nitrogen fixing

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Nitrogen fixation is a chemical process by which molecular
nitrogen Nitrogen is the chemical element with the Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the pnictogens. It is a common element in the ...
(), with a strong triple
covalent bond A covalent bond is a chemical bond A chemical bond is a lasting attraction between atoms or ions that enables the formation of Molecule, molecules and crystals. The bond may result from the Coulomb's law, electrostatic force between oppos ...
, in the
air The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing fo ...
is converted into
ammonia Ammonia is an inorganic chemical compound, compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a dis ...
() or related nitrogenous compounds, typically in soil or aquatic systems but also in industry. Atmospheric nitrogen is molecular dinitrogen, a relatively nonreactive molecule that is metabolically useless to all but a few microorganisms. Biological nitrogen fixation or ''diazotrophy'' is an important microbials mediated process that converts dinitrogen (N2) gas to ammonia (NH3) using the
nitrogenase Nitrogenases are enzymes () that are produced by certain bacteria, such as cyanobacteria (blue-green bacteria) and rhizobacteria. These enzymes are responsible for the Organic redox reaction, reduction of nitrogen (N2) to ammonia (NH3). Nitrog ...
protein complex (Nif). Nitrogen fixation is essential to life because fixed inorganic nitrogen compounds are required for the
biosynthesis Biosynthesis is a multi-step, enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts ...
of all nitrogen-containing
organic compounds In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bonds. Due to carbon's ability to Catenation, catenate (form chains with other carbon atoms), millions of organic c ...
, such as
amino acids Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ami ...
and
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metabo ...
s,
nucleoside triphosphate A nucleoside triphosphate is a nucleoside containing a nitrogenous base bound to a 5-carbon sugar (either ribose or deoxyribose), with three phosphate groups bound to the sugar. They are the molecular precursors of both DNA and RNA, which are chai ...
s and
nucleic acid Nucleic acids are biopolymers, macromolecules, essential to all Organism, known forms of life. They are composed of nucleotides, which are the monomers made of three components: a pentose, 5-carbon sugar, a phosphate group and a nitrogenous base. ...
s. As part of the
nitrogen cycle The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmosphere, atmospheric, terrestrial ecosystem, terrestrial, and marine ecosystems. The conversion of nitrogen can ...
, it is essential for
agriculture Agriculture or farming is the practice of cultivating Plant, plants and livestock. Agriculture was the key development in the rise of Sedentism, sedentary human civilization, whereby farming of Domestication, domesticated species created food ...
and the manufacture of
fertilizer A fertilizer (American English) or fertiliser (British English; American and British English spelling differences#-ise, -ize (-isation, -ization), see spelling differences) is any material of natural or synthetic origin that is applied to soil ...
. It is also, indirectly, relevant to the manufacture of all nitrogen chemical compounds, which includes some explosives, pharmaceuticals, and dyes. Nitrogen fixation is carried out naturally in
soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organism In biology, an organism () is any life, living system that functions as an individual entity. All organi ...
by
microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
s termed
diazotroph Diazotrophs are bacteria and archaea that Nitrogen fixation, fix gaseous nitrogen in the atmosphere into a more usable form such as ammonia. A diazotroph is a microorganism that is able to grow without external sources of fixed nitrogen. Examples o ...
s that include
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometre The micrometre (Amer ...
, such as ''
Azotobacter ''Azotobacter'' is a genus of usually Motility, motile, oval or spherical bacteria that form thick-walled cysts (and also has hard crust) and may produce large quantities of capsular Mucus, slime. They are aerobic, free-living soil Microorganism ...
,'' and
archaea Archaea ( ; singular archaeon ) is a Domain (biology), domain of Unicellular organism, single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially Taxonomy (biology), classified as bacter ...
. Some nitrogen-fixing bacteria have
symbiotic Symbiosis (from Ancient Greek, Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different Organism, biological organisms, be it Mutualism (biolog ...
relationships with plant groups, especially
legume A legume () is a plant in the family Fabaceae (or Leguminosae), or the fruit or seed of such a plant. When used as a dry grain, the seed is also called a pulse. Legumes are grown agriculturally, primarily for human consumption, for livestock for ...
s. Looser non-symbiotic relationships between diazotrophs and plants are often referred to as associative, as seen in nitrogen fixation on
rice Rice is the seed of the Poaceae, grass species ''Oryza sativa'' (Asian rice) or less commonly ''Oryza glaberrima'' (African rice). The name wild rice is usually used for species of the genera ''Zizania (genus), Zizania'' and ''Porteresia'', bo ...
roots. Nitrogen fixation occurs between some
termite Termites are small insects that live in colonies and have distinct castes (Eusociality, eusocial) and feed on wood or other dead plant matter. Termites comprise the infraorder Isoptera, or alternatively the Taxonomic rank#All ranks, epifamily ...
s and
fungi A fungus (plural, : fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified ...
. It occurs naturally in the air by means of NOx production by
lightning Lightning is a naturally occurring electrostatic discharge during which two electric charge, electrically charged regions, both in the atmosphere or with one on the land, ground, temporarily neutralize themselves, causing the instantaneous ...
. All biological reactions involving the process of nitrogen fixation are catalyzed by enzymes called
nitrogenase Nitrogenases are enzymes () that are produced by certain bacteria, such as cyanobacteria (blue-green bacteria) and rhizobacteria. These enzymes are responsible for the Organic redox reaction, reduction of nitrogen (N2) to ammonia (NH3). Nitrog ...
s. These enzymes contain
iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundance ...
, often with a second metal, usually
molybdenum Molybdenum is a chemical element with the Symbol (chemistry), symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from New Latin, Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since i ...
but sometimes
vanadium Vanadium is a chemical element with the Symbol (chemistry), symbol V and atomic number 23. It is a hard, silvery-grey, malleable transition metal. The elemental metal is rarely found in nature, but once isolated artificially, the formation of an ...
.

