Editing Nitrogen fixation

{{Short description|Conversion of molecular nitrogen into biologically accessible nitrogen compounds}}
{{cs1 config|name-list-style=vanc|display-authors=6}}
{{Use dmy dates|date=March 2021}}

'''Nitrogen fixation''' is a [[chemical process]] by which molecular [[dinitrogen]] ({{chem|N|2}}) is converted into [[ammonia]] ({{chem|NH|3}}).<ref name=Rees>{{cite journal |doi=10.1021/acs.chemrev.0c00067 |title=Structural Enzymology of Nitrogenase Enzymes |date=2020 |last1=Einsle |first1=Oliver |last2=Rees |first2=Douglas C. |journal=Chemical Reviews |volume=120 |issue=12 |pages=4969–5004 |pmid=32538623 |pmc=8606229 }}</ref> It occurs both biologically and  [[abiological nitrogen fixation|abiologically]] in [[chemical industry|chemical industries]].  Biological nitrogen fixation or ''[[diazotroph]]y'' is [[catalyze]]d by [[enzyme]]s called [[nitrogenase]]s.<ref>{{Cite journal| vauthors = Burris RH, Wilson PW |date=June 1945|title=Biological Nitrogen Fixation |journal=Annual Review of Biochemistry|language=en|volume=14|issue=1|pages=685–708|doi=10.1146/annurev.bi.14.070145.003345|issn=0066-4154}}</ref> These enzyme complexes are encoded by the [[Nif gene|''Nif'' gene]]s (or ''Nif'' [[homolog]]s) and contain [[iron]], often with a second metal (usually [[molybdenum]], but sometimes [[vanadium]]).<ref name="Wagner">{{cite journal | vauthors = Wagner SC | date = 2011 | title = Biological Nitrogen Fixation | journal = Nature Education Knowledge | volume = 3 | issue = 10 | page = 15 | url = https://www.nature.com/scitable/knowledge/library/biological-nitrogen-fixation-23570419 | access-date = 29 January 2019 | archive-url = https://web.archive.org/web/20180913194741/http://www.nature.com/scitable/knowledge/library/biological-nitrogen-fixation-23570419 | archive-date = 13 September 2018 | url-status = live }}</ref>

Some nitrogen-fixing bacteria have [[symbiotic]] relationships with [[plant]]s, especially [[legume]]s, [[moss]]es and [[aquatic fern]]s such as ''[[Azolla]]''.<ref name="Zahran">{{cite journal | vauthors = Zahran HH | title = Rhizobium-legume symbiosis and nitrogen fixation under severe conditions and in an arid climate | journal = Microbiology and Molecular Biology Reviews | volume = 63 | issue = 4 | pages = 968–89, table of contents | date = December 1999 | pmid = 10585971 | pmc = 98982 | doi = 10.1128/MMBR.63.4.968-989.1999 }}</ref> Looser non-symbiotic relationships between diazotrophs and plants are often referred to as associative, as seen in nitrogen fixation on [[rice]] roots. Nitrogen fixation occurs between some [[termite]]s and [[fungus|fungi]].<ref name="Sapountzis">{{cite journal | vauthors = Sapountzis P, de Verges J, Rousk K, Cilliers M, Vorster BJ, Poulsen M | title = Potential for Nitrogen Fixation in the Fungus-Growing Termite Symbiosis | journal = Frontiers in Microbiology | volume = 7 | pages = 1993 | date = 2016 | pmid = 28018322 | pmc = 5156715 | doi = 10.3389/fmicb.2016.01993 | doi-access = free }}</ref> It occurs naturally in the air by means of [[NOx|NO<sub>x</sub>]] production by [[lightning]].<ref>{{cite book| vauthors = Slosson E |title=Creative Chemistry|url=https://archive.org/details/creativechemist00slosgoog|year=1919|publisher=The Century Co.|location=New York, NY|pages=[https://archive.org/details/creativechemist00slosgoog/page/n42 19]–37}}</ref><ref>{{cite journal | vauthors = Hill RD, Rinker RG, Wilson HD |date=1979|title=Atmospheric Nitrogen Fixation by Lightning|journal=J. Atmos. Sci.|volume=37|issue=1|pages=179–192|doi=10.1175/1520-0469(1980)037<0179:ANFBL>2.0.CO;2 |bibcode=1980JAtS...37..179H|doi-access=free}}</ref>

Fixed nitrogen is essential to [[life]] on [[Earth]]. All nitrogen-containing [[organic compounds]] such as [[DNA]] and [[protein]]s contain nitrogen. Industrial nitrogen fixation underpins the manufacture of all nitrogenous [[industrial product]]s, which include [[fertilizer]]s, [[pharmaceutical]]s, [[textile]]s, [[dye]]s and [[explosive]]s.

