual biotic nitrogen fixation is estimated to be approximately 200 million tons per year.
Atmospheric nitrogen is transformed into ammonia. In a first phase of the reaction, the Ntwo is reduced to NH3 (ammonia). In this form it is incorporated into amino acids.
In this process, an enzymatic complex with various oxidation-reduction centers is involved. This nitrogenase complex is made up of a reductase (provides electrons) and a nitrogenase. The latter uses electrons to reduce Ntwo to NH3. A large amount of ATP is consumed in the process.
The nitrogenase complex is irreversibly inhibited in the presence of high concentrations of Otwo. In radical nodules, a protein (leghemoglobin) is present that keeps the O content very low.two. This protein is produced by the interaction between the roots and the bacteria.
Plants that do not have a symbiotic association with N-fixing bacteriatwo, They take nitrogen from the soil. The absorption of this element is carried out in the form of nitrates through the roots.
Once nitrates enter the plant, some of it is used by the root cells. Another part is distributed by the xylem to the entire plant.
When it is to be used, nitrate is reduced to nitrite in the cytoplasm. This process is catalyzed by the enzyme nitrate reductase. Nitrites are transported to chloroplasts and other plastids, where they are reduced to ammonium ion (NH4).
Ammonium ion in large quantities is toxic to the plant. So it is quickly incorporated into carbonate skeletons to form amino acids and other molecules..
In the case of consumers, nitrogen is obtained by feeding directly from plants or other animals.
In this process, the nitrogenous compounds present in the soil are degraded to simpler chemical forms. Nitrogen is contained in dead organic matter and wastes such as urea (mammalian urine) or uric acid (bird droppings).
The nitrogen contained in these substances is in the form of complex organic compounds. Microorganisms use the amino acids contained in these substances to produce their proteins. In this process, they release excess nitrogen in the form of ammonia or ammonium ion..
These compounds are available in the soil for other microorganisms to act in the following phases of the cycle.
During this phase, soil bacteria oxidize ammonia and ammonium ion. In the process, energy is released that is used by the bacteria in their metabolism..
In the first part, the nitrosifying bacteria of the genus Nitrosomes they oxidize ammonia and ammonium ion to nitrite. The enzyme ammonia mooxygenase is found in the membrane of these microorganisms. It oxidizes the NH3 to hydroxylamine, which is then oxidized to nitrite in the periplasm of the bacteria.
Subsequently, the nitrating bacteria oxidize the nitrites to nitrates using the enzyme nitrite oxidoreductase. Nitrates remain available in the soil, where they can be absorbed by plants.
In this stage, oxidized forms of nitrogen (nitrites and nitrates) are converted back to Ntwo and to a lesser extent nitrous oxide.
The process is carried out by anaerobic bacteria, which use nitrogenous compounds as electron acceptors during respiration. The rate of denitrification depends on several factors, such as available nitrate and soil saturation and temperature..
When the soil is saturated with water, the Otwo it is no longer readily available and the bacteria use NO3 as an electron acceptor. When temperatures are very low, microorganisms cannot carry out the process.
This phase is the only way that nitrogen is removed from an ecosystem. In this way, the Ntwo that was fixed returns to the atmosphere and the balance of this element is maintained.
This cycle has great biological relevance. As we explained previously, nitrogen is an important part of living organisms. Through this process it becomes biologically usable.
In the development of crops, the availability of nitrogen is one of the main limitations of productivity. Since the beginning of agriculture, the soil has been enriched with this element.
The cultivation of legumes to improve soil quality is a common practice. Likewise, the sowing of rice in flooded soils promotes the environmental conditions necessary for the use of nitrogen..
During the 19th century, guano (bird excreta) was widely used as an external source of nitrogen in crops. However, by the end of this century it was insufficient to increase food production.
The German chemist Fritz Haber, in the late 19th century, developed a process that was later commercialized by Carlo Bosch. This consists of reacting Ntwo and hydrogen gas to form ammonia. It is known as the Haber-Bosch process.
This form of artificial production of ammonia is one of the main sources of nitrogen that can be used by living beings. It is considered that 40% of the world population depends on these fertilizers for their food.
The current anthropic production of ammonia is approximately 85 tons per year. This has negative consequences on the nitrogen cycle..
Due to the high use of chemical fertilizers, there is contamination of soils and aquifers. It is considered that more than 50% of this contamination is a consequence of the Haber-Bosch synthesis.
Nitrogen excesses lead to eutrification (nutrient enrichment) of water bodies. Anthropic eutrification is very fast and causes accelerated growth mainly of algae.
They consume a lot of oxygen and can accumulate toxins. Due to the lack of oxygen, the other organisms present in the ecosystem end up dying.
In addition, the use of fossil fuels releases a large amount of nitrous oxide into the atmosphere. This reacts with ozone and forms nitric acid, which is one of the components of acid rain..
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