# History

Biological nitrogen fixation was discovered by
Jean-Baptiste Boussingault Jean-Baptiste Joseph Dieudonné Boussingault (2 February 1801 – 11 May 1887) was a French chemist who made significant contributions to agricultural science, petroleum science and metallurgy. Biography Jean-Baptiste Boussingault – an agric ...
in 1838. Later, in 1880, the process by which it happens was discovered by German
agronomist An agriculturist, agriculturalist, agrologist, or agronomist (abbreviated as agr.), is a professional in the Agricultural science, science, practice, and management of Farming, agriculture and agribusiness. It is a regulated profession in Canada ...
Hermann Hellriegel and and was fully described by Dutch microbiologist
Martinus Beijerinck Martinus Willem Beijerinck (, 16 March 1851 – 1 January 1931) was a Dutch microbiologist and botanist who was one of the founders of history of virology, virology and environmental microbiology. He is credited with the discovery of viruses, ...
. "The protracted investigations of the relation of plants to the acquisition of nitrogen begun by Saussure, Ville, Lawes and Gilbert and others culminated in the discovery of symbiotic fixation by Hellriegel and Wilfarth in 1887." "Experiments by Bossingault in 1855 and Pugh, Gilbert & Lawes in 1887 had shown that nitrogen did not enter the plant directly. The discovery of the role of nitrogen fixing bacteria by Herman Hellriegel and Herman Wilfarth in 1886-8 would open a new era of soil science." In 1901 Beijerinck showed that ''
Azotobacter chroococcum ''Azotobacter chroococcum'' is a bacterium that has the ability to fix atmospheric nitrogen. It was discovered by Martinus Beijerinck in 1901, and was the first aerobic, free-living nitrogen fixer discovered. ''A. chroococcum'' could be useful ...
'' was able to fix atmospheric nitrogen. This was the first species of the
azotobacter ''Azotobacter'' is a genus of usually Motility, motile, oval or spherical bacteria that form thick-walled cysts (and also has hard crust) and may produce large quantities of capsular Mucus, slime. They are aerobic, free-living soil Microorganism ...
genus, so-named by him. It is also the first known
diazotroph Diazotrophs are bacteria and archaea that Nitrogen fixation, fix gaseous nitrogen in the atmosphere into a more usable form such as ammonia. A diazotroph is a microorganism that is able to grow without external sources of fixed nitrogen. Examples o ...
, species that use
diatomic Diatomic molecules () are molecules composed of only two atoms, of the same or different chemical elements. If a diatomic molecule consists of two atoms of the same element, such as hydrogen () or oxygen (), then it is said to be homonuclear mol ...
nitrogen as a step in the complete
nitrogen cycle The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmosphere, atmospheric, terrestrial ecosystem, terrestrial, and marine ecosystems. The conversion of nitrogen can ...
.