== History ==
[[File:Nitrogen Cycle.svg|thumb|320px|right|Schematic representation of the [[nitrogen cycle]]. Abiotic nitrogen fixation has been omitted.]]
Biological nitrogen fixation was discovered by [[Jean-Baptiste Boussingault]] in 1838.<ref>{{cite journal |last1=Boussingault |title=Recherches chimiques sur la vegetation, entreprises dans le but d'examiner si les plantes prennent de l'azote à l'atmosphere |journal=Annales de Chimie et de Physique |date=1838 |volume=67 |pages=5–54 |url=https://babel.hathitrust.org/cgi/pt?id=iau.31858046218297&view=1up&seq=9&skin=2021 |series=2nd series |trans-title=Chemical investigations into vegetation, undertaken with the goal of examining whether plants take up nitrogen in the atmosphere |language=French}} and [https://babel.hathitrust.org/cgi/pt?id=hvd.hx3dx7&view=1up&seq=357&skin=2021 '''69''': 353–367].</ref><ref>{{cite book | vauthors = Smil V |year=2001|title=Enriching the Earth|publisher=Massachusetts Institute of Technology}}</ref> Later, in 1880, the process by which it happens was discovered by German [[Agronomy|agronomist]] [[Hermann Hellriegel]] and {{interlanguage link|Hermann Wilfarth|de}}<ref name=Hellriegel>{{cite book| vauthors = Hellriegel H, Wilfarth H |year=1888|title=Untersuchungen über die Stickstoffnahrung der Gramineen und Leguminosen|trans-title=Studies on the nitrogen intake of Gramineae and Leguminosae|publisher=Buchdruckerei der "Post" Kayssler & Co.|location=Berlin, Germany|url= https://www.biodiversitylibrary.org/item/69506#page/7/mode/1up|language=German}}</ref> and was fully described by Dutch microbiologist [[Martinus Beijerinck]].<ref name="Beijerinck">{{cite journal| vauthors = Beijerinck MW |year=1901|title=Über oligonitrophile Mikroben|trans-title=On oligonitrophilic microbes|journal=Centralblatt für Bakteriologie, Parasitenkunde, Infektionskrankheiten und Hygiene|volume=7|issue=16|pages=561–582|url=https://books.google.com/books?id=tcBFAQAAMAAJ&pg=PA561|language=German}}</ref>

"The protracted investigations of the relation of plants to the acquisition of nitrogen begun by [[Nicolas de Saussure|de Saussure]], [[Georges Ville|Ville]], [[John Bennet Lawes|Lawes]], [[Joseph Henry Gilbert|Gilbert]] and others, and culminated in the discovery of symbiotic fixation by Hellriegel and Wilfarth in 1887."<ref>Howard S. Reed (1942) ''A Short History of Plant Science'', page 230, Chronic Publishing</ref>

"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 1886–1888 would open a new era of [[soil science]]."<ref>[[Margaret Rossiter]] (1975) ''The Emergence of Agricultural Science'', page 146, [[Yale University Press]]</ref>

In 1901, Beijerinck showed that ''[[Azotobacter chroococcum]]'' was able to fix atmospheric nitrogen. This was the first species of the ''[[azotobacter]]'' genus, so-named by him. It is also the first known [[diazotroph]], species that use [[diatomic molecule|diatomic]] nitrogen as a step in the complete [[nitrogen cycle]].<ref>{{Cite journal |last1=Al-Baldawy |first1=Muneer Saeed M. |last2=Matloob |first2=Ahed A. A. H. |last3=Almammory |first3=Mohammed K. N. |date=2023-11-01 |title=The Importance of Nitrogen-Fixing Bacteria Azotobacter chroococcum in Biological Control to Root Rot Pathogens (Review) |journal=IOP Conference Series: Earth and Environmental Science |volume=1259 |issue=1 |pages=012110 |doi=10.1088/1755-1315/1259/1/012110 |issn=1755-1307|doi-access=free |bibcode=2023E&ES.1259a2110A }}</ref>

== Biological ==
Biological nitrogen fixation (BNF) occurs when atmospheric nitrogen is converted to ammonia by a [[nitrogenase]] enzyme.<ref name=Rees/> The overall reaction for BNF is:

{{chem2|N2 + 16ATP + 16H2O + 8e- + 8H+}} → {{chem2|2NH3 +H2 + 16ADP + 16P_{i}|}}

The process is coupled to the [[hydrolysis]] of 16 equivalents of [[adenosine triphosphate|ATP]] and is accompanied by the co-formation of one equivalent of {{chem|H|2}}. The conversion of {{chem|N|2}} into ammonia occurs at a [[metal cluster]] called [[FeMoco]], an abbreviation for the iron-[[molybdenum]] cofactor. The mechanism proceeds via a series of [[protonation]] and reduction steps wherein the FeMoco [[active site]] [[hydrogenate]]s the {{chem|N|2}} substrate.<ref name=Rees/> In free-living [[diazotroph]]s, nitrogenase-generated ammonia is assimilated into [[glutamate]] through the [[glutamine synthetase]]/glutamate synthase pathway. The microbial [[nif gene]]s required for nitrogen fixation are widely distributed in diverse environments.<ref>{{cite journal | vauthors = Gaby JC, Buckley DH | title = A global census of nitrogenase diversity | journal = Environmental Microbiology | volume = 13 | issue = 7 | pages = 1790–9 | date = July 2011 | pmid = 21535343 | doi = 10.1111/j.1462-2920.2011.02488.x | bibcode = 2011EnvMi..13.1790G }}</ref>

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 organism|anaerobic]] conditions, respiring to draw down oxygen levels, or binding the oxygen with a [[protein]] such as [[leghemoglobin]].<ref name="postgate">{{cite book | vauthors = Postgate J |year=1998 |title= Nitrogen Fixation|edition= 3rd |publisher=Cambridge University Press|location= Cambridge}}</ref><ref>{{cite journal | vauthors = Streicher SL, Gurney EG, Valentine RC | title = The nitrogen fixation genes | journal = Nature | volume = 239 | issue = 5374 | pages = 495–9 | date = October 1972 | pmid = 4563018 | doi = 10.1038/239495a0 | bibcode = 1972Natur.239..495S | s2cid = 4225250 }}</ref>

