Nitrogen Cycle

Humans and the nitrogen cycle

Resources

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.

Resources

BOOKS

Freedman, Jeri. Climate Change: Human Effects On The Nitrogen Cycle. New York: Rosen Publishing Group, 2006.

Leigh, G.J. The World’s Greatest Fix: A History of Nitrogen and Agriculture. Oxford: Oxford University Press, 2004.

Lovelock, James. The Revenge of Gaia: Earth’s Climate Crisis and the Fate of Humanity. New York: Basic Books, 2006.

Bill Freedman

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:

1. 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.
2. 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.
3. 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.
4. 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.
5. 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 ).
6. 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.

See also Nitrogen fixation.

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

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.

Nitrogen cycle

Nitrogen is a macronutrient essential to all living organisms. It is an integral component of amino acids which are the building blocks of proteins; it forms part of the nitrogenous bases common to DNA and RNA; it helps make up ATP, and it is a major component of the chlorophyll molecule in plants. In essence, life as we know it cannot exist without nitrogen.

Although nitrogen is readily abundant as a gas (it comprises 79 percent of atmospheric gases by volume), most organisms cannot use it in this state. It must be converted to a chemically usable form such as ammonia (NH₃) or nitrate (NO₃) for most plants, and amino acids for all animals. The processes involved in the conversion of nitrogen to its various forms comprise the nitrogen cycle. Of all the nutrient cycles, this is considered the most complex and least well understood scientifically. The processes that make up the nitrogen cycle include nitrogen fixation , ammonification, nitrification , and denitrification .

Nitrogen fixation refers to the conversion of atmospheric nitrogen gas (N₂) to ammonia (NH₃) or nitrate (NO₃). The latter is formed when lightning or sometimes cosmic radiation causes oxygen and nitrogen to react in the atmosphere . Farmers are usually delighted when electrical storms move through their areas because it supplies "free" nitrogen to their crops, thus saving money on fertilizer . Ammonia is produced from N₂ by a special group of microbes in a process called biological fixation, which accounts for about 90 percent of all fixed N₂ each year worldwide. This process is accomplished by a relatively small number of species of bacteria and blue-green algae, or blue-green bacteria. The most well known of these nitrogen-fixing organisms are the bacteria in the genus Rhizobium which are associated with the root nodules of legumes. The legumes attract these bacteria by secreting a chemical into the soil that stimulates the bacteria to multiply and enter the root hair tips. The resultant swellings contain millions of bacteria and are called root nodules, and here, near the soil's surface, they actively convert atmospheric N₂ to NH₃, which is taken up by the plant. This is an example of a symbiotic relationship, where both organisms benefit. The bacteria benefit from the physical location in which to grow, and they also utilize sugars supplied by the plant photosynthate to reduce the N₂ to NH₃. The legumes, in turn, benefit from the NH₃ produced. The energetic cost for this nutrient is quite high, however, and legumes typically take their nitrogen directly from the soil rather than from their bacteria when the soil is fertilized.

Although nitrogen fixation by legume bacteria is the major source of biological fixation, other species of bacteria are also involved in this process. Some are associated with non-legume plants such as toyon (Ceanothus ), silverberry (Elaeagnus ), water fern (Azolla ), and alder (Alnus ). With the exception of the water fern, these plants are typically pioneer species growing in low-nitrogen soil. Other nitrogen-fixing bacteria live as free-living species in the soil. These include microbes in the genera Azotobacter and Clostridium, which live in aerobic and anaerobic sediments, respectively.

Blue-green algae are the other major group of living organisms which fix atmospheric N₂. They include approximately forty species in such genera as Aphanizomenon, Anabaena, Calothrix, Gloeotrichia, and Nostoc. They inhabit both soil and freshwater and can tolerate adverse and even extreme conditions. For example, some species grow in hot springs where the water is 212°F (100°C), whereas other species inhabit glaciers where the temperature is 32°F (0°C). The characteristic bluish-green coloration is a telltale sign of their presence. Some blue-green algae are found as pioneer species invading barren soil devoid of nutrients, particularly nitrogen, either as solitary individuals or associated with other organisms such as lichens . Flooded rice fields are another prime location for nitrogen-fixing blue-green algae.

Perhaps the most common environments where blue-green algae are found are lakes and ponds, particularly when the body of water is eutrophic—containing high concentrations of nutrients, especially phosphorus . Algae can reach bloom proportions during the warm summer months and are often considered a nuisance because they float on the surface, forming dense scum. The resultant odor following decomposition is usually pungent, and fish such as catfish often acquire an off-flavor taste from ingesting these algae.

