TheInfoList

Nitrogen fixation is a chemical process by which molecular
nitrogen Nitrogen is the chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

(), with a strong triple
covalent bond A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they s ...
, in the
air The atmosphere of Earth is the layer of gas Gas is one of the four fundamental states of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (ph ...
is converted into
ammonia Ammonia is a chemical compound, compound of nitrogen and hydrogen with the chemical formula, formula NH3. A Binary compounds of hydrogen, stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct ch ...

() 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 Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life Life is a characteristic that distinguishes physical entities that have biological processes, such as signaling and self-sustaining processes, ...

useless to all but a few microorganisms. Biological nitrogen fixation or diazotrophy is an important microbially mediated process that converts (N2) gas to ammonia (NH3) using the nitrogenase 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 (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and ...

of all nitrogen-containing
organic compounds In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...
, such as
amino acids Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reactions, a ...

and
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

s,
nucleoside triphosphate Nucleosides are glycosylamine 120px, Cyclic hemiaminal ether bond derived from an aldehyde Glycosylamines are a class of biochemical compounds consisting of a Glycosyl, glycosyl group attached to an amine, amino group, -NR2. They are also known ...
s and
nucleic acid Nucleic acids are biopolymer Biopolymers are natural polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecule ...

s. As part of the
nitrogen cycle The nitrogen cycle is the biogeochemical cycle In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physi ...

, it is essential for
agriculture Agriculture is the science, art and practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary Image:Family watching television 1958.jpg, Exercise trends, Increases in sedentary behaviors su ...

and the manufacture of
fertilizer A fertilizer (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Currently, American E ...

. 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 is a mixture In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, comp ...

by
microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes ...
s termed
diazotrophDiazotrophs are bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometres in lengt ...
s that include
bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

such as ''
Azotobacter ''Azotobacter'' is a genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer ...
'' and
archaea Archaea ( ; singular archaeon ) constitute a domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a pa ...

. 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 Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can ...

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 A seed is an embryonic ''Embryonic'' is the twelfth studio album by experimental rock band the Flaming Lips released on October 13, 2009, on Warner Bros. Records, Warner Bros. The band's first double album, it was relea ...

roots. Nitrogen fixation occurs between some
termite Termites are eusocial Eusociality (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its populati ...

s and
fungi A fungus (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full ...

. It occurs naturally in the air by means of production by
lightning Lightning is a naturally occurring electrostatic discharge Electrostatic discharge (ESD) is the sudden flow of electricity Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan ...

. All biological reactions involving the process of nitrogen fixation are catalysed by enzymes called
nitrogenase Nitrogenases are enzymes () that are produced by certain bacteria, such as cyanobacteria (blue-green bacteria). These enzymes are responsible for the Organic redox reaction, reduction of nitrogen (N2) to ammonia (NH3). Nitrogenases are the only ...

s. These enzymes contain
iron Iron () is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behav ...

, often with a second metal, usually
molybdenum Molybdenum is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behav ...

but sometimes
vanadium Vanadium is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science tha ...

.

# History

Biological nitrogen fixation was discovered by
Jean-Baptiste Boussingault Jean-Baptiste Joseph Dieudonné Boussingault (1 February 1801 – 11 May 1887) was a French chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin ''alchemist'') is a scientist A scientist is a pers ...

in 1838. Later, in 1880, the process by which it happens was discovered by German
agronomist Agronomy is the science and technology of producing and using plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical ...
Hermann Hellriegel Hermann Hellriegel (October 21, 1831 – September 24, 1895) was a German agricultural chemist who discovered that leguminous plants assimilate the free nitrogen Nitrogen is the chemical element with the Symbol (chemistry), symbol N and a ...

and and was fully described by Dutch microbiologist
Martinus Beijerinck Martinus Willem Beijerinck (, 16 March 1851 – 1 January 1931) was a Dutch microbiologist A microbiologist (from Greek ) is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge i ...

