Nitrogen Fixation

Biological nitrogen (N₂) fixation is the reduction of atmospheric nitrogen gas to ammonia, according to the equation:

N₂+ 10H⁺ 8 e ^- 16ATP ⁺2NH₄⁺ + H₂ + 16ADP + 16P_i

The reaction is mediated by an oxygen-sensitive enzyme nitrogenase and requires energy, as indicated by the consumption of adenosine triphosphate (ATP ). This conversion of inert N₂ gas into a form utilized by most organisms is the second most important biological process on Earth after photosynthesis. It contributes 175 million tons of nitrogen per year to the global nitrogen economy and accounts for 65 percent of the nitrogen used in agriculture. In Brazil alone, N₂ fixation contributes the equivalent of 2.5 million tons of fertilizer nitrogen annually to agricultural production and is essential to a country with limited natural gas reserves for fertilizer nitrogen production.

This article emphasizes symbiotic N₂ fixation in grain and pasture legumes in the family Fabaceae. N₂ fixation also occurs in leguminous and actinorhizal trees, sugarcane, and rice.

N₂-Fixing Organisms and Variation in Their Rates of Fixation

The ability to fix N₂ is restricted to prokaryotic organisms. Within this group the ability occurs in many different species. These include cyano-bacteria and actinomycetes , as well as eubacteria, including heterotrophic (e.g., Azotobacter ), autotrophic (Thiobacillus ), aerobic (Bacillus ), anaerobic (Clostridium ), and photosynthetic (Rhodospirillum ) species.

N₂-fixing organisms can live free in nature (e.g., Azotobacter ), enter loose (associative) symbiosis with plants or animals (Acetobacter and sugarcane), or establish longer-term relationships within specialized structures provided by their host (Rhizobium and the legume nodule).

Some free-living organisms fix enough N₂ in vitro to grow without added nitrogen, but limited energy supply can limit N₂ fixation in nature. For instance, non-symbiotic organisms in primary successional areas of the Hawaii Volcanoes National Park were found to fix only 0.3 to 2.8 kilograms of N₂ per hectare per year, and non-symbiotic N₂ fixation in soil rarely exceeds 15 kilograms per hectare per year. Higher levels in tidal flats and rice paddies are largely due to photosynthetic bacteria and cyanobacteria.

The importance of energy supply for fixation can be seen by comparing these rates to those found in legumes, where the symbiotic bacteria are supplied with high-energy products from photosynthesis. Rates of symbiotic N₂ fixation in legumes vary with plant species and cultivar, growing season, and soil fertility. Some forage legumes can fix 600 kilograms per hectare per year but more common values are 100 to 300 kilograms per hectare per year. Rates for grain legumes are often lower. Inclusion of legumes in crop rotations is generally thought to improve soil nitrogen levels, but benefits depend on the level of N₂ fixed and the amount of nitrogen removed in grain or forage. A good soybean crop might fix 180 kilograms per hectare but remove 210 kilograms per hectare in the grain.

Nodule Formation and Structure in Legumes

The most-studied symbiotic system is between N₂-fixing bacteria known as rhizobia and legumes such as clover and soybean. Rhizobia produce stem or root nodules on their host(s), and within these nodules receive protection from external stresses and energy for growth and N₂ fixation. The host receives most of the nitrogen it needs for growth. Six genera of rhizobia (Rhizobium, Azorhizobium, Mesorhizobium, Bradyrhizobium, Sinorhizobium, and Allorhizobium ) are recognized.

Rhizobia use several different mechanisms to infect their host, but only infection via root hairs is described here. Infection is initiated with the attachment of suitable rhizobia to newly emerged root hairs and leads to localized hydrolysis of the root hair cell wall. Root hair curling and deformation results, with many of the root hairs taking the shape of a shepherd's crook. Hydrolysis of the cell wall allows rhizobia to enter their host, but they never really gain intracellular access. Plant-derived material is deposited about them, and as they move down the root hair toward the root cortex they remain enclosed within a plant-derived infection thread. Even within the nodule they are separated from their host by a host-derived peribacteroid membrane. This separation is usually seen as a mechanism to suppress plant defense responses likely to harm the bacteria.