=== Importance of nitrogen ===
{{biogeochemical cycle sidebar|nutrient}}
Atmospheric nitrogen cannot be metabolized by most organisms,<ref>{{cite book | vauthors = Delwiche CC | chapter = Cycling of Elements in the Biosphere|date=1983 | title = Inorganic Plant Nutrition|pages=212–238| veditors = Läuchli A, Bieleski RL |series=Encyclopedia of Plant Physiology|place=Berlin, Heidelberg|publisher=Springer|language=en|doi=10.1007/978-3-642-68885-0_8|isbn=978-3-642-68885-0 }}</ref> because its triple covalent bond is very strong. Most take up fixed nitrogen from various sources. 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,<ref name="REDFIELD 1958 230A–221">{{Cite journal| vauthors = Redfield AC |title=The Biological Control of Chemical Factors in the Environment|date=1958|url=https://www.jstor.org/stable/27827150|journal=American Scientist|volume=46|issue=3|pages=230A–221|jstor=27827150|issn=0003-0996}}</ref> who determined the stoichiometric relationship between C:N:P atoms, The Redfield Ratio, to be 106:16:1.<ref name="REDFIELD 1958 230A–221"/>

=== Nitrogenase ===
{{Main|Nitrogenase}}
The protein complex nitrogenase is responsible for [[Catalysis|catalyzing]] the reduction of nitrogen gas (N<sub>2</sub>) to ammonia (NH<sub>3</sub>).<ref>{{cite journal |doi=10.1021/acs.chemrev.9b00556 |title=Reduction of Substrates by Nitrogenases |date=2020 |last1=Seefeldt |first1=Lance C. |last2=Yang |first2=Zhi-Yong |last3=Lukoyanov |first3=Dmitriy A. |last4=Harris |first4=Derek F. |last5=Dean |first5=Dennis R. |last6=Raugei |first6=Simone |last7=Hoffman |first7=Brian M. |journal=Chemical Reviews |volume=120 |issue=12 |pages=5082–5106 |pmid=32176472 |pmc=7703680 }}</ref><ref>{{cite journal |doi=10.1002/1873-3468.14534 |title=Biological nitrogen fixation in theory, practice, and reality: A perspective on the molybdenum nitrogenase system |date=2023 |last1=Threatt |first1=Stephanie D. |last2=Rees |first2=Douglas C. |journal=FEBS Letters |volume=597 |issue=1 |pages=45–58 |pmid=36344435 |pmc=10100503 }}</ref> In [[cyanobacteria]], this [[enzyme]] system is housed in a specialized cell called the [[heterocyst]].<ref>{{cite journal | vauthors = Peterson RB, Wolk CP | title = High recovery of nitrogenase activity and of Fe-labeled nitrogenase in heterocysts isolated from Anabaena variabilis | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 75 | issue = 12 | pages = 6271–6275 | date = December 1978 | pmid = 16592599 | pmc = 393163 | doi = 10.1073/pnas.75.12.6271 | doi-access = free | bibcode = 1978PNAS...75.6271P }}</ref> 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).<ref>{{cite journal | vauthors = Beversdorf LJ, Miller TR, McMahon KD | title = The role of nitrogen fixation in cyanobacterial bloom toxicity in a temperate, eutrophic lake | journal = PLOS ONE | volume = 8 | issue = 2 | pages = e56103 | date = 2013-02-06 | pmid = 23405255 | pmc = 3566065 | doi = 10.1371/journal.pone.0056103 | doi-access = free | bibcode = 2013PLoSO...856103B }}</ref><ref>{{Cite journal| vauthors = Gallon JR |date=2001-03-01|title=N2 fixation in phototrophs: adaptation to a specialized way of life |journal=Plant and Soil|language=en|volume=230|issue=1|pages=39–48 |doi=10.1023/A:1004640219659|bibcode=2001PlSoi.230...39G |s2cid=22893775|issn=1573-5036}}</ref><ref>{{cite journal | vauthors = Paerl H | title = The cyanobacterial nitrogen fixation paradox in natural waters | journal = F1000Research | volume = 6 | pages = 244 | date = 2017-03-09 | pmid = 28357051 | pmc = 5345769 | doi = 10.12688/f1000research.10603.1 | doi-access = free }}</ref> Additionally, the combined concentrations of both ammonium and nitrate are thought to inhibit N<sub>Fix</sub>, specifically when intracellular concentrations of 2-[[Ketoglutaric acid|oxoglutarate]] (2-OG) exceed a critical threshold.<ref>{{cite journal | vauthors = Li JH, Laurent S, Konde V, Bédu S, Zhang CC | title = An increase in the level of 2-oxoglutarate promotes heterocyst development in the cyanobacterium Anabaena sp. strain PCC 7120 | journal = Microbiology | volume = 149 | issue = Pt 11 | pages = 3257–3263 | date = November 2003 | pmid = 14600238 | doi = 10.1099/mic.0.26462-0 | doi-access = free }}</ref> The specialized heterocyst cell is necessary for the performance of nitrogenase as a result of its sensitivity to ambient oxygen.<ref>{{cite book | vauthors = Wolk CP, Ernst A, Elhai J | chapter = Heterocyst Metabolism and Development|date=1994 | title = The Molecular Biology of Cyanobacteria |pages=769–823| veditors = Bryant DA |series=Advances in Photosynthesis|place=Dordrecht|publisher=Springer Netherlands|language=en|doi=10.1007/978-94-011-0227-8_27|isbn=978-94-011-0227-8 }}</ref>