# Biological

Biological nitrogen fixation (BNF) occurs when atmospheric nitrogen is converted to ammonia by a
nitrogenase Nitrogenases are enzymes () that are produced by certain bacteria, such as cyanobacteria (blue-green bacteria) and rhizobacteria. These enzymes are responsible for the Organic redox reaction, reduction of nitrogen (N2) to ammonia (NH3). Nitrog ...
enzyme. The overall reaction for BNF is: :N2 + 16ATP + 16H2O + 8e- + 8H+ -> 2NH3 +H2 + 16ADP + 16$\text_i$ The process is coupled to the
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...
of 16 equivalents of ATP and is accompanied by the co-formation of one equivalent of . The conversion of into ammonia occurs at a metal cluster called
FeMoco FeMoco ( cofactor) is the primary cofactor of nitrogenase Nitrogenases are enzymes () that are produced by certain bacteria, such as cyanobacteria (blue-green bacteria) and rhizobacteria. These enzymes are responsible for the Organic redox ...
, an abbreviation for the iron-
molybdenum Molybdenum is a chemical element with the Symbol (chemistry), symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from New Latin, Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since i ...
cofactor. The mechanism proceeds via a series of
protonation In chemistry, protonation (or hydronation) is the adding of a proton#Hydrogen ion, proton (or hydron (chemistry), hydron, or hydrogen cation), (H+) to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is ...
and reduction steps wherein the FeMoco
active site In biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of Cell (biology), ce ...
hydrogenate Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a Catalysis, catalyst such as nickel, palladium or platinum. The process is commonly employed to redox, reduce or S ...
s the substrate. In free-living
diazotroph Diazotrophs are bacteria and archaea that Nitrogen fixation, fix gaseous nitrogen in the atmosphere into a more usable form such as ammonia. A diazotroph is a microorganism that is able to grow without external sources of fixed nitrogen. Examples o ...
s, nitrogenase-generated ammonia is assimilated into
glutamate Glutamic acid (symbol Glu or E; the ionic form is known as glutamate) is an α-amino acid that is used by almost all living beings in the biosynthesis of proteins. It is a Essential amino acid, non-essential nutrient for humans, meaning that th ...
through the glutamine synthetase/glutamate synthase pathway. The microbial
nif gene The ''nif'' genes are genes encoding enzymes involved in the nitrogen fixation, fixation of atmospheric nitrogen into a form of nitrogen available to living organisms. The primary enzyme encoded by the ''nif'' genes is the nitrogenase complex which ...
s required for nitrogen fixation are widely distributed in diverse environments. For example, decomposing wood, which generally has a low nitrogen content, has been shown to host a diazotrophic community. The bacteria enrich the wood substrate with nitrogen through fixation, thus enabling deadwood decomposition by fungi. Nitrogenases are rapidly degraded by oxygen. For this reason, many bacteria cease production of the enzyme in the presence of oxygen. Many nitrogen-fixing organisms exist only in
anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: *Adhesive#Anaerobic, Anaerobic ad ...
conditions, respiring to draw down oxygen levels, or binding the oxygen with a
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metabo ...
such as
leghemoglobin 3rd Leghemoglobin (also leghaemoglobin or legoglobin) is an oxygen-carrying phytoglobin found in the nitrogen-fixing root nodule Root nodules are found on the roots of plants, primarily legumes, that form a symbiosis with nitrogen-fixing ba ...
.