Presence of the rhizobia causes multiplication and enlargement of root cortical cells and gives the nodule a characteristic shape and structure: either round as in soybean or elongated as in alfalfa or clover. Such nodules have several distinct regions. The area of active N₂ fixation is either pink or red in color due to the presence of hemoglobin needed for oxygen transport. In most legumes nodules are visible within six to ten days of inoculation; N₂ fixation as evidenced by improved plant growth and coloration of the nodules can occur within three weeks.

Molecular Changes Associated with Nodulation and N₂ Fixation

The signs of infection are paralleled at a molecular level by signaling between host and rhizobia. Nodulation genes in Rhizobium are borne on extra-chromosomal (plasmid) deoxyribonucleic acid (DNA). They include both common genes found in all rhizobia and host-specific genes involved in the nodulation of specific legumes. Most are only expressed in the presence of a suitable host. Substances termed flavonoids present in the root exudate trigger this response, with legumes differing in the flavonoids each produced. Rhizobia also differ in their response to these compounds .

More than fifty nodulation genes have been identified. Some are involved in the regulation of nodulation, but most function in the synthesis of a chitin -like lipo-chito-oligosaccharide or nod factor. These molecules all have the same core structure (coded for by the common nodulation genes), but they vary in the side chains each carries, affecting host range. They are powerful plant hormones, which at low concentration can initiate most of the changes found during nodule development.

Interaction of host and rhizobia is also accompanied by the expression of nodule-specific proteins or nodulins. Several nodulins have now been found in actinorhizal and mycorrhizal symbiosis, and together with pea mutants that neither nodulate nor form mycorrhizal associations indicate some common elements in symbiosis.

Nodulin expression can vary temporally and spatially. Early nodulins are involved in infection or nodule development and may be expressed within six hours of inoculation. Later nodulins are involved in nodule function, carbon and nitrogen metabolism, or to O₂ transport. Nodule hemoglobin is an obvious example of this group.

Specificity in Nodulation

Given the complex signaling involved, specificity in nodulation is to be expected. Each rhizobium has the ability to nodulate some, but not all, legumes. Host range can vary, with one rhizobia only nodulating a particular species of clover, for example, while another will nodulate many different legumes. A consequence of this specificity is that legumes being introduced into new areas will usually need to be inoculated with appropriate rhizobia before seeding. In the early 1900s this was often achieved by mixing seed with soil from an area where the crop had been grown before. Today, more than one hundred different inoculant preparations are needed for the different crop, tree, and pasture legumes used in agriculture and conservation. Most are grown in culture and sold commercially. The legumes for which inoculant preparations are available, and the methods used to prepare, distribute, and apply these cultures, are detailed on the Rhizobium Research Laboratory Web site (http://www.Rhizobium.umn.edu).

When properly carried out, legume inoculation should result in abundant nodulation and high levels of N₂ fixation. Reinoculation should not be necessary because large numbers of rhizobia will be released from nodules at the end of the growing season and establish themselves in the soil. Problems with the culture used and environmental and soil factors can limit response, especially in the lesser-developed countries. Common concerns include:

• poor-quality inoculant strains weak in N₂ fixation and noncompetitive or nonpersistent in soil
• inoculants with low rhizobial numbers because of problems in production or packaging or during shipment
• inappropriate use of fertilizer or pesticides injurious to the rhizobia
• soil acidity, drought, or temperature conditions that affect strain survival or nodulation and N₂ fixation.