Nitrogenase consist of two proteins, a catalytic iron-dependent protein, commonly referred to as MoFe protein and a reducing iron-only protein (Fe protein). Three iron-dependent proteins are known: [[molybdenum]]-dependent, [[vanadium]]-dependent, and [[iron]]-only, with all three nitrogenase protein variations containing an iron protein component. Molybdenum-dependent nitrogenase is most common.<ref name=Rees/> The different types of nitrogenase can be determined by the specific iron protein component.<ref>{{cite book | vauthors = Schneider K, Müller A | title = Catalysts for Nitrogen Fixation| chapter = Iron-Only Nitrogenase: Exceptional Catalytic, Structural and Spectroscopic Features|date=2004 |pages=281–307| veditors = Smith BE, Richards RL, Newton WE |series=Nitrogen Fixation: Origins, Applications, and Research Progress|place=Dordrecht|publisher=Springer Netherlands|language=en|doi=10.1007/978-1-4020-3611-8_11|isbn=978-1-4020-3611-8 }}</ref> Nitrogenase is highly conserved. [[Gene expression]] through [[DNA sequencing]] can distinguish which protein complex is present in the microorganism and potentially being expressed. Most frequently, the [[Nif gene|''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.<ref>{{Cite journal| vauthors = Knoche KL, Aoyama E, Hasan K, Minteer SD |date=2017|title=Role of Nitrogenase and Ferredoxin in the Mechanism of Bioelectrocatalytic Nitrogen Fixation by the Cyanobacteria Anabaena variabilis SA-1 Mutant Immobilized on Indium Tin Oxide (ITO) Electrodes|url=https://www.cheric.org/research/tech/periodicals/view.php?seq=1531452|journal=Electrochimica Acta|language=ko|volume=232|pages=396–403|doi=10.1016/j.electacta.2017.02.148|url-access=subscription}}</ref> In studying the ecology and evolution of [[Diazotroph|nitrogen-fixing bacteria]], the ''nifH'' gene is the [[biomarker]] most widely used.<ref>{{cite journal | vauthors = Raymond J, Siefert JL, Staples CR, Blankenship RE | title = The natural history of nitrogen fixation | journal = Molecular Biology and Evolution | volume = 21 | issue = 3 | pages = 541–554 | date = March 2004 | pmid = 14694078 | doi = 10.1093/molbev/msh047 | doi-access = free | author4-link = Robert E. Blankenship }}</ref> ''nif''H has two similar genes ''anf''H and vnfH that also encode for the nitrogenase reductase component of the nitrogenase complex.<ref>{{cite journal | vauthors = Schüddekopf K, Hennecke S, Liese U, Kutsche M, Klipp W | title = Characterization of anf genes specific for the alternative nitrogenase and identification of nif genes required for both nitrogenases in Rhodobacter capsulatus | journal = Molecular Microbiology | volume = 8 | issue = 4 | pages = 673–684 | date = May 1993 | pmid = 8332060 | doi = 10.1111/j.1365-2958.1993.tb01611.x | s2cid = 42057860 }}</ref>

=== Evolution of nitrogenase ===
Nitrogenase is thought to have evolved sometime between 1.5-2.2 billion years ago (Ga),<ref>{{Cite journal |last1=Garcia |first1=Amanda K. |last2=McShea |first2=Hanon |last3=Kolaczkowski |first3=Bryan |last4=Kaçar |first4=Betül |date=May 2020 |title=Reconstructing the evolutionary history of nitrogenases: Evidence for ancestral molybdenum-cofactor utilization |journal=Geobiology |language=en |volume=18 |issue=3 |pages=394–411 |doi=10.1111/gbi.12381 |issn=1472-4677 |pmc=7216921 |pmid=32065506|bibcode=2020Gbio...18..394G }}</ref><ref>{{Cite journal |last1=Boyd |first1=E. S. |last2=Anbar |first2=A. D. |last3=Miller |first3=S. |last4=Hamilton |first4=T. L. |last5=Lavin |first5=M. |last6=Peters |first6=J. W. |date=May 2011 |title=A late methanogen origin for molybdenum-dependent nitrogenase |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1472-4669.2011.00278.x |journal=Geobiology |language=en |volume=9 |issue=3 |pages=221–232 |doi=10.1111/j.1472-4669.2011.00278.x |pmid=21504537 |bibcode=2011Gbio....9..221B |issn=1472-4677|url-access=subscription }}</ref> although some isotopic support showing nitrogenase evolution as early as around 3.2 Ga.<ref>{{Cite journal |last1=Stüeken |first1=Eva E. |last2=Buick |first2=Roger |last3=Guy |first3=Bradley M. |last4=Koehler |first4=Matthew C. |date=April 2015 |title=Isotopic evidence for biological nitrogen fixation by molybdenum-nitrogenase from 3.2 Gyr |url=https://www.nature.com/articles/nature14180 |journal=Nature |language=en |volume=520 |issue=7549 |pages=666–669 |doi=10.1038/nature14180 |pmid=25686600 |bibcode=2015Natur.520..666S |issn=0028-0836|url-access=subscription }}</ref> Nitrogenase appears to have evolved from [[maturase]]-like proteins, although the function of the preceding protein is currently unknown.<ref>{{Cite journal |last1=Garcia |first1=Amanda K |last2=Kolaczkowski |first2=Bryan |last3=Kaçar |first3=Betül |date=2022-03-02 |editor-last=Archibald |editor-first=John |title=Reconstruction of Nitrogenase Predecessors Suggests Origin from Maturase-Like Proteins |journal=Genome Biology and Evolution |language=en |volume=14 |issue=3 |doi=10.1093/gbe/evac031 |issn=1759-6653 |pmc=8890362 |pmid=35179578}}</ref>

