Nitrogen Cycle

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Nitrogen cycle

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, and to a somewhat lesser degree in freshwater . Consequently, plants in many ecosystems will be more productive if the supply of available nitrogen is increased through fertilization. This is why fertilization is such a common practice in agriculture, and why nitrogen is by far the most commonly applied nutrient.

Most plants obtain their nitrogen by assimilating it from their environment, mostly as nitrate or ammonium dissolved in soil water that is taken up by roots, or as gaseous nitrogen oxides that are taken up by plant leaves from the atmosphere. However, some plants live in a symbiotic relationship with microorganisms that have the ability to fix atmospheric nitrogen (properly called dinitrogen) into ammonia , and these plants benefit greatly from access to an increased supply of nitrogen.

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.

Chemical forms of nitrogen

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. The most prominent of the inorganic molecules of nitrogen are:

Dinitrogen (N2). Sometimes this molecule is less accurately called "nitrogen," although that term should properly be restricted to nitrogen atoms . Dinitrogen is a gas, and it is very unreactive because its two nitrogen atoms are held together by a relatively strong, triple bond. About 79% 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 . As is explained below, N2 must be "fixed" into other forms before it can be assimilated by most organisms.
Nitrate (NO3 ). This 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 (NH + 4 ). Ammonia usually occurs as a gas, vapor, or liquid, while ammonium is a positively charged ion (or cation ), formed by the addition of a hydrogen ion (H+) to ammonia. 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). This gas is emitted to the atmosphere mostly as a result of combustions. The NO of combustions is generated by two mechanisms: (a) oxidation of the organic nitrogen of the fuel of biomass , oil, or coal , and (b) 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 its products. In the atmosphere, emitted NO is oxidized to NO2, which is therefore considered to be a secondary, non-emitted, gas. NO and NO2 are often considered together, as NOx. In the atmosphere, gaseous NOx may be directly taken in by plant foliage, or it may be oxidized to nitrate. The nitrate may combine with atmospheric cations, such as ammonium, to form an ammonium nitrate salt . Fine aerosols of ammonium nitrate particulates may directly deposit in ecosystems, or they may serve as condensation nuclei for the formation of ice crystals, which may eventually become snowflakes or raindrops that are delivered to ecosystems as precipitation . By these mechanisms, biologically useful forms of fixed nitrogen are deposited from the atmosphere. It should also be mentioned that atmospheric nitrate can also combine with hydrogen ion, and can occur as a dilute solution of nitric acid . This is the reason why nitrate derived from NOx is such an important contributor to acidic precipitation (see entry on acid rain ).
Nitrous oxide (N2O). The major source of atmospheric N2O is denitrification occurring in wet, nitrate-rich soils. N2O is a relatively unreactive and persistent gas in the atmosphere. However, N2O is very slowly oxidized to NOxgases under the influence of sunlight and atmospheric catalysts.

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.

Dinitrogen fixation

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 catalytically breaking the triple bond of N2, generating two molecules of ammonia for each molecule of dinitrogen that is reacted. Because the activation energy for this chemical reaction is rather high, it takes quite a lot of biological energy to fix atmospheric dinitrogen in this way, about 12-15 moles of adenosine triphosphate (ATP) per mole of dinitrogen that is converted. Despite the high energy costs, the dinitrogen fixation reaction is still very favorable ecologically, because access is gained to ammonia and ammonium, chemical forms of nitrogen that organisms can utilize for their nutrition. 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 microorganisms that can fix atmospheric dinitrogen, including species of true bacteria , blue-green algae or bacteria, and actinomycetes. 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 microorganisms 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 NOx gases by plants.

Ammonification and nitrification

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 (NO-₂ ), 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

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. In intensive agricultural systems, rates of fertilization often exceed 1,103 lb (500 kg) of N per hectare per year. 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 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 water-bodies. 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 NOx gases to the atmosphere. The most important sources of emission are automobiles, power plants, home furnaces, and factories. The emitted NOx is an important air pollutant, because it is critical in the photochemical oxidative reactions by which toxic ozone is formed, and because the NOx is an important source of nitrate in acidic precipitation.

The nitrogen cycle is a critical biogeochemical process, and is the ultimate means of entry and exit of biologically available forms of nitrogen in ecosystems. However, there are many ways by which human activities can substantially change the rates of the processes in this cycle, causing a great deal of environmental damage. The key to sustainable management of the nitrogen cycle is the recognition of the minimum and maximum rates that represent a healthy degree of ecological function. Human influences must then be kept within those bounds of desirable rates of nitrogen cycling.

Resources

books

Atlas, R. M., and R. Bartha. Microbial Ecology. Menlo Park, CA: Benjamin/Cummings, 1987.

Freedman, B. Environmental Ecology. 2nd ed. San Diego: Academic Press, 1995.

Bill Freedman

KEY TERMS

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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 Nitrosomonasbacteria 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.