. "The protracted investigations of the relation of plants to the acquisition of nitrogen begun by Saussure,
Ville ''Ville'' () is the French word nowadays meaning "city A city is a large .Goodall, B. (1987) ''The Penguin Dictionary of Human Geography''. London: Penguin.Kuper, A. and Kuper, J., eds (1996) ''The Social Science Encyclopedia''. 2nd edition ...
, and Gilbert and others culminated in the discover 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 was able to fix atmospheric nitrogen. This was the first species of the
azotobacter ''Azotobacter'' is a genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer ...
genus, so-named by him. It is also the first known
diazotrophDiazotrophs are bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometres in lengt ...
, species that use
diatomic Diatomic molecules are molecule A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position I ...
nitrogen as a step in the complete
nitrogen cycle The nitrogen cycle is the biogeochemical cycle In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physi ...

. 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). These enzymes are responsible for the Organic redox reaction, reduction of nitrogen (N2) to ammonia (NH3). Nitrogenases are the only ...

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 Substitution may refer to: Arts and media *Chord substitution, in music, swapping one chord fo ...

of 16 equivalents of
ATP ATP may refer to: Companies and organizations * Association of Tennis Professionals * American Technical Publishers * ', a Danish pension * Armenia Tree Project * Association for Transpersonal Psychology * ATP architects engineers office * ATP ...

and is accompanied by the co-formation of one equivalent of . The conversion of into ammonia occurs at a
metal cluster A metal (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 mil ...
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). These enzymes are responsible for the Organic redox reaction, reduction o ...

, an abbreviation for the iron-
molybdenum Molybdenum is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behav ...

cofactor. The mechanism proceeds via a series of
protonation In chemistry, protonation (or hydronation) is the adding of a proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutrons ...
and reduction steps wherein the FeMoco
active site Active may refer to: Music * ''Active'' (album), a 1992 album by Casiopea * Active Records Active Records was a record label, record sublabel of RCA Records founded in 1980. The label focused mainly on heavy metal music. The label was disso ...

hydrogenates the substrate. In free-living
diazotrophDiazotrophs are bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometres in lengt ...
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 Amino acids are organic compounds that contain amino (–NH2) and Carboxylic acid, carboxyl (–COOH) functional groups, along with a Substituent, sid ...
through the
glutamine synthetase Glutamine synthetase (GS) () is an enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substr ...

/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 biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

such as
leghemoglobin Leghemoglobin (also leghaemoglobin or legoglobin) is an oxygen-carrying phytoglobin found in the nitrogen-fixing root nodule Root nodules are found on the root In vascular plants, the roots are the plant organ, organs of a plant that are mod ...
.

## 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 Heterocysts or heterocytes are specialized nitrogen fixation, nitrogen-fixing cells formed during nitrogen starvation by some filamentation, filamentous cyanobacteria, such as ''Nostoc punctiforme'', ''Cylindrospermum stagnale'', and ''Anabaena sph ...
. 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 (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

including
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical ...

(e.g. the highly significant '' Trichodesmium'' and '' Cyanothece''), as well as
green sulfur bacteria The green sulfur bacteria (Chlorobiaceae) are a family In , family (from la, familia) is a of people related either by (by recognized birth) or (by marriage or other relationship). The purpose of families is to maintain the well-being ...
, Azotobacteraceae,
rhizobia '' bacteria Rhizobia are diazotrophic bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a ...

and ''
Frankia ''Frankia'' is a genus of nitrogen-fixing bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typic ...
''. Several obligately anaerobic bacteria fix nitrogen including many (but not all) ''
Clostridium ''Clostridium'' is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, virus ...
'' spp. Some
archaea Archaea ( ; singular archaeon ) constitute a domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a pa ...

also fix nitrogen, including several
methanogen Methanogens are microorganisms that produce methane Methane (, ) is a chemical compound with the chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular ...
ic
taxa In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechani ...
, which are significant contributors to nitrogen fixation in oxygen-deficient soils.
Cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical ...