Nitrogenase has three different forms (''Nif, Anf, and Vnf'') that correspond with the metal found in the active site of the protein (molybdenum, iron, and vanadium respectively).<ref>{{Cite journal |last=Eady |first=Robert R. |date=1996-01-01 |title=Structure−Function Relationships of Alternative Nitrogenases |url=https://pubs.acs.org/doi/10.1021/cr950057h |journal=Chemical Reviews |language=en |volume=96 |issue=7 |pages=3013–3030 |doi=10.1021/cr950057h |pmid=11848850 |issn=0009-2665|url-access=subscription }}</ref> Marine metal abundances over Earth's geologic timeline are thought to have driven the relative abundance of which form of nitrogenase was most common.<ref>{{Cite journal |last1=Anbar |first1=A. D. |last2=Knoll |first2=A. H. |date=2002-08-16 |title=Proterozoic Ocean Chemistry and Evolution: A Bioinorganic Bridge? |url=https://www.science.org/doi/10.1126/science.1069651 |journal=Science |language=en |volume=297 |issue=5584 |pages=1137–1142 |doi=10.1126/science.1069651 |pmid=12183619 |bibcode=2002Sci...297.1137A |issn=0036-8075}}</ref> Currently, there is no conclusive agreement on which form of nitrogenase arose first.

===Microorganisms===
{{Main|Diazotroph}}
Diazotrophs are widespread within domain [[Bacteria]] including [[cyanobacteria]] (e.g. the highly significant ''[[Trichodesmium]]'' and ''[[Cyanothece]]''), [[green sulfur bacteria]], [[purple sulfur bacteria]], [[Azotobacteraceae]], [[rhizobia]] and ''[[Frankia]].''<ref>{{Cite web|url=https://scitechdaily.com/nitrogen-inputs-in-the-ancient-ocean-underappreciated-bacteria-step-into-the-spotlight/|title=Nitrogen Inputs in the Ancient Ocean: Underappreciated Bacteria Step Into the Spotlight|first=Max Planck|last=Institute|date=6 August 2021}}</ref><ref name="Mus-2016">{{cite journal | vauthors = Mus F, Crook MB, Garcia K, Garcia Costas A, Geddes BA, Kouri ED, Paramasivan P, Ryu MH, Oldroyd GE, Poole PS, Udvardi MK, Voigt CA, Ané JM, Peters JW | title = Symbiotic Nitrogen Fixation and the Challenges to Its Extension to Nonlegumes | journal = Applied and Environmental Microbiology | volume = 82 | issue = 13 | pages = 3698–3710 | date = July 2016 | pmid = 27084023 | pmc = 4907175 | doi = 10.1128/AEM.01055-16 | bibcode = 2016ApEnM..82.3698M | veditors = Kelly RM }}</ref> Several obligately anaerobic bacteria fix nitrogen including many (but not all) ''[[Clostridium]]'' spp. Some [[archaea]] such as ''[[Methanosarcina acetivorans]]'' also fix nitrogen,<ref>{{cite journal | vauthors = Dhamad AE, Lessner DJ | title = A CRISPRi-dCas9 System for Archaea and Its Use To Examine Gene Function during Nitrogen Fixation by Methanosarcina acetivorans | journal = Applied and Environmental Microbiology | volume = 86 | issue = 21 | pages = e01402–20 | date = October 2020 | pmid = 32826220 | pmc = 7580536 | doi = 10.1128/AEM.01402-20 | bibcode = 2020ApEnM..86E1402D | veditors = Atomi H }}</ref> and several other [[methanogen]]ic [[taxa]], are significant contributors to nitrogen fixation in oxygen-deficient soils.<ref>{{cite journal | vauthors = Bae HS, Morrison E, Chanton JP, Ogram A | title = Methanogens Are Major Contributors to Nitrogen Fixation in Soils of the Florida Everglades | journal = Applied and Environmental Microbiology | volume = 84 | issue = 7 | pages = e02222–17 | date = April 2018 | pmid = 29374038 | pmc = 5861825 | doi = 10.1128/AEM.02222-17 | bibcode = 2018ApEnM..84E2222B }}</ref>