The next major component of the nitrogen cycle is ammonification. It involves the breakdown of organic matter (in this case amino acids) by decomposer organisms to NH₃, yielding energy. It is therefore the reverse reaction of amino acid synthesis. Dead plant and animal tissues and waste materials are broken down to amino acids and eventually NH₃ by the saprophagous bacteria and fungi in both soil and water.

Nitrification is a biological process where NH₃ is oxidized in two steps, first to NO₃ and next to nitrate (NO2). It is accomplished by two genera of bacteria, Nitrosomonas and Nitrobacter in the soil, and Nitrosococcus and Nitrococcus in salt water. Since nitrification is an oxidation reaction, it requires oxygenated environments.

Dentrification is the reverse reaction of nitrification and occurs under anaerobic conditions. It involves the breakdown of nitrates and nitrites into gaseous N₂ by microorganisms and fungi. Bacteria in the genus Pseudomonas (e.g., P. dentrificans ) reduce NO₃ in the soil.

The cycling of nitrogen in an ecosystem is obviously complex. In aquatic ecosystems, nitrogen can enter the food chain through various sources, primarily surface runoff into lakes or rivers, mixing of nutrient-rich bottom waters (normally only during spring and fall turnovers in north temperate lakes), and biological fixation of atmospheric nitrogen by blue-green algae. Phytoplankton (microscopic algae) then rapidly take up the available nitrogen in the form of NH₃ or NO₃ and assimilate it into their tissues, primarily as amino acids. Some nitrogen is released by leakage through cell membranes. Herbivorous zooplankton that ingest these algae convert their amino acids into different amino acids and excrete the rest. Carnivorous or omnivorous zooplankton and fish that eat the herbivores do the same. Excretion (usually as NH₃ and urea) is thus a valuable nutrient recycling mechanism in aquatic ecosystems. Different species of phytoplankton actively compete for these nutrients when they are limited. Decomposing bacteria in the lake, particularly in the top layer of the bottom sediments, play an important role in the breakdown of dead organic matter which sinks to the bottom. The cycle is thus complete.

The cycling of nitrogen in marine ecosystems is similar to that in lakes, except that nitrogen lost to the sediments in the deep open water areas is essentially lost. Recycling only occurs in the nearshore regions, usually through a process called upwelling. Another difference is that marine phytoplankton prefer to take up nitrogen in the form of NH₃ rather than NO₃.

In terrestrial ecosystems, NH₃ and NO₃ in the soil is taken up by plants and assimilated into amino acids. As in aquatic habitats, the nitrogen is passed through the food chain/web from plants to herbivores to carnivores, which manufacture new amino acids. Upon death, decomposers begin the breakdown process, converting the organic nitrogen to inorganic NH₃. Bacteria are the main decomposers of animal matter and fungi are the main group that break down plants. Shelf and bracket fungi, for example, grow rapidly on fallen trees in forests. The action of termites, bark beetles, and other insects that inhabit these trees greatly speed up the process of decomposition.

There are three major differences between nitrogen cycling in aquatic versus terrestrial ecosystems. First, the nitrogen reserves are usually much greater in terrestrial habitats because nutrients contained in the soil remain accessible, whereas nitrogen released in water and not taken up by phytoplankton sinks to the bottom where it can be lost or held for a long time. Secondly, nutrient recycling by herbivores is normally a more significant process in aquatic ecosystems. Thirdly, terrestrial plants prefer to take up nitrogen as NO₃ and aquatic plants prefer NH₃.

Forces in nature normally operate in a balance, and gains are offset by losses. So it is with the nitrogen cycle in freshwater and terrestrial ecosystems. Losses of nitrogen by detritrification, runoff, sedimentation , and other releases equal gains by fixation and other sources.

Humans, however, have an influence on the nitrogen cycle that can greatly change normal pathways. Fertilizers used in excess on residential lawns and agricultural fields add tremendous amounts of nitrogen (typically as urea or ammonium nitrate) to the target area. Some of the nitrogen is taken up by the vegetation, but most washes away as surface runoff, entering streams, ponds, lakes, and the ocean. This contributes to the accelerated eutrophication of these bodies of water. For example, periodic unexplained blooms of toxic dinoflagellates off the coast of southern Norway have been blamed on excess nutrients, particularly nitrogen, added to the ocean by the fertilizer runoff from agricultural fields in southern Sweden and northern Denmark. These algae have caused massive dieoffs of salmon in the mariculture pens popular along the coast, resulting in millions of dollars of damage. Similar circumstances have contributed to blooms of other species of dinoflagellates, creating what are known as red tides. When filter-feeding shellfish ingest these algae, they become toxic, both to other fishes and humans. Paralytic shellfish poisoning may result within thirty minutes, leading to impairment of normal nerve conduction, difficulty in breathing, and possible death. Saxotoxin, the toxin produced by the dinoflagellate Gonyaulax, is fifty times more lethal than strychnine and curare.