, 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 In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physi ...

of the
biosphere The biosphere (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is ap ...
. In general, cyanobacteria can use various inorganic and organic sources of combined nitrogen, such as
nitrate Nitrate is a polyatomic ion A polyatomic ion, also known as a molecular ion, is a covalently bonded A covalent bond is a chemical bond A chemical bond is a lasting attraction between atoms, ions or molecules that enables the format ...

,
nitrite The nitrite ion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects tha ...

,
ammonium The ammonium cation An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects t ...

,
urea Urea, also known as carbamide, is an organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the prop ...

, or some
amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reactions, a ...

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 one of the four geology, geologic eon (geology), eons of Earth, Earth's history, occurring (4 to 2.5 Billion years, Gya). During the Archean, the Earth's Crust (geology), crust had ...

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'' 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 Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can ...

family In human society A society is a Social group, group of individuals involved in persistent Social relation, social interaction, or a large social group sharing the same spatial or social territory, typically subject to the same Politic ...
Fabaceae The Fabaceae or Leguminosae,International Code of Nomenc ...

— with
taxa In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechani ...
such as
kudzu Kudzu (also called Japanese arrowroot or Chinese arrowroot) is a group of climbing, coiling, and trailing perennial vines native to much of East Asia East Asia is the eastern region of Asia Asia () is Earth's largest and most po ...

,
clover Clover or trefoil are common names for plants of the genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining ( ...

,
soybean The soybean, soy bean, or soya bean (''Glycine max'') is a species of legume A legume () is a plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation ...

,
alfalfa Alfalfa () (''Medicago sativa''), also called lucerne, is a perennial A perennial plant or simply perennial is a plant Plants are predominantly photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the ...

,
lupin ''Lupinus'', commonly known as lupin, lupine, or regionally bluebonnet etc., is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology Biology is the natur ...
,
peanut The peanut, also known as the groundnut, goober (US), pindar (US) or monkey nut (UK), and taxonomically classified as ''Arachis hypogaea'', is a legume A legume () is a plant Plants are predominantly photosynthetic Photosynth ...

and
rooibos Rooibos ( ; ; ''Aspalathus linearis''), meaning "red bush"; is a Broom (shrub), broom-like member of the plant family Fabaceae that grows in South Africa's fynbos. The leaves are used to make a herbal tea that is called by the names: rooibos ( ...

. 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 '' bacteria Rhizobia are diazotrophic bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a ...

bacteria within nodules in their
root systems In vascular plant Vascular plants (from Latin ''vasculum'': duct), also known as Tracheophyta (the tracheophytes , from Greek τραχεῖα ἀρτηρία ''trācheia artēria'' 'windpipe' + φυτά ''phutá'' 'plants'), form a large grou ...

, 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 is a mixture In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, comp ...

. The great majority of legumes have this association, but a few
genera Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circumscribing) and classifying gr ...
(e.g., ''
Styphnolobium ''Styphnolobium'' is a small genus of three or four species of small tree In botany, a tree is a perennial plant with an elongated Plant stem, stem, or trunk (botany), trunk, supporting branches and leaves in most species. In some usages, ...
'') 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 Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining ( ...

. Fixation efficiency in soil is dependent on many factors, including the
legume A legume () is a plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can ...

and air and soil conditions. For example, nitrogen fixation by red clover can range from .