[[Cyanobacteria]], commonly known as blue-green algae, inhabit nearly all illuminated environments on Earth and play key roles in the carbon and [[nitrogen cycle]] of the [[biosphere]]. In general, cyanobacteria can use various inorganic and organic sources of combined nitrogen, such as [[nitrate]], [[nitrite]], [[ammonium]], [[urea]], or some [[amino acid]]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]] eon.<ref>{{cite journal | vauthors = Latysheva N, Junker VL, Palmer WJ, Codd GA, Barker D | title = The evolution of nitrogen fixation in cyanobacteria | journal = Bioinformatics | volume = 28 | issue = 5 | pages = 603–606 | date = March 2012 | pmid = 22238262 | doi = 10.1093/bioinformatics/bts008 | doi-access = free }}</ref> Nitrogen fixation not only naturally occurs in soils but also aquatic systems, including both freshwater and marine.<ref name="Pierella Karlusich-2021">{{cite journal | vauthors = Pierella Karlusich JJ, Pelletier E, Lombard F, Carsique M, Dvorak E, Colin S, Picheral M, Cornejo-Castillo FM, Acinas SG, Pepperkok R, Karsenti E, de Vargas C, Wincker P, Bowler C, Foster RA | title = Global distribution patterns of marine nitrogen-fixers by imaging and molecular methods | journal = Nature Communications | volume = 12 | issue = 1 | pages = 4160 | date = July 2021 | pmid = 34230473 | pmc = 8260585 | doi = 10.1038/s41467-021-24299-y | bibcode = 2021NatCo..12.4160P }}</ref><ref>{{Cite journal| vauthors = Ash C |date=2021-08-13| veditors = Ash C, Smith J |title=Some light on diazotrophs |journal=Science|language=en|volume=373|issue=6556|pages=755.7–756|doi=10.1126/science.373.6556.755-g|bibcode=2021Sci...373..755A|s2cid=238709371|issn=0036-8075}}</ref> Indeed, the amount of nitrogen fixed in the ocean is at least as much as that on land.<ref>{{cite journal | vauthors = Kuypers MM, Marchant HK, Kartal B | title = The microbial nitrogen-cycling network | journal = Nature Reviews. Microbiology | volume = 16 | issue = 5 | pages = 263–276 | date = May 2018 | pmid = 29398704 | doi = 10.1038/nrmicro.2018.9 | hdl-access = free | s2cid = 3948918 | hdl = 21.11116/0000-0003-B828-1 }}</ref> 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.<ref>{{cite journal | vauthors = Bergman B, Sandh G, Lin S, Larsson J, Carpenter EJ | title = Trichodesmium--a widespread marine cyanobacterium with unusual nitrogen fixation properties | journal = FEMS Microbiology Reviews | volume = 37 | issue = 3 | pages = 286–302 | date = May 2013 | pmid = 22928644 | pmc = 3655545 | doi = 10.1111/j.1574-6976.2012.00352.x }}</ref> Marine surface lichens and non-photosynthetic bacteria belonging in Proteobacteria and Planctomycetes fixate significant atmospheric nitrogen.<ref>{{Cite web|url=https://www.sciencedaily.com/releases/2018/06/180611133453.htm|title=Large-scale study indicates novel, abundant nitrogen-fixing microbes in surface ocean|website=ScienceDaily|access-date=8 June 2019|archive-url=https://web.archive.org/web/20190608024940/https://www.sciencedaily.com/releases/2018/06/180611133453.htm|archive-date=8 June 2019|url-status=live}}</ref> Species of nitrogen fixing cyanobacteria in fresh waters include: ''[[Aphanizomenon]]'' and [[Dolichospermum flosaquae|''Dolichospermum'']] (previously Anabaena).<ref>{{Cite journal| vauthors = Rolff C, Almesjö L, Elmgren R |date=2007-03-05|title=Nitrogen fixation and abundance of the diazotrophic cyanobacterium Aphanizomenon sp. in the Baltic Proper|url=http://www.int-res.com/abstracts/meps/v332/p107-118/|journal=Marine Ecology Progress Series|language=en|volume=332|pages=107–118|doi=10.3354/meps332107 |bibcode=2007MEPS..332..107R|doi-access=free}}</ref> Such species have specialized cells called [[Heterocyst|heterocytes]], in which nitrogen fixation occurs via the nitrogenase enzyme.<ref>{{Cite journal| vauthors = Carmichael WW |date=12 Oct 2001|title=Health Effects of Toxin-Producing Cyanobacteria: "The CyanoHABs" |journal=Human and Ecological Risk Assessment|language=en|volume=7|issue=5|pages=1393–1407|doi=10.1080/20018091095087|bibcode=2001HERA....7.1393C |s2cid=83939897|issn=1080-7039}}</ref><ref>{{cite journal | vauthors = Bothe H, Schmitz O, Yates MG, Newton WE | title = Nitrogen fixation and hydrogen metabolism in cyanobacteria | journal = Microbiology and Molecular Biology Reviews | volume = 74 | issue = 4 | pages = 529–551 | date = December 2010 | pmid = 21119016 | pmc = 3008169 | doi = 10.1128/MMBR.00033-10 }}</ref>