Because of earlier problems in inoculant production and quality, many countries have now developed regulations governing the quality of inoculant cultures. In the United States, inoculant quality control still rests with the producer.

see also Atmosphere and Plants; Biogeochemical Cycles; Cyanobacteria; Eubacteria; Fabaceae; Fertilizer; Flavonoids; Mycorrhizae; Nutrients; Roots.

Peter H. Graham

Bibliography

Graham, P. H. "Biological Dinitrogen Fixation: Symbiotic." In Principles and Applications of Soil Microbiology, eds. D. Sylvia et al. Upper Saddle River, NJ: Prentice-Hall, 1998.

Young, J. P. W. "Phylogenetic Classification of Nitrogen-Fixing Organisms." In Biological Nitrogen Fixation, eds. G. Stacey et al. New York: Chapman & Hall, 1992.

Nitrogen Fixation

Resources

Nitrogen fixation refers to the chemical conversion of nitrogen gas (dinitrogen, N₂) to an oxidized form (a form with fewer electrons), usually nitric oxide (NO) or ammonia (NH₃). Nitrogen fixation can occur through inorganic reactions that do not involve living organisms or as a result of biological processes. Because the nitrogen atoms in dinitrogen are bound by a very strong triple bond, this gas is very stable and cannot be utilized as a source of nutrition by any but a few highly specialized microorganisms. These nitrogen–fixing microbes are critically important ecologically because nitrogen fixation is the ultimate source of the capital of available organic nitrogen in ecosystems.

Land–based ecosystems fix about 135 million metric tons of nitrogen each year. Marine ecosystems fix about 40 million metric tons annually. Atmospheric dinitrogen can be fixed by inorganic reactions occurring in combustion, lightning strikes, and other circumstances involving high temperatures. Overall, naturally occurring non-biological fixation has been estimated to be equivalent to 10–20% of biological fixation of dinitrogen.

Nitrogen fixation is also associated with certain human activities. The internal combustion engines of automobiles burn fuels under conditions of very high temperature and pressure which favors the oxidation of nitrogen gas to nitric oxide, an important air pollutant and source of fixed nitrogen. The combustion of coal and other organic fuels also results in the formation of nitric oxide through both the fixation of nitrogen gas and the oxidation of the organic nitrogen of the fuel. Also, nitrogen gas is fixed industrially in enormous amounts to provide agricultural fertilizers.

Biological nitrogen fixation is accomplished through the action of an enzyme known as nitrogenase. Nitrogenase consists of two distinct proteins which contain molybdenum, iron, and sulfur. Because the nitrogenase proteins are denatured by exposure to oxygen (O₂), they can only operate in an anaerobic environment.

The nitrogenase enzyme can cleave other triple bonds in addition to that of dinitrogen gas. One example is the triple bond that exists in acetylene. Most assays of nitrogen fixation rates take advantage of this fact, measuring nitrogenase activity through the rate at which ethylene is generated by the reaction of acetylene with nitrogenase.

Only certain microorganisms can synthesize the nitrogenase enzyme. Some of these microbes are free–living in soils and water, while others occur in mutualisms with fungi or plants.

A large number of free–living bacteria of anaerobic environments have the ability to fix nitrogen, including the genus Clostridium. Fewer genera of aerobic bacteria have this ability. Free–living nitrogen fixation occurs most vigorously in habitats containing large quantities of carbon–rich organic debris such as rotting logs, heaps of sawdust, and compost piles.

Blue-green bacteria are free-living, photosynthetic microbes that fix nitrogen in aquatic habitats and moist soil, including the genera Anabaena, Nostoc, and Calothrix. Strains of these microbes also live in mutualisms with certain fungi in a symbiosis known as lichens which have an ability to fix nitrogen. Anabaena and Nostoc also occur in a mutualism with the floating aquatic fern Azolla which can be cultivated on the surface of rice paddies to fix nitrogen at a rate as great as 100–150 kg N/ha.yr, saving on the use of synthetic fertilizer.