Other forms of human intrusion into the nitrogen cycle include harvesting crops, logging , sewage, animal wastes, and exhaust from automobiles and factories. Harvesting crops and logging remove nitrogen from the system. The other processes are point sources of excess nitrogen. Autos and factories produce nitrous oxides (NO_x) such as nitrogen dioxide (NO₂), a major air pollutant. NO₂ contributes to the formation of smog , often irritating eyes and leading to breathing difficulty. It also reacts with water vapor in the atmosphere to form weak nitric acid (HNO₃), one of the major components of acid rain .

See also Agricultural pollution; Air pollution; Aquatic weed control; Marine pollution; Nitrogen waste; Soil fertility; Urban runoff

[John Korstad ]

RESOURCES

BOOKS

Ehrlich, P. R., A. H. Ehrlich, and J. P. Holdren. Ecoscience: Population, Resources, Environment. New York: W. H. Freeman, 1977.

Ricklefs, R. E. Ecology. 3rd ed. New York: W. H. Freeman, 1990.

Smith, R. E. Ecology and Field Biology. 4th ed. New York: Harper and Row, 1990.

PERIODICALS

Brill, W. J. "Biological Nitrogen Fixation." Scientific American 236 (1977): 68–81.

Delwiche, C. C. "The Nitrogen Cycle." Scientific American 223 (1970): 136–46.

Nitrogen cycle

The term nitrogen cycle refers to a series of reactions in which the element nitrogen and its compounds pass continuously through Earth's atmosphere, lithosphere (crust), and hydrosphere (water component). The major components of the nitrogen cycle are shown in the accompanying figure. In this diagram, elemental nitrogen is represented by the formula N₂, indicating that each molecule of nitrogen consists of two nitrogen atoms. In this form, nitrogen is more correctly called dinitrogen.

Nitrogen fixation

Nitrogen is the most abundant single gas in Earth's atmosphere. It makes up about 80 percent of the atmosphere. This fact is important because plants require nitrogen for their growth and, in turn, animals depend on plants for their survival. The problem is, however, that plants are unable to use nitrogen in its elemental form—as dinitrogen. Any process by which elemental dinitrogen is converted to a compound is known as nitrogen fixation.

Dinitrogen is converted from an element to a compound by a number of naturally occurring processes. When lightning passes through the atmosphere, it prompts a reaction between nitrogen and oxygen; oxides of nitrogen—primarily nitric oxide (NO) and nitrogen dioxide (NO₂)—are formed. Both oxides then combine with water vapor in the atmosphere to form nitric acid (HNO₃). Nitric acid is carried to the ground in rain and snow, where it is converted to nitrites and nitrates. Nitrites and nitrates are both compounds of nitrogen and oxygen, the latter containing more oxygen than the former. Naturally occurring minerals such as saltpeter (potassium nitrate; KNO₃) and Chile saltpeter (sodium nitrate; NaNO₃) are the most common nitrates found in Earth's crust.

Certain types of bacteria also have the ability to convert elemental dinitrogen to nitrates. Probably the best known of these bacteria are the rhizobium, which live in nodules on the roots of leguminous plants such as peas, beans, clover, and the soya plant.

Finally, dinitrogen is now converted to nitrates on very large scales by human processes. In the Haber process, for example, nitrogen and hydrogen are combined to form ammonia, which is then used in the manufacture of synthetic fertilizers, most of which contain nitrates.

Words to Know

Ammonification: The conversion of nitrogen compounds from plants and animals to ammonia and ammonium; this conversion occurs in soil or water and is carried out by bacteria.

Denitrification: The conversion of nitrates to dinitrogen (or nitrous oxide) by bacteria.

Dinitrogen fixation (nitrogen fixation): The conversion of elemental dinitrogen (N2) in the atmosphere to a compound of nitrogen deposited on Earth's surface.

Nitrification: The process by which bacteria oxidize ammonia and ammonium compounds to nitrites and nitrates.

Ammonification, nitrification, and denitrification

Nitrogen that has been fixed by one of the mechanisms described above can then be taken in by plants through their roots and used to build new stems, leaves, flowers, and other structures. Almost all animals obtain the nitrogen they require, in turn, by eating plants and taking in the plant's organic forms of nitrogen.

The nitrogen stored in plants and animals is eventually returned to Earth by one of two processes: elimination (in the case of animals) or death (in the case of both animals and plants). In whatever form the nitrogen occurs in the dead plant or animal, it is eventually converted to ammonia (NH₃) or one of its compounds. Compounds formed from ammonia are known as ammonium compounds. This process of ammonification is carried out (as the plant or animal decays) by a number of different microorganisms that occur naturally in the soil.