=== Algae ===
One type of [[organelle]], originating from [[cyanobacteria]]l [[endosymbiont]]s called [[UCYN-A]]2,<ref name="Thompson_2012" /><ref>{{Cite journal |last1=Thompson |first1=Anne |last2=Carter |first2=Brandon J. |last3=Turk-Kubo |first3=Kendra |last4=Malfatti |first4=Francesca |last5=Azam |first5=Farooq |last6=Zehr |first6=Jonathan P. |date=October 2014 |title=Genetic diversity of the unicellular nitrogen-fixing cyanobacteria UCYN-A and its prymnesiophyte host: UCYN-A genetic diversity |url=https://cloudfront.escholarship.org/dist/prd/content/qt4687q7k8/qt4687q7k8.pdf?t=nx0365 |journal=Environmental Microbiology |language=en |volume=16 |issue=10 |pages=3238–3249 |doi=10.1111/1462-2920.12490 |pmid=24761991 |s2cid=24822220}}</ref> can turn nitrogen gas into a biologically available form. This [[nitroplast]] was discovered in [[algae]], particularly in the marine algae [[Braarudosphaera bigelowii]].<ref>{{Cite journal |last=Wong |first=Carissa |date=2024-04-11 |title=Scientists discover first algae that can fix nitrogen — thanks to a tiny cell structure |url=https://www.nature.com/articles/d41586-024-01046-z |journal=Nature |volume=628 |issue=8009 |page=702 |language=en |doi=10.1038/d41586-024-01046-z|pmid=38605201 |bibcode=2024Natur.628..702W |url-access=subscription }}</ref>

[[Diatom]]s in the family ''Rhopalodiaceae'' also possess [[cyanobacteria]]l [[endosymbiont]]s called spheroid bodies or diazoplasts.<ref>{{cite journal |last1=Moulin |first1=Solène L. Y. |last2=Frail |first2=Sarah |last3=Braukmann |first3=Thomas |last4=Doenier |first4=Jon |last5=Steele-Ogus |first5=Melissa |last6=Marks |first6=Jane C. |last7=Mills |first7=Matthew M. |last8=Yeh |first8=Ellen |date=15 April 2024 |title=The endosymbiont of Epithemia clementina is specialized for nitrogen fixation within a photosynthetic eukaryote |journal=ISME Communications |volume=4 |pages=ycae055 |doi=10.1093/ismeco/ycae055 |pmc=11070190 |pmid=38707843 |doi-access=free}}</ref> These endosymbionts have lost photosynthetic properties, but have kept the ability to perform nitrogen fixation, allowing these diatoms to fix atmospheric nitrogen.<ref>{{Cite journal |last1=Schvarcz |first1=Christopher R. |last2=Wilson |first2=Samuel T. |last3=Caffin |first3=Mathieu |last4=Stancheva |first4=Rosalina |last5=Li |first5=Qian |last6=Turk-Kubo |first6=Kendra A. |last7=White |first7=Angelicque E. |last8=Karl |first8=David M. |last9=Zehr |first9=Jonathan P. |last10=Steward |first10=Grieg F. |date=2022-02-10 |title=Overlooked and widespread pennate diatom-diazotroph symbioses in the sea |journal=Nature Communications |language=en |volume=13 |issue=1 |pages=799 |bibcode=2022NatCo..13..799S |doi=10.1038/s41467-022-28065-6 |issn=2041-1723 |pmc=8831587 |pmid=35145076}}</ref><ref>{{Cite journal |pmc=4128115 |year=2014 |last1=Nakayama |first1=T. |title=Complete genome of a nonphotosynthetic cyanobacterium in a diatom reveals recent adaptations to an intracellular lifestyle |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=111 |issue=31 |pages=11407–11412 |last2=Kamikawa |first2=R. |last3=Tanifuji |first3=G. |last4=Kashiyama |first4=Y. |last5=Ohkouchi |first5=N. |last6=Archibald |first6=J. M. |last7=Inagaki |first7=Y. |pmid=25049384 |doi=10.1073/pnas.1405222111 |bibcode=2014PNAS..11111407N |doi-access=free}}</ref> Other diatoms in symbiosis with nitrogen-fixing cyanobacteria are among the genera ''Hemiaulus'', ''Rhizosolenia'' and ''Chaetoceros''.<ref>{{Cite journal |last1=Pierella Karlusich |first1=Juan José |last2=Pelletier |first2=Eric |last3=Lombard |first3=Fabien |last4=Carsique |first4=Madeline |last5=Dvorak |first5=Etienne |last6=Colin |first6=Sébastien |last7=Picheral |first7=Marc |last8=Cornejo-Castillo |first8=Francisco M. |last9=Acinas |first9=Silvia G. |last10=Pepperkok |first10=Rainer |last11=Karsenti |first11=Eric |date=2021-07-06 |title=Global distribution patterns of marine nitrogen-fixers by imaging and molecular methods |journal=Nature Communications |language=en |volume=12 |issue=1 |pages=4160 |doi=10.1038/s41467-021-24299-y |issn=2041-1723 |pmc=8260585 |pmid=34230473 |bibcode=2021NatCo..12.4160P}}</ref>