Nitrogen-fixing bacteria can also occur in mutualisms with certain vascular plants. The best known of these associations are with species of legumes (order Leguminosae). About one–half of the world’s 10,000 species of legumes occur in a nitrogen–fixing mutualism with bacteria in the genus Rhizobium. About 50 of these species are utilized in agriculture, for example, the food crops garden pea (Pisum sativum ), broad bean (Vicia faba ), and soybean (Glycine max ), and the soil conditioners red clover (Trifolium pratense ) and alfalfa (Medicago sativa ). The strain of Rhizobium is specific to each legume species.

Rhizobium occurs in specialized nodules on the roots of the legumes. These are developed when the soil–dwelling Rhizobium invades a root hair, stimulating the plant to form a nodule. Nodule development is inhibited in acidic soils and if the concentrations of nitrate in soil are large. To protect the nitrogenase enzyme, the interior of the nodules is anaerobic. This is due in part to the presence of an oxygen–binding pigment known as leghaemoglobin which colors the nodule interior a dark red. The leghaemoglobin also serves as an oxygen carrier important for other aspects of bacterial metabolism, in this way performing a similar function as the haemoglobin of animals.

Several hundred species in other plant families also develop root nodules that can fix nitrogen. The best known of these are the woody plants known as alders (Alnus spp. ), which develop nodules containing actinomycetes in the genus Frankia. that are known to fix nitrogen include the genera Casuarina, Ceanothus, Myrica, Dryas, and Shepherdia, variously associated with fungi, actinomycetes, or bacteria.

Some species of grasses occur in a relatively loose, non-obligate mutualism with nitrogen–fixing microorganisms. These associative symbioses include those of the crabgrass Digitaria with the bacterium Spirillum and that of another weedy grass, Paspalum, with Azotobacter. Although these nitrogen–fixing symbioses do not involve agriculturally important species of plants, they may nevertheless prove to be significant. Research is being conducted into the possibility of transferring the associative ability of nitrogen fixation of these species into grasses that are important in agriculture, such as corn (Zea mays ) and wheat (Triticum aestivum ). If this feat of bioengineering could be accomplished, it could lead to significant savings of nitrogen fertilizer which must be manufactured using non-renewable resources.

One last example of a nitrogen–fixing mutualism involves termites. These insects have a bacterium, Enterobacter agglomerans, in their gut that fixes nitrogen. This is important to termites because their diet of cellulose is highly deficient in nitrogen.

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 fixation

Nitrogen fixation refers to the chemical conversion of nitrogen gas (dinitrogen, N2) to some oxidized form, usually nitric oxide (NO) or ammonia (NH3). Nitrogen fixation can occur through inorganic reactions or as a result of biological processes. Because the nitrogen atoms in dinitrogen are bound by a very strong triple bond, this gas is very stable and cannot be utilized as a source of nutrition by any but a few highly specialized microorganisms . These nitrogen-fixing microbes are critically important ecologically because nitrogen fixation is the ultimate source of the capital of available organic nitrogen in ecosystems.

It has been estimated that terrestrial ecosystems fix about 135 million metric tons of nitrogen/year (106 tons N/yr), and marine ecosystems 40 × 106 tons N/yr. The terrestrial fixation most commonly occurs in agroecosystems in which legumes are cultivated which fix an estimated 44 × 106 tons N/yr, while grasslands fix 45 × 106 tons N/yr, forests 40 × 106 tons N/yr, and all other terrestrial ecosystems 10 × 106 tons N/yr. Industrial fixation of nitrogen for the manufacturing of synthetic fertilizers contributed another 30 × 106 tons N/yr in the early 1980s, while fixation during combustion accounts for 19 × 106 tons N/yr. Because of increasing demands for nitrogen fertilizers for agriculture, it has been estimated that industrial fixation could increase to 100 × 106 tons N/yr by the year 2000.