### Non-leguminous

Other nitrogen fixing families include: * Some
cycad :''For the insect, see Cicada.'' Cycads are seed plants that typically have a stout and woody (ligneous) trunk (botany), trunk with a crown (botany), crown of large, hard, stiff, evergreen and (usually) pinnate leaves. The species are dioecious ...

s. * ''
Parasponia ''Trema'' is a genus of about 15 species of evergreen trees closely related to the hackberries (''Celtis''), occurring in subtropical and tropical regions of southern Asia, northern Australasia, Africa, South America, South and Central America, a ...
'', a tropical genus in the family
Cannabaceae Cannabaceae is a small family In human society A society is a Social group, group of individuals involved in persistent Social relation, social interaction, or a large social group sharing the same spatial or social territory, typi ...
, which are able to interact with rhizobia and form nitrogen-fixing nodules *
Actinorhizal plantActinorhizal plants are a group of angiosperms characterized by their ability to form a symbiosis with the nitrogen fixing actinobacteria ''Frankia''. This association leads to the formation of nitrogen-fixing root nodules. Classification Actinorhi ...
s such as
alder Alder is the common name of a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (ta ...

and
bayberry ''Myrica'' is a genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer to a ...

can form nitrogen-fixing nodules, thanks to a symbiotic association with ''
Frankia ''Frankia'' is a genus of nitrogen-fixing bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typic ...
'' bacteria. These plants belong to 25 genera distributed across eight families. The ability to fix nitrogen is present in other families that belong to the
order Order, ORDER or Orders may refer to: * Orderliness Orderliness is a quality that is characterized by a person’s interest in keeping their surroundings and themselves well organized, and is associated with other qualities such as cleanliness a ...
s
Cucurbitales The Cucurbitales are an order of flowering plants, included in the rosid group of dicotyledons. This order mostly belongs to tropical areas, with limited presence in subtropic and temperate regions. The order includes shrubs and trees, together wi ...
,
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 Rosales is an order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, dirt, trash, or waste, and t ...

, which together with the
Fabales The Fabales are an order (biology), order of flowering plants included in the Rosids, rosid group of the eudicots in the APG II system, Angiosperm Phylogeny Group II classification system. In the APG II circumscription, this order includes the f ...
form a 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 In , family (from la, familia) is a of people related either by (by recognized birth) or (by marriage or other relationship). The purpose of families is to maintain the well-being ...
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 File:Cladogram imaginary birds.jpg, Imaginary cladogram. The yellow mask is a plesiomorphy for each living masked species, because it is ancestral. It is also a symplesiomorphy for them. But for the four living species as a whole, it is an apomorph ...
and subsequently lost in most descendants of the original nitrogen-fixing plant; however, it may be that the basic and
physiological Physiology (; ) is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in the real world. ...
requirements were present in an incipient state in the
most recent common ancestor In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanis ...
s of all these plants, but only evolved to full function in some of them. Several nitrogen-fixing symbiotic associations involve
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical ...

(such as ''
Nostoc ''Nostoc'', also known as star jelly Star jelly (also called astromyxin, astral jelly) is a gelatinous substance sometimes found on grass or even on branches of trees. According to folklore, it is deposited on the Earth during meteor showers. St ...

''): * Some lichens such as ''
Lobaria ''Lobaria'' is a genus of lichens commonly known as "lung wort" or "lungmoss" as their physical shape somewhat resembles a lung, and their ecological niche is similar to that of moss. ''Lobaria'' are unusual in that they have a three-part symbio ...

'' and '''' *
Mosquito fern ''Azolla'' (mosquito fern, duckweed fern, fairy moss, water fern) is a genus of seven species of aquatic plant, aquatic ferns in the family Salviniaceae. They are extremely reduced in form and specialized, looking nothing like other typical ferns ...
(''
Azolla ''Azolla'' (mosquito fern, duckweed fern, fairy moss, water fern) is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of ...

'' species) *
Cycad :''For the insect, see Cicada.'' Cycads are seed plants that typically have a stout and woody (ligneous) trunk (botany), trunk with a crown (botany), crown of large, hard, stiff, evergreen and (usually) pinnate leaves. The species are dioecious ...

s *''
Gunnera ''Gunnera'' is the sole genus of herbaceous flowering plants in the family Gunneraceae, which contains 63 species. Some species have extremely large leaves. Species in the genus are variously native to Latin America, Australia, New Zealand, Pa ...
'' *'' Blasia'' (
liverwort The Marchantiophyta are a division of non-vascular land plants commonly referred to as hepatics or liverworts. Like mosses and hornworts, they have a gametophyte A gametophyte () is one of the two alternation of generations, alternating m ...