## Importance of nitrogen

Atmospheric nitrogen is inaccessible to most organisms, because its triple covalent bond is very strong. Life takes up fixed nitrogen in various ways. Considering atom acquisition, for every 100 atoms of carbon, roughly 2 to 20 atoms of nitrogen are assimilated. The atomic ratio of carbon (C) : nitrogen (N) : phosphorus (P) observed on average in planktonic biomass was originally described by Alfred Redfield. The Redfield Ratio, the stoichiometric relationship between C:N:P atoms, is 106:16:1.

## Nitrogenase

The protein complex nitrogenase is responsible for catalyzing the reduction of nitrogen gas (N2) to ammonia (NH3). In Cyanobacteria, this enzyme system is housed in a specialized cell called the heterocyst. The production of the nitrogenase complex is genetically regulated, and the activity of the protein complex is dependent on ambient oxygen concentrations, and intra- and extracellular concentrations of ammonia and oxidized nitrogen species (nitrate and nitrite). Additionally, the combined concentrations of both ammonium and nitrate are thought to inhibit NFix, specifically when intracellular concentrations of 2-oxoglutarate (2-OG) exceed a critical threshold. The specialized heterocyst cell is necessary for the performance of nitrogenase as a result of its sensitivity to ambient oxygen. Nitrogenase consist of two proteins, a catalytic iron-dependent protein, commonly referred to as MoFe protein and a reducing iron-only protein (Fe protein). There are three different iron dependent proteins, molybdenum-dependent, vanadium-dependent, and iron-only, with all three nitrogenase protein variations containing an iron protein component. Molybdenum-dependent nitrogenase is the most commonly present nitrogenase. The different types of nitrogenase can be determined by the specific iron protein component. Nitrogenase is highly conserved. Gene expression through DNA sequencing can distinguish which protein complex is present in the microorganism and potentially being express. Most frequently, the ''nif''H gene is used to identify the presence of molybdenum-dependent nitrogenase, followed by closely related nitrogenase reductases (component II) ''vnf''H and ''anf''H representing vanadium-dependent and iron-only nitrogenase, respectively. In studying the ecology and evolution of nitrogen-fixing bacteria, the ''nifH'' gene is the biomarker most widely used. ''nif''H has two similar genes ''anf''H and vnfH that also encode for the nitrogenase reductase component of the nitrogenase complex.