Inorganic nitrogen fixation

Atmospheric dinitrogen can be fixed by inorganic reactions occurring in combustion, lightning strikes, and other circumstances involving high temperatures. Overall, naturally occurring non-biological fixation has been estimated to be equivalent to 10-20% of biological fixation of dinitrogen.

Nitrogen fixation is also associated with certain activities of humans. The internal combustion engines of automobiles burn fuels under conditions of very high temperature and pressure which favors the oxidation of nitrogen gas to nitric oxide, an important air pollutant and source of fixed nitrogen. The combustion of coal and other organic fuels also results in the formation of nitric oxide through both the fixation of nitrogen gas and the oxidation of the organic nitrogen of the fuel. Also, as noted previously, nitrogen gas is fixed industrially in enormous amounts to provide agricultural fertilizers. The industrial reaction is known as the Haber process, and it involves the use of hydrogen gas generated from the methane of natural gas to fix ammonium from dinitrogen.

Biological nitrogen fixation

Biological nitrogen fixation is accomplished through the catalytic action of an enzyme known as nitrogenase. Nitrogenase consists of two distinct proteins which contain molybdenum, iron , and sulfur . Because the nitrogenase proteins are denatured by exposure tooxygen (O2), they can only operate in an anaerobic environment. Nitrogen fixation using nitrogenase requires rather large inputs of energy to drive the process, equivalent to about 150 calories of energy per mole of nitrogen gas that is fixed. Although nitrogen fixation is energetically expensive, it is nevertheless worthwhile for plants that grow in nitrogen-deficient habitats.

The nitrogenase enzyme can cleave other triple bonds in addition to that of dinitrogen gas, for example, that of acetylene. Most assays of nitrogen fixation rates take advantage of this fact, measuring nitrogenase activity through the rate at which ethylene is generated by the reaction of acetylene with this enzyme.

Only certain microorganisms can synthesize the nitrogenase enzyme. Some of these microbes are free-living in soils and water , while others occur in mutualisms with fungi or plants.

A large number of free-living bacteria of anaerobic environments have the ability to fix nitrogen, including the genus Clostridium. Fewer genera of aerobic bacteria have this ability. Free-living nitrogen fixation occurs most vigorously in habitats containing large quantities of carbon-rich organic debris such as rotting logs, heaps of sawdust, and compost piles.

Blue-green bacteria are free-living, photosynthetic microbes that fix nitrogen in aquatic habitats and moist soil , including the genera Anabaena, Nostoc, and Calothrix. Strains of these microbes also live in mutualisms with certain fungi in a symbiosis known as lichens which have an ability to fix nitrogen. Anabaena and Nostoc also occur in a mutualism with the floating aquatic fern Azolla which can be cultivated on the surface of rice paddies to fix nitrogen at a rate as great as 100-150 kg N/ha.yr, saving on the use of synthetic fertilizer.

Nitrogen-fixing bacteria can also occur in mutualisms with certain vascular plants. The best known of these associations are with species of legumes (order Leguminosae). About one-half of the world's 10,000 species of legumes occur in a nitrogen-fixing mutualism with bacteria in the genus Rhizobium. About 50 of these species are utilized in agriculture, for example, the food crops garden pea (Pisum sativum), broad bean (Vicia faba), and soybean (Glycine max), and the soil conditioners red clover (Trifolium pratense) and alfalfa (Medicago sativa). The strain of Rhizobium is specific to each legume species.

The Rhizobium occurs in specialized nodules on the roots of the legumes. These are developed when the soil-dwelling Rhizobium invades a root hair, stimulating the plant to form a nodule. Nodule development is inhibited in acidic soils and if the concentrations of nitrate in soil are large. To protect the nitrogenase enzyme, the interior of the nodules is anaerobic. This is due in part to the presence of an oxygen-binding pigment known as leghaemoglobin which colors the nodule interior a dark red. The leghaemoglobin also serves as an oxygen carrier important for other aspects of bacterial metabolism , in this way performing a similar function as the haemoglobin of animals.