Ammonia and ammonium compounds, in their turn, are then converted to yet another form, first to nitrites and then to nitrates. The transformation of ammonia and ammonium to nitrite and nitrate is an oxidation process that takes place through the action of various bacteria such as those in the genus Nitrosomonas and Nitrobacter. The conversion of ammonia and ammonium compounds to nitrites and nitrates is called nitrification.

In the final stage of the nitrogen cycle, oxygen is removed from nitrates by bacteria in a process known as denitrification. Denitrification converts nitrogen from its compound form to its original elemental form as dinitrogen, and the cycle is ready to begin once again.

Nitrogen Cycle

The nitrogen cycle describes the stages in which the important gas nitrogen is converted and circulated from the nonliving world to the living world and back again. There are five main steps in the nitrogen cycle, four of which are carried out by bacteria. Since nitrogen cannot be used by the cells of living things until it has been converted by bacteria into a useful form, these nitrogen-fixing bacteria not only play a key role in the nitrogen cycle but are in fact essential to all life on Earth.

All plants and animals need a certain amount of nitrogen in their systems in order to live. Nitrogen is an important component of amino acids, which are the building blocks of life, and nucleic acids, which make up genetic material. Although nitrogen is a free gas (it remains a gas under normal atmospheric conditions) that makes up nearly 80 percent of Earth's atmosphere, only bacteria can use it in this form. This means that for every plant and animal on Earth, the abundant nitrogen in the atmosphere is entirely useless unless it is somehow combined with hydrogen or oxygen. Bacteria that produce compounds of nitrogen, called nitrates, work to form this necessary combination. Most nitrates are produced by bacteria in the soil and are therefore absorbed by plants. Animals obtain the nitrates either by eating plants or other animals that eat plants. When plants and animals die, the nitrates in their systems are returned to the atmosphere and back to the beginning of the nitrogen cycle.

THE FIVE STEPS OF THE NITROGEN CYCLE

The nitrogen cycle has five main steps: fixation, nitrification, assimilation, ammonification, and denitrification. All except the third step (assimilation) are carried out by bacteria.

Nitrogen Fixation. The first step, nitrogen fixation, is probably the most important since it frees what was previously unusable nitrogen. Only a few types of bacteria are able to break the chemical bonds that hold the paired atoms in nitrogen gas together. Nitrogen-fixing bacteria that live in the soil have an enzyme called nitrogenase that they use to break that bond or "fix" and capture nitrogen gas from the atmosphere and convert it to useable ammonia. Blue-green algae, called cyanobacteria, is one example of bacteria that can change atmospheric nitrogen to ammonia.

Nitrification. The second step in the cycle, called nitrification, occurs when the converted nitrogen is combined with oyxgen and hydrogen. This combination results in the production of nitrogen compounds, like nitrates,

that other organisms can use. During the third stage, assimilation, plants take up these nitrates with their roots and use the nitrates to build proteins and nucleic acids. In other words, the plants use the nitrates to grow. Animals, like plants, also need proteins to function and grow, and proteins are composed of amino acids that contain nitrogen atoms.

Ammonification. The fourth stage in the nitrogen cycle is called ammonification. This is the process in which these nitrogen compounds are returned to the soil and reconverted to ammonia by other bacteria. Ammonification occurs in two ways. The first is when a plant or animal dies and their organic matter is acted upon by decomposer bacteria. These bacteria play an important role because the nitrogen trapped in dead animals and plants would be useless and wasted were it not converted. A second way for ammonification to occur is when animals leave their urine and feces on the ground. Almost immediately by the same decomposer bacteria work to release the nitrogen found in these waste products.

Denitrification. The last step in the nitrogen cycle is called denitrification and involves the reduction of ammonia to nitrogen gas. Again, this process is carried out by a certain type of bacteria whose actions eventually release nitrogen gas back into the atmosphere. The complete nitrogen cycle is, therefore, a process in which nitrogen from the atmosphere is captured and converted, passes through a number of organisms (including bacteria), and is finally returned to the atmosphere to enter the cycle again.

There is a more dramatic way that nitrogen is converted to a usable form. The atmosphere of Earth is roughly four parts nitrogen to one part oxygen, and these two gases never normally react with one another. However, during a thunderstorm when lightning strikes, the temperature and the pressure of the air is such that the two gases react chemically and form nitrous oxide gases. In the rainwater that accompanies the storm, nitrous oxide is dissolved and falls to Earth as nitric acid. This enters the nitrogen cycle when it soaks into the soil and forms nitrites and nitrates. However, far more nitrates are formed by bacteria in the soil than by atmospheric lightning flashes.