## Microorganisms

Bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometre The micrometre (Amer ...
including
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum (biology), phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's ...
(e.g. the highly significant ''
Trichodesmium ''Trichodesmium'', also called sea sawdust, is a genus of Filamentation, filamentous cyanobacteria. They are found in nutrient poor tropical and subtropical ocean waters (particularly around Australia and in the Red Sea, where they were first des ...
'' and ''
Cyanothece ''Cyanothece'' is a genus of unicellular, diazotrophic, oxygenic photosynthesizing cyanobacteria. Modern organisms and cellular organization In 1976, Jiří Komárek defined the prokaryotic cyanobacteria genus ''Cyanothece'' as distinct from '' ...
''),
green sulfur bacteria The green sulfur bacteria are a phylum of obligately anaerobic organism, anaerobic photoautotrophic bacteria that metabolize sulfur. Green sulfur bacteria are motility, nonmotile (except ''Chloroherpeton thalassium'', which may glide) and capabl ...
, Azotobacteraceae,
rhizobia Rhizobia are diazotrophic bacteria that Nitrogen fixation, fix nitrogen after becoming established inside the root nodules of legumes (Fabaceae). To express genes for nitrogen fixation, rhizobia require a plant Host (biology), host; they cannot i ...
and ''
Frankia ''Frankia'' is a genus of Nitrogen fixation, nitrogen-fixing Filamentation, bacteria that live in symbiosis with actinorhizal plants, similar to the ''Rhizobium'' bacteria found in the root nodules of legumes in the family Fabaceae. ''Frankia'' ...
.'' Several obligately anaerobic bacteria fix nitrogen including many (but not all) ''
Clostridium ''Clostridium'' is a genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biolo ...
'' spp. Some
archaea Archaea ( ; singular archaeon ) is a Domain (biology), domain of Unicellular organism, single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially Taxonomy (biology), classified as bacter ...
such as '' Methanosarcina acetivorans'' also fix nitrogen,. and several other
methanogen Methanogens are microorganisms that produce methane as a Metabolism, metabolic byproduct in Hypoxia (environmental), hypoxic conditions. They are Prokaryote, prokaryotic and belong to the Domain (biology), domain Archaea. All known methanogens are ...
ic
taxa In biology, a taxon (back-formation from ''Taxonomy (biology), taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known ...
, are significant contributors to nitrogen fixation in oxygen-deficient soils.
Cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum (biology), phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's ...
, commonly known as blue-green algae, inhabit nearly all illuminated environments on Earth and play key roles in the carbon and
nitrogen cycle The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmosphere, atmospheric, terrestrial ecosystem, terrestrial, and marine ecosystems. The conversion of nitrogen can ...
of the
biosphere The biosphere (from Ancient Greek, Greek βίος ''bíos'' "life" and σφαῖρα ''sphaira'' "sphere"), also known as the ecosphere (from Greek οἶκος ''oîkos'' "environment" and σφαῖρα), is the worldwide sum of all ecosystems. ...
. In general, cyanobacteria can use various inorganic and organic sources of combined nitrogen, such as
nitrate Nitrate is a polyatomic ion with the chemical formula . salt (chemistry), Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are solubility, soluble in wat ...
,
nitrite The nitrite polyatomic ion, ion has the chemical formula . Nitrite (mostly sodium nitrite) is widely used throughout chemical and pharmaceutical industries. The nitrite anion is a pervasive intermediate in the nitrogen cycle in nature. The name ...
,
ammonium The ammonium cation is a positively-charged polyatomic ion with the chemical formula or . It is formed by the protonation of ammonia (). Ammonium is also a general name for positively charged or protonated substituted amines and quaternary amm ...
,
urea Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two Amine, amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid. Urea serves an impor ...
, or some
amino acid Amino acids are organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bonds. Due to carbon's ability to Catenation, catenate (form chains with ot ...
s. Several cyanobacteria strains are also capable of diazotrophic growth, an ability that may have been present in their last common ancestor in the
Archean The Archean Eon ( , also spelled Archaean or Archæan) is the second of four geologic eons of Earth's history, representing the time from . The Archean was preceded by the Hadean Eon and followed by the Proterozoic. The Earth Ear ...
eon. Nitrogen fixation not only naturally occurs in soils but also aquatic systems, including both freshwater and marine. Indeed, the amount of nitrogen fixed in the ocean is at least as much as that on land. The colonial marine cyanobacterium ''
Trichodesmium ''Trichodesmium'', also called sea sawdust, is a genus of Filamentation, filamentous cyanobacteria. They are found in nutrient poor tropical and subtropical ocean waters (particularly around Australia and in the Red Sea, where they were first des ...
'' is thought to fix nitrogen on such a scale that it accounts for almost half of the nitrogen fixation in marine systems globally. Marine surface lichens and non-photosynthetic bacteria belonging in Proteobacteria and Planctomycetes fixate significant atmospheric nitrogen. Species of nitrogen fixing cyanobacteria in fresh waters include: '' Aphanizomenon'' and ''Dolichospermum'' (previously Anabaena). Such species have specialized cells called heterocytes, in which nitrogen fixation occurs via the nitrogenase enzyme.