Several hundred species in other plant families also develop root nodules that can fix nitrogen. The best known of these are the woody plants known as alders (Alnus spp.), which develop nodules containing actinomycetes in the genus Frankia. Red alder (Alnus rubra) reaches tree-size, and its stands can fix hundreds of kilograms of nitrogen per year. Other non-leguminous plants that are known to fix nitrogen include the genera Casuarina, Ceanothus, Myrica, Dryas, and Shepherdia, variously associated with fungi, actinomycetes, or bacteria.

Some species of grasses occur in a relatively loose, non-obligate mutualism with nitrogen-fixing microorganisms. These associative symbioses include those of the crabgrass Digitaria with the bacterium Spirillum and that of another weedy grass, Paspalum, with Azotobacter. Although these nitrogen-fixing symbioses do not involve agriculturally important species of plants, they may nevertheless prove to be significant. Research is being conducted into the possibility of transferring the associative ability of nitrogen fixation of these species into grasses that are important in agriculture, such as corn (Zea mays) and wheat (Triticum aestivum). If this feat of bioengineering could be accomplished, it could lead to significant savings of nitrogen fertilizer which must be manufactured using non-renewable resources.

One last example of a nitrogen-fixing mutualism involves termites . These insects have a bacterium, Enterobacter agglomerans, in their gut that fixes nitrogen. This is important to termites because their diet of cellulose is highly deficient in nitrogen.

See also Nitrogen cycle.

Resources

books

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

Bill Freedman

KEY TERMS

Mutualism

—A relationship between two or more organisms that is mutually beneficial.

Nitrogen fixation (dinitrogen fixation)

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

Nitrogenase

—An enzyme synthesized by certain microorganisms that is capable of cleaving the triple bond of nitrogen gas generating ammonium, a biologically useful type of fixed nitrogen.

Nitrogen Fixation

Nitrogen fixation refers to the conversion of atmospheric nitrogen gas (N₂) into a form usable by plants and other organisms. Nitrogen fixation is conducted by a variety of bacteria, both as free-living organisms and in symbiotic association with plants. Because it is the principal source of the nitrogen in the soil, nitrogen that plants need to grow, nitrogen fixation is one of the most important biochemical processes on Earth. Even modern agricultural systems depend on nitrogen fixation by alfalfa, clover, and other legumes to supplement chemical nitrogen fertilizers.

Living organisms need nitrogen because it is a part of the amino acids that make up proteins , and the nucleic acids that make up DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). Nitrogen within living organisms is eventually decomposed and converted to atmospheric nitrogen (N₂). This form, however, is highly stable and unreactive chemically, and is therefore not available for use by most organisms. Some species of bacteria, though, can convert N₂ into NH₃ (ammonia) or other usable forms of nitrogen. These nitrogen-fixing bacteria include species of the genera Rhizobium, Anabaena, Azotobacter, and Clostridium, as well as others.

Each of the nitrogen-fixing bacteria employs the same enzyme , nitrogenase. The nitrogenase enzyme is shaped something like a butterfly, and contains an atom of molybdenum at its core that is crucial for the reaction. Soils deficient in molybdenum cannot sustain effective nitrogen fixation, and monitoring soil for this element is important to ensure maximum fixation in managed fields or pastures.

Nitrogenase requires a large amount of energy to convert N₂ to NH₃. Free-living bacteria must obtain the nutrients for supplying this energy themselves. Other bacteria have developed symbiotic associations with plants to provide them with sugars, supplying both a source of energy and a source of carbon for the bacterium's own synthetic reactions. The bacteria, in turn, supply the plant with some of the fixed nitrogen. For instance, the nitrogen-fixing Anabaena lives symbiotically with a water fern, Azolla. Azolla is grown in rice paddies early in the season. As the rice grows above the water surface, it shades out the fern, which dies, releasing the stored nitrogen. In this way, the paddy is fertilized without application of chemical fertilizers.

