Nitrogen Cycle

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Nitrogen Cycle

The nitrogen cycle refers to the movment of nitrogen through the air, water, and soil of Earth.

Nitrogen is a critically important nutrient for organisms, being one of the most abundant elements in their tissues, and an integral component of many biochemicals, including amino acids, proteins, and nucleic acids.

The availability of biologically useful forms of nitrogen is a common limiting factor in the productivity of plants. This is especially true of plants growing in terrestrial and marine environments, but is also the case for plants in freshwater environments. Many plants in a variety of ecosystems are more productive if the supply of available nitrogen is increased through fertilization with nitrogen-containing compounds. This is why fertilization is such a common practice in agriculture, and why nitrogen is the most commonly applied nutrient.

Most plants obtain their nitrogen by assimilating it from their environment as nitrate or ammonium dissolved in the water that is taken up by roots, or as a gas (nitrogen oxide) taken up from the air by leaves. However, some plants live in a symbiotic relationship with microorganisms that have the ability to fix atmospheric nitrogen into ammonia. The symbiosis between tobacco plants and Rhizobium leguminosarum is one example.

Almost all animals obtain the nitrogen they require by eating plants and assimilating the plant’s organic forms of nitrogen, which are then broken down metabolically and used by the animal to synthesize their own necessary biochemicals. A few animals, however, can utilize inorganic sources of nitrogen. For example, ruminants such as the cow can utilize urea or ammonia, because microorganisms that live in their forestomachs can assimilate these inorganic chemicals and synthesize amino acids and proteins, which the cattle can then use.

Nitrogen (N) can occur in many chemical forms in the environment. Organic nitrogen refers to a very diverse array of nitrogen-containing organic molecules, ranging from simple amino acids through proteins and nucleic acids, to large and complex molecules such as humic substances in soil and water. A smaller number of inorganic forms of nitrogen occur in the environment, but these are very important ecologically. Dinitrogen (N₂) is a gas, and it is very unreactive because its two nitrogen atoms are held together by a relatively strong, triple bond. About 80% of the volume of Earth’s

atmosphere consists of dinitrogen, but because of its almost inert character few organisms can directly use this gas in their nutrition. N₂ must be “fixed” into other forms before it can be assimilated by most organisms.

Nitrate (NO₃^–) is a negatively charged ion (or anion), and is highly soluble in water. Nitrate is the preferred form of nitrogen nutrition for most species of plants. If nitrate is not assimilated by plants or microorganisms, it is readily leached from soils.

Ammonia (NH₃) and ammonium (NH₄⁺) are other forms of nitrogen. Ammonium is soluble in water, although it is also electrochemically attracted to negatively charged surfaces associated with clays and organic matter in soil, and is therefore not as mobile as nitrate. Plants that are adapted to acidic soils can utilize ammonium as their source of nitrogen nutrition, although most species of non-acidic soils can only utilize nitrate.

Nitric oxide (NO) is a gas that is emitted to the atmosphere mostly as a result of combustions. The NO of combustions is generated by oxidation of the organic nitrogen of the fuel of biomass, oil, or coal, and combination of atmospheric dinitrogen with oxygen under conditions of high temperature and pressure, as occurs, for example, in the internal combustion engine of automobiles. NO is produced in large quantities by both of these reactions, and it is typically emitted into the atmosphere.

Nitrogen dioxide (NO₂) and nitrous oxide (N₂O) are tow other nitrogen sources.

Depending upon environmental and biological conditions, all of these various organic and inorganic compounds containing nitrogen can be variously transformed. The diverse manners in which the movements and transformations of these chemicals occur is known as the nitrogen cycle.

Atmospheric N₂ is very abundant, but this molecule is highly unreactive and cannot be assimilated by most organisms and used in their nutrition. To be useful to plants, dinitrogen must be “fixed” into inorganic forms that can be taken up by roots or leaves. Dinitrogen fixation can occur through non-biological processes. For example, during a lightning strike there is a brief occurrence of a very high temperature and high pressure, conditions that favor the combination of atmospheric N₂ and O₂ to form NO.

However, biological fixation of dinitrogen is typically more important. A few species of microorganisms can synthesize an enzyme, called nitrogenase, which is capable of breaking the triple bond of N₂, generating two molecules of ammonia for each molecule of dinitrogen that is reacted. Because the nitrogenase enzyme is denatured by oxygen (O₂), the fixation reaction can only occur under anaerobic conditions, where oxygen is not present.

There are numerous species of free-living micro-organisms that can fix atmospheric dinitrogen. These microorganisms are most abundant in wet or moist environments, especially in situations where nutrients other than nitrate or ammonium are relatively abundant, for example, in rotting logs or other dead biomass, or in lakes that are well fertilized with phosphate from sewage. Such conditions are typically relatively deficient in available forms of nitrogen, and are commonly anoxic, creating obviously favorable opportunities for species of microorganisms that can utilize dinitrogen.