## Root nodule symbioses

### Legume family

Plants that contribute to nitrogen fixation include those of the
legume A legume () is a plant in the family Fabaceae (or Leguminosae), or the fruit or seed of such a plant. When used as a dry grain, the seed is also called a pulse. Legumes are grown agriculturally, primarily for human consumption, for livestock for ...
family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...
Fabaceae The Fabaceae or Leguminosae,International Code of Nomenc ...
— with
taxa In biology, a taxon (back-formation from ''Taxonomy (biology), taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known ...
such as
kudzu Kudzu (; also called Japanese arrowroot or Chinese arrowroot) is a group of climbing, coiling, and trailing deciduous perennial vines native to much of East Asia, Southeast Asia, and some Pacific islands, but invasive species, invasive in many ...
,
clover Clover or trefoil are common names for plants of the genus ''Trifolium'' (from Latin ''tres'' 'three' + ''folium'' 'leaf'), consisting of about 300 species In biology, a species is the basic unit of Taxonomy (biology), classification and a ...
,
soybean The soybean, soy bean, or soya bean (''Glycine max'') is a species of legume native to East Asia, widely grown for its edible bean, which has numerous uses. Traditional unfermented food uses of soybeans include soy milk, from which tofu and ...
,
alfalfa Alfalfa () (''Medicago sativa''), also called lucerne, is a perennial plant, perennial flowering plant in the legume family Fabaceae. It is cultivated as an important forage crop in many countries around the world. It is used for grazing, hay, ...
,
lupin ''Lupinus'', commonly known as lupin, lupine, or regionally bluebonnet etc., is a genus of plants in the legume family Fabaceae. The genus includes over 199 species, with center of diversity, centers of diversity in North and South America. Sm ...
,
peanut The peanut (''Arachis hypogaea''), also known as the groundnut, goober (US), pindar (US) or monkey nut (UK), is a legume crop grown mainly for its edible Seed, seeds. It is widely grown in the tropics and subtropics, important to both small ...
and
rooibos Rooibos ( ; , meaning "red bush"), or ''Aspalathus linearis'', is a broom (shrub), broom-like member of the plant family Fabaceae that grows in South Africa's fynbos biome. The leaves are used to make a herbal tea that is called rooibos (esp ...
. They contain
symbiotic Symbiosis (from Ancient Greek, Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different Organism, biological organisms, be it Mutualism (biolog ...
rhizobia Rhizobia are diazotrophic bacteria that Nitrogen fixation, fix nitrogen after becoming established inside the root nodules of legumes (Fabaceae). To express genes for nitrogen fixation, rhizobia require a plant Host (biology), host; they cannot i ...
bacteria within nodules in their root systems, producing nitrogen compounds that help the plant to grow and compete with other plants. When the plant dies, the fixed nitrogen is released, making it available to other plants; this helps to fertilize the
soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organism In biology, an organism () is any life, living system that functions as an individual entity. All organi ...
. The great majority of legumes have this association, but a few
genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
(e.g., ''
Styphnolobium ''Styphnolobium'' is a small genus of three or four species of small trees and shrubs in the subfamily Faboideae of the pea family Fabaceae, formerly included within a broader interpretation of the genus ''Sophora''. It was recently assigned to t ...
'') do not. In many traditional farming practices, fields are rotated through various types of crops, which usually include one consisting mainly or entirely of
clover Clover or trefoil are common names for plants of the genus ''Trifolium'' (from Latin ''tres'' 'three' + ''folium'' 'leaf'), consisting of about 300 species In biology, a species is the basic unit of Taxonomy (biology), classification and a ...
. Fixation efficiency in soil is dependent on many factors, including the
legume A legume () is a plant in the family Fabaceae (or Leguminosae), or the fruit or seed of such a plant. When used as a dry grain, the seed is also called a pulse. Legumes are grown agriculturally, primarily for human consumption, for livestock for ...
and air and soil conditions. For example, nitrogen fixation by red clover can range from .