) *
Hornwort Hornworts are a group of bryophyte Bryophytes are an informal group consisting of three divisions Division or divider may refer to: Mathematics *Division (mathematics) Division is one of the four basic operations of arithmetic, the ways th ...
s

## Eukaryotic nitrogenase engineering

Some scientists are working towards introducing the genes responsible for nitrogen fixation directly into plant DNA. As all known examples of nitrogen fixation takes place in prokaryotes, transferring the functionality to eukaryotes such as plant is a challenge; one team is using yeast as their eukaryotic test organism. A major problem to overcome is the oxygen-sensitivity of the produced enzymes, as well as the energy requirements. Having the process taking place inside of mitochondria or chloroplasts is being considered.

# Industrial processes

The possibility that atmospheric nitrogen reacts with certain chemicals was first observed by Desfosses in 1828. He observed that mixtures of
alkali metal The alkali metals consist of the chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structur ...
oxides and carbon react with nitrogen at high temperatures. With the use of
barium carbonate Barium carbonate is the inorganic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, ...

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. A method for nitrogen fixation was first described by
Henry Cavendish Henry Cavendish FRS FRS may also refer to: Government and politics * Facility Registry System, a centrally managed Environmental Protection Agency database that identifies places of environmental interest in the United States * Family Resourc ...
in 1784 using electric arcs reacting nitrogen and oxygen in air. This method was implemented in the
Birkeland–Eyde process The Birkeland–Eyde process was one of the competing industrial processes in the beginning of Nitrogen fertilizer, nitrogen based fertilizer production. It is a multi-step nitrogen fixation reaction that uses electrical arcs to react atmospheric n ...
. The fixation of nitrogen by lightning is a very similar natural occurring process.

## Frank-Caro process

In 1898
Frank Frank may refer to: People As a name * Frank (given name) * Frank (surname) Groups of people * A member of the medieval Germanic people, the Franks * Crusaders in medieval Middle Eastern history * Levantines (Latin Christians) known as Franco ...

and
Caro Caro may refer to: Places * Caro, Michigan, United States * Caro, Morbihan, France * Çaro, Pyrénées-Atlantiques, France Other uses * Caro (surname), including a list of people with the name * Caro (drink), a drink by Nestlé * Caro (ho ...
developed a way to fix nitrogen in the form of
calcium cyanamide Calcium cyanamide is the inorganic compound with the formula CaCN2. It is the calcium salt of the cyanamide () anion. This chemical is used as fertilizer and is commercially known as nitrolime. It was first synthesized in 1898 by Adolph Frank and ...

. The Frank-Caro and Ostwald processes dominated industrial fixation until the discovery of the
Haber process The Haber process, also called the Haber–Bosch process, is an artificial nitrogen fixation process and is the main industrial procedure for the ammonia production, production of ammonia today. It is named after its inventors, the German chemist ...

in 1909.

## Haber process

The most common ammonia production method is the
Haber process The Haber process, also called the Haber–Bosch process, is an artificial nitrogen fixation process and is the main industrial procedure for the ammonia production, production of ammonia today. It is named after its inventors, the German chemist ...

. The Haber-Bosch nitrogen reduction process for industrial fertilizer production revolutionized modern day technology. Fertilizer production is now the largest source of human-produced fixed nitrogen in the terrestrial
ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...