Some species of plants live in an intimate and mutually beneficial symbiosis with microorganisms that have the capability of fixing dinitrogen. The plants benefit from the symbiosis by having access to a dependable source of fixed nitrogen, while the micro-organisms benefit from energy and habitat provided by the plant. The best known symbioses involve many species in the legume family (Fabaceae) and strains of a bacterium known as Rhizobium japonicum. Some plants in other families also have dinitrogen–fixing symbioses, for example, red alder (Alnus rubra ) and certain actinomycetes, a type of microorganism. Many species of lichens, which consist of a symbiotic relationship between a fungus and a blue–green bacterium, can also fix dinitrogen.

Biological dinitrogen fixation is an ecologically important process, being ultimately responsible for most of the fixed nitrogen that occurs in the biomass of organisms and ecosystems. The only other significant sources of fixed nitrogen to ecosystems are atmospheric depositions of nitrate and ammonium with precipitation and dustfall, and the direct uptake of NO_x gases by plants.

Ammonification is the process by which the organically bound nitrogen of microbial, plant, and animal biomass is recycled after their death. Ammonification is carried out by a diverse array of microorganisms that perform ecological decay services, and its product is ammonia or ammonium ion. Ammonium is a suitable source of nutrition for many species of plants, especially those living in acidic soils. However, most plants cannot utilize ammonium effectively, and they require nitrate as their essential source of nitrogen nutrition.

Nitrate is synthesized from ammonium by an important bacterial process known as nitrification. The first step in nitrification is the oxidation of ammonium to nitrite (NO₂^–), a function carried out by bacteria in the genus Nitrosomonas. Once formed, the nitrite is rapidly oxidized further to nitrate, by bacteria in the genus Nitrobacter. The bacteria responsible for nitrification are very sensitive to acidity, so this process does not occur at significant rates in acidic soil or water. This is the reason why plants of acidic habitats must be capable of utilizing ammonium as their source of nitrogen nutrition.

Denitrification is another bacterial process, carried out by a relatively wide range of species. In denitrification, nitrate is reduced to either nitrous oxide or dinitrogen, which is then emitted to the atmosphere. Denitrification occurs under conditions where oxygen is not present, and its rate is largest when concentrations of nitrate are large. Consequently, fertilized agricultural fields that are wet or flooded can have quite large rates of denitrification. In some respects, denitrification can be considered to be an opposite process to dinitrogen fixation. In fact, the global rates of dinitrogen fixation and denitrification are in an approximate balance, meaning that the total quantity of fixed nitrogen in Earth’s ecosystems is neither increasing nor decreasing substantially over time.

Humans and the nitrogen cycle

One of the major influences of humans on the nitrogen cycle occurs through the use of nitrogen–containing fertilizers in agriculture. Under conditions in which agricultural plants have access to as much water as they require, their productivity is usually constrained by the rate at which they can obtain nitrogen in available forms, particularly nitrate, and sometimes ammonium. Under such conditions, farmers attempt to increase the availability of these nutrients, usually by applying fertilizers. This represents an enormously larger rate of nitrogen input than occurs naturally by atmospheric deposition and dinitrogen fixation, and it is also much larger than the rates in which ammonium and nitrate are made naturally available

KEY TERMS

Ammonification —The microbial conversion of organic nitrogen to ammonium in soil or water.

Denitrification —The anaerobic, microbial reduction of nitrate to gaseous nitrous oxide (N₂O) or nitrogen gas (N₂) which are then emitted to the atmosphere.

Dinitrogen fixation (nitrogen fixation) —The conversion of atmospheric dinitrogen (N₂) to ammonia or an oxide of nitrogen. This process can occur inorganically at high temperature and/or pressure, and biologically through the action of the microbial enzyme, nitrogenase.

Leaching —The removal of dissolved substances in soil in percolating water.

Nitrification —The process by which Nitrosomonas bacteria oxidize ammonium to nitrite, which is then oxidized by Nitrobacter to nitrate.

Nitrogenase —The microbial enzyme that fixes dinitrogen, by cleaving its triple bond and forming ammonia.

Symbiosis —A biological relationship between two or more organisms that is mutually beneficial. The relationship is obligate, meaning that the partners cannot successfully live apart in nature.

by ammonification and nitrification. Consequently, the ability of the crop and ecosystem to assimilate this much nitrogen input is satiated, and some of the input, especially nitrate, leaches from the site into groundwater and surface waters such as streams and rivers. This can cause a severe contamination of these waters in agricultural areas, which can lead to risks for human health through drinking nitrate–rich ground water, and contribute to increased productivity of surface waters through eutrophication. In addition, when soils with large nitrate concentrations become wet, the rate of denitrification is greatly increased. This represents a loss of fixed nitrogen capital, and the emitted nitrous oxide may contribute to an enhancement of Earth’s greenhouse effect.

Humans also influence the nitrogen cycle by dumping sewage and other types of organic matter into waterbodies. There is a great deal of environmental damage associated with these practices, including lowered dissolved oxygen levels associated with microbial oxidation of the organic matter, and the presence of fecal pathogens and parasites. The accidental fertilization of waterbodies with large amounts of nitrogen contributes greatly to eutrophication.

Humans also affect the nitrogen cycle through the emissions of large quantities of NO_x gases to the atmosphere. The most important sources of emission are automobiles, power plants, home furnaces, and factories. The emitted NO_x is an important air pollutant, because it is critical in the photochemical oxidative reactions by which toxic ozone is formed, and because the NO_x is an important source of nitrate in acidic precipitation.