### Non-leguminous

The ability to fix nitrogen in nodules is present in actinorhizal plants such as
alder Alders are trees comprising the genus ''Alnus'' in the birch family Betulaceae. The genus comprises about 35 species of monoecious trees and shrubs, a few reaching a large size, distributed throughout the Temperate climate, north temperate z ...
and bayberry, with the help of ''
Frankia ''Frankia'' is a genus of Nitrogen fixation, nitrogen-fixing Filamentation, bacteria that live in symbiosis with actinorhizal plants, similar to the ''Rhizobium'' bacteria found in the root nodules of legumes in the family Fabaceae. ''Frankia'' ...
'' bacteria. They are found in 25 genera in the orders Cucurbitales,
Fagales The Fagales are an order (biology), order of flowering plants, including some of the best-known trees. The order name is derived from genus ''Fagus'', beeches. They belong among the rosid group of dicotyledons. The families and genera currently ...
and Rosales, which together with the Fabales form a ''nitrogen-fixing clade'' of eurosids. The ability to fix nitrogen is not universally present in these families. For example, of 122
Rosaceae Rosaceae (), the rose family, is a medium-sized family (biology), family of flowering plants that includes 4,828 known species in 91 genera. The name is derived from the type genus ''Rose, Rosa''. Among the most species-rich genera are ''Alche ...
genera, only four fix nitrogen. Fabales were the first lineage to branch off this nitrogen-fixing clade; thus, the ability to fix nitrogen may be
plesiomorphic In phylogenetics, a plesiomorphy ("near form") and symplesiomorphy are synonyms for an ancestral Phenotypic trait, character shared by all members of a clade, which does not distinguish the clade from other clades. Plesiomorphy, symplesiomorph ...
and subsequently lost in most descendants of the original nitrogen-fixing plant; however, it may be that the basic genetic and
physiological Physiology (; ) is the science, scientific study of function (biology), functions and mechanism (biology), mechanisms in a life, living system. As a branches of science, sub-discipline of biology, physiology focuses on how organisms, organ syst ...
requirements were present in an incipient state in the
most recent common ancestor In biology and genetic genealogy, the most recent common ancestor (MRCA), also known as the last common ancestor (LCA) or concestor, of a set of organisms is the most recent individual from which all the organisms of the set are Common descent, ...
s of all these plants, but only evolved to full function in some of them. In addition, '' Trema'' (''Parasponia''), a tropical genus in the family
Cannabaceae Cannabaceae is a small family (biology), family of flowering plants, known as the hemp family. As now Circumscription (taxonomy), circumscribed, the family includes about 170 species grouped in about 11 genera, including ''Cannabis'' (hemp), ''H ...
, is unusually able to interact with rhizobia and form nitrogen-fixing nodules.

## Other plant symbionts

Some other plants live in association with a cyanobiont (cyanobacteria such as '' Nostoc'') which fix nitrogen for them: * Some lichens such as '' Lobaria'' and '' Peltigera'' * Mosquito fern (''
Azolla ''Azolla'' (mosquito fern, duckweed fern, fairy moss, water fern) is a genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV c ...
'' species) *
Cycad Cycads are seed plant A spermatophyte (; ), also known as phanerogam (taxon Phanerogamae) or phaenogam (taxon Phaenogamae), is any plant that produces seeds, hence the alternative name seed plant. Spermatophytes are a subset of the embryop ...
s *'' Gunnera'' *'' Blasia'' ( liverwort) *
Hornwort Hornworts are a group of non-vascular Embryophytes (land plants) constituting the division Anthocerotophyta (). The common name refers to the elongated horn-like structure, which is the sporophyte. As in mosses and liverworts, hornworts have ...
s Some symbiotic relationships involving agriculturally-important plants are: *
Sugarcane Sugarcane or sugar cane is a species of (often hybrid) tall, Perennial plant, perennial grass (in the genus ''Saccharum'', tribe Andropogoneae) that is used for sugar Sugar industry, production. The plants are 2–6 m (6–20 ft) tall with ...
and unclear
endophyte An endophyte is an endosymbiont, often a bacterium or fungus, that lives within a plant for at least part of its life cycle without causing apparent disease. Endophytes are ubiquitous and have been found in all species of plants studied to date; h ...
s *
Foxtail millet Foxtail millet, scientific name ''Setaria italica'' (synonym ''Panicum italicum'' L.), is an annual grass grown for human food. It is the second-most widely planted species of millet, and the most grown millet species in Asia. The oldest evidenc ...
and '' Azospirillum brasilense'' * Kallar grass and '' Azoarcus'' sp. strain BH72 *
Rice Rice is the seed of the Poaceae, grass species ''Oryza sativa'' (Asian rice) or less commonly ''Oryza glaberrima'' (African rice). The name wild rice is usually used for species of the genera ''Zizania (genus), Zizania'' and ''Porteresia'', bo ...
and '' Herbaspirillum seropedicae'' *
Wheat Wheat is a Poaceae, grass widely Agriculture, cultivated for its seed, a cereal grain that is a worldwide staple food. The Taxonomy of wheat, many species of wheat together make up the genus ''Triticum'' ; the most widely grown is common wheat ...
and ''
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'' *
Maize Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American English, North American and Australian English), is a cereal grain first domesticated by indigenous peoples of Mexico, indigenous ...
landrace 'Sierra Mixe' / 'olotón' and various
Bacteroidota The phylum (biology), phylum Bacteroidota (synonym Bacteroidetes) is composed of three large classes of Gram-negative bacteria, Gram-negative, nonsporeforming, anaerobic or aerobic, and rod-shaped bacteria that are widely distributed in the envir ...
and
Pseudomonadota Pseudomonadota (synonym Proteobacteria) is a major phylum of Gram-negative bacteria. The renaming of phyla in 2021 remains controversial among microbiologists, many of whom continue to use the earlier names of long standing in the literature. The ...