The bacterial genera Rhizobium and Bradyrhizobium have developed a large number of symbioses with members of the Fabaceae (legume) family. Fabaceae includes alfalfa, clover, beans and peas of all kinds, mesquites, acacias, and dozens of other species both domesticated and wild. The roots of the host plant become infected with the bacteria as seedlings, and respond by surrounding the bacteria with root hairs. The relationship between a particular host species and a particular bacterium is highly specific, and is regulated by a series of recognition events that prevent the wrong species of bacterium from taking up residence in the wrong plant.

The plant eventually develops a specialized structure known as a nodule, while the bacteria inside grow into enlarged forms known as bacteroids. The oxygen concentration inside the nodule must be closely regulated, since oxygen inhibits nitrogenase. This regulation is aided by the presence of leghemoglobin, an oxygen-binding protein similar to hemoglobin . The heme (oxygen-binding) portion is produced by the bacterium, while the globin (protein) portion is produced by the host plant, again illustrating the closeness of the symbiotic relationship.

Richard Robinson

Bibliography

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants, 6th ed. New York: W. H. Freeman and Company, 1999.

Nitrogen fixation

Nitrogen fixation is the biological process by which atmospheric nitrogen gas (N₂) is converted into ammonia (NH₃). Nitrogen is an essential nutrient for the growth of all organisms, as it is a component of proteins, nucleic acids, amino acids, and other cellular constituents. Nearly 79% of the earth's atmosphere is N₂, but N₂ is unavailable for use by most organisms, because it is nearly inert due to the triple bonds between the two nitrogen atoms. To utilize nitrogen, organisms require that it be fixed (combined) in the form of ammonium (NH₄) or nitrate ions (NO₃).

The bacteria that accomplish nitrogen fixation are either free-living or form symbiotic associations with plants or other organisms such as termites or protozoa. The free-living bacteria include both aerobic and anaerobic bacteria, including some species that are photosynthetic.

The most well-known nitrogen-fixing symbiotic relationships are those of legumes (e.g., peas, beans, clover, soybeans, and alfalfa) and the bacteria Rhizobium, which is present in nodules on the roots of the legumes. The plants provide the bacteria with carbohydrates for energy and a stable environment for growth, while the bacteria provide the plants with usable nitrogen and other essential nutrients. The bacteria invade the plant and cause formation of the nodule by inducing growth of the plant host cells. The Rhizobium are separated from the plant cells by being enclosed in a membrane. The cultivation of legumes as green manure crops to add nitrogen by incorporation into the soil is a long-established agricultural process.

Frankia, a type of bacteria belonging to the bacterial group actinomycetes, form nitrogen-fixing root nodules with several woody plants, including alder and sea buckhorn. These trees are pioneer species that invade nutrient-poor soils.

The photosynthetic nitrogen-fixing bacteria cyanobacteria (also known as blue-green algae) live as free-living organisms or as symbionts with lichens in pioneer habitats such as desert soils. They also form a symbiotic association with the fern Azolla, which is also found in lakes, swamps, and streams. In rice paddies, Azolla has been grown as a green manure crop; it is plowed into the soil to release nitrogen before planting of the rice crop.

The nitrogen fixed by the nitrogen-fixing bacteria is utilized by other living organisms. After their death, the nitrogen is released into the environment through decomposition processes and continues to move through the nitrogen cycle .

[Judith L. Sims ]

RESOURCES

BOOKS

Postgate, John R., and J. R. Postgate. Nitrogen Fixation. Cambridge, UK: Cambridge University Press, 1998.

OTHER

Biology Teaching Organization. The Microbial World: The Nitrogen Cycle and Nitrogen Fixation. Edinburgh School of Biology, The University of Edinburgh. [cited June 21, 2002]. <http://helios.bto.ed.ac.uk/bto/microbes/nitrogen.htm>.