. Ammonia is a required precursor to
fertilizer A fertilizer (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Currently, American E ...

s,
explosive An explosive (or explosive material) is a reactive substance that contains a great amount of potential energy that can produce an explosion An explosion is a rapid expansion in volume Volume is a expressing the of enclosed by a . ...

s, 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 reducing 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 complex Transition metal dinitrogen complexes are coordination compounds that contain transition metals as ion centers the Nitrogen, dinitrogen molecules (N2) as ligands. Historical background Transition metal complexes of N2 have been studied since ...
es. The first dinitrogen
complex The UCL Faculty of Mathematical and Physical Sciences is one of the 11 constituent faculties of University College London , mottoeng = Let all come who by merit deserve the most reward , established = , type = Public university, Public rese ...
to be reported was ()2+. Some soluble complexes do catalyze nitrogen fixation.

# Lightning

Nitrogen can be fixed by
lightning Lightning is a naturally occurring electrostatic discharge Electrostatic discharge (ESD) is the sudden flow of electricity Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan ...

converting nitrogen gas () and oxygen gas () in the atmosphere into (
nitrogen oxides Nitrogen oxide may refer to a binary compound In materials chemistry, a binary phase or binary compound is a chemical compound containing two different elements. Some binary phase compounds are molecular, e.g. carbon tetrachloride (CCl4). More ty ...
). The molecule is highly stable and nonreactive due to the
triple bond A triple bond in chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they un ...

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 Nitrous acid (molecular formula ) is a weak and monoprotic acid An acid is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings ...

) or (
nitric acid Nitric acid (), also known as ''aqua fortis'' (Latin for "strong water") and spirit of niter, is a highly corrosive mineral acid. The pure compound is colorless, but older samples tend to acquire a yellow cast due to decomposition into nitroge ...

). When these acids seep into the soil, they make ), which is of use to plants.

*
Birkeland–Eyde process The Birkeland–Eyde process was one of the competing industrial processes in the beginning of Nitrogen fertilizer, nitrogen based fertilizer production. It is a multi-step nitrogen fixation reaction that uses electrical arcs to react atmospheric n ...
: an industrial fertiliser production process *
George Washington Carver George Washington Carver ( 1864 – January 5, 1943) was an American Agricultural science, agricultural scientist and inventor who promoted alternative crops to cotton and methods to prevent soil depletion. He was the most prominent black scie ...

: an American botanist *
Denitrification Denitrification is a microbially facilitated process where nitrate (NO3−) is reduced and ultimately produces molecular nitrogen Nitrogen is the chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic tab ...
: an organic process of nitrogen release *
Heterocyst Heterocysts or heterocytes are specialized nitrogen fixation, nitrogen-fixing cells formed during nitrogen starvation by some filamentation, filamentous cyanobacteria, such as ''Nostoc punctiforme'', ''Cylindrospermum stagnale'', and ''Anabaena sph ...
*
Nitrification '' Nitrification'' is the biological oxidation (mild reducing agent) are added to powdered potassium permanganate Potassium permanganate is an inorganic compound with the chemical formula KMnO4 and composed of potassium ion, K+ and permangan ...
: biological production of nitrogen *
Nitrogen cycle The nitrogen cycle is the biogeochemical cycle In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physi ...

: the flow and transformation of nitrogen through the environment * Nitrogen deficiency * Nitrogen fixation package for quantitative measurement of nitrogen fixation by plants *
Nitrogenase Nitrogenases are enzymes () that are produced by certain bacteria, such as cyanobacteria (blue-green bacteria). These enzymes are responsible for the Organic redox reaction, reduction of nitrogen (N2) to ammonia (NH3). Nitrogenases are the only ...

: enzymes used by organisms to fix nitrogen *
Ostwald processThe Ostwald process is a chemical process used for making nitric acid Nitric acid (), also known as ''aqua fortis'' (Latin for "strong water") and spirit of niter, is a highly corrosive mineral acid. The pure compound is colorless, but older sam ...
: a chemical process for making nitric acid () * Push–pull technology: the use of both repellent and attractive organisms in agriculture *Carbon fixation