# Industrial processes

## Historical

A method for nitrogen fixation was first described by Henry Cavendish in 1784 using electric arcs reacting nitrogen and oxygen in air. This method was implemented in the Birkeland–Eyde process of 1903. The fixation of nitrogen by lightning is a very similar natural occurring process. The possibility that atmospheric nitrogen reacts with certain chemicals was first observed by Desfosses in 1828. He observed that mixtures of alkali metal oxides and carbon react with nitrogen at high temperatures. With the use of barium carbonate as starting material, the first commercial process became available in the 1860s, developed by Margueritte and Sourdeval. The resulting barium cyanide reacts with steam, yielding ammonia. In 1898 Adolph Frank, Frank and Nikodem Caro, Caro developed what is known as the Frank–Caro process to fix nitrogen in the form of calcium cyanamide. The process was eclipsed by the Haber process, which was discovered in 1909.

## Haber process

The dominant industrial method for producing ammonia is the Haber process also known as the Haber-Bosch process. Fertilizer production is now the largest source of human-produced fixed nitrogen in the terrestrial ecosystem. Ammonia is a required precursor to
fertilizer A fertilizer (American English) or fertiliser (British English; American and British English spelling differences#-ise, -ize (-isation, -ization), see spelling differences) is any material of natural or synthetic origin that is applied to soil ...
s, explosives, and other products. The Haber process requires high pressures (around 200 atm) and high temperatures (at least 400 °C), which are routine conditions for industrial catalysis. This process uses natural gas as a hydrogen source and air as a nitrogen source. The ammonia product has resulted in an intensification of nitrogen fertilizer globally and is credited with supporting the expansion of the human population from around 2 billion in the early 20th century to roughly 8 billion people now.

## Homogeneous catalysis

Much research has been conducted on the discovery of catalysts for nitrogen fixation, often with the goal of lowering energy requirements. However, such research has thus far failed to approach the efficiency and ease of the Haber process. Many compounds react with atmospheric nitrogen to give dinitrogen complexes. The first dinitrogen Complex (chemistry), complex to be reported was pentaamine(dinitrogen)ruthenium(II) chloride, ()2+. Some soluble complexes do catalyze nitrogen fixation.

# Lightning

Nitrogen can be fixed by
lightning Lightning is a naturally occurring electrostatic discharge during which two electric charge, electrically charged regions, both in the atmosphere or with one on the land, ground, temporarily neutralize themselves, causing the instantaneous ...
converting nitrogen gas () and oxygen gas () in the atmosphere into (nitrogen oxides). The molecule is highly stable and nonreactive due to the triple bond between the nitrogen atoms. Lightning produces enough energy and heat to break this bond allowing nitrogen atoms to react with oxygen, forming . These compounds cannot be used by plants, but as this molecule cools, it reacts with oxygen to form , which in turn reacts with water to produce (nitrous acid) or (nitric acid). When these acids seep into the soil, they make nitrate, (nitrate), which is of use to plants.