===Root nodule symbioses===
{{Main|Root nodule}}

====Legume family====
[[Image:Root nodules on fava bean plant.jpg|thumb|right|Nodules are visible on this broad bean root]]
Plants that contribute to nitrogen fixation include those of the [[legume]] [[family (biology)|family]]—[[Fabaceae]]— with [[taxa]] such as [[kudzu]], [[clover]], [[soybean]], [[alfalfa]], [[lupin]], [[peanut]] and [[rooibos]].<ref name="Mus-2016" /> They contain [[symbiosis|symbiotic]] [[rhizobia]] bacteria within [[root nodule|nodules]] in their [[root|root systems]], producing nitrogen compounds that help the plant to grow and compete with other plants.<ref>{{cite journal | vauthors = Kuypers MM, Marchant HK, Kartal B | title = The microbial nitrogen-cycling network | journal = Nature Reviews. Microbiology | volume = 16 | issue = 5 | pages = 263–276 | date = May 2018 | pmid = 29398704 | doi = 10.1038/nrmicro.2018.9 | hdl = 21.11116/0000-0003-B828-1 | s2cid = 3948918 | hdl-access = free }}</ref> When the plant dies, the fixed nitrogen is released, making it available to other plants; this helps to fertilize the [[soil]].<ref name=postgate/><ref>{{cite book | vauthors = Smil V |year=2000 |title=Cycles of Life |publisher=Scientific American Library}}</ref> The great majority of legumes have this association, but a few [[genera]] (e.g., ''[[Styphnolobium]]'') do not. In many traditional farming practices, fields are [[Crop Rotation|rotated]] through various types of crops, which usually include one consisting mainly or entirely of [[clover]].{{citation needed|date=August 2019}}

Fixation efficiency in soil is dependent on many factors, including the [[legume]] and air and soil conditions. For example, nitrogen fixation by red clover can range from {{convert|50|to|200|lb/acre|abbr=on}}.<ref>{{Cite web|url=http://www1.foragebeef.ca/$Foragebeef/frgebeef.nsf/all/frg90/$FILE/fertilitylegumefixation.pdf|title=Nitrogen Fixation and Inoculation of Forage Legumes|archive-url=https://web.archive.org/web/20161202170130/http://www1.foragebeef.ca/$Foragebeef/frgebeef.nsf/all/frg90/$FILE/fertilitylegumefixation.pdf|archive-date=2 December 2016|url-status=dead}}</ref>

==== Non-leguminous ====
[[Image:A sectioned alder root nodule gall.JPG|right|thumb|A sectioned alder tree root nodule]]

The ability to fix nitrogen in nodules is present in [[actinorhizal plant]]s such as [[alder]] and [[bayberry]], with the help of ''[[Frankia]]'' bacteria. They are found in 25 genera in the [[order (biology)|order]]s [[Cucurbitales]], [[Fagales]] and [[Rosales]], which together with the [[Fabales]] form a ''nitrogen-fixing clade'' of [[eurosid]]s. The ability to fix nitrogen is not universally present in these families. For example, of 122 [[Rosaceae]] 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]] and subsequently lost in most descendants of the original nitrogen-fixing plant; however, it may be that the basic [[genetics|genetic]] and [[physiological]] requirements were present in an incipient state in the [[most recent common ancestor]]s of all these plants, but only evolved to full function in some of them.<ref>{{cite book | vauthors = Dawson JO | chapter = Ecology of Actinorhizal Plants |title=Nitrogen-fixing Actinorhizal Symbioses |volume=6 |pages=199–234 |doi=10.1007/978-1-4020-3547-0_8 |year=2008 |publisher=Springer |series=Nitrogen Fixation: Origins, Applications, and Research Progress |isbn=978-1-4020-3540-1 }}</ref>

In addition, ''[[Trema (plant)|Trema]]'' (''Parasponia''), a tropical genus in the family [[Cannabaceae]], is unusually able to interact with rhizobia and form nitrogen-fixing nodules.<ref>{{cite journal | vauthors = Op den Camp R, Streng A, De Mita S, Cao Q, Polone E, Liu W, Ammiraju JS, Kudrna D, Wing R, Untergasser A, Bisseling T, Geurts R | title = LysM-type mycorrhizal receptor recruited for rhizobium symbiosis in nonlegume Parasponia | journal = Science | volume = 331 | issue = 6019 | pages = 909–12 | date = February 2011 | pmid = 21205637 | doi = 10.1126/science.1198181 | author-link11 = Ton Bisseling | s2cid = 20501765 | bibcode = 2011Sci...331..909O }}</ref>

{| class="wikitable"
|+Non-legumious nodulating plants
!Family
!Genera
!Species
|-
|[[Betulaceae]]
|{{plainlist|
* [[Alnus]] (alders)
}}
|Most or all species
|-
|[[Boraginaceae]]
|{{plainlist|
* [[Phacelia]]
}}
|{{plainlist|
* [[Phacelia tanacetifolia]]
}}
|-
|[[Cannabaceae]]
|{{plainlist|
* [[Trema (plant)|Trema (Parasponia)]]
}}
|{{plainlist|
* [[Trema orientale]]
* [[Trema lamarckiana]]
}}
|-
|[[Casuarinaceae]]
|{{plainlist|
* [[Allocasuarina]]
* [[Casuarina]]
* [[Ceuthostoma]]
* [[Gymnostoma]]
}}
|
|-
|[[Coriariaceae]]
|{{plainlist|
* [[Coriaria]]
}}
|{{plainlist|
* [[Coriaria arborea]]
* [[Coriaria myrtifolia]]
}}
|-
|[[Datiscaceae]]
|{{plainlist|
* [[Datisca]]
}}
|
|-
|[[Elaeagnaceae]]
|{{plainlist|
* [[Elaeagnus]] (silverberries)
* [[Hippophae]] (sea-buckthorns)
* [[Shepherdia]] (buffaloberries)
}}
|
|-
|[[Myricaceae]]
|{{plainlist|
* [[Comptonia (plant)|Comptonia]] (sweetfern)
* [[Myrica]] (babyberries)
}}
|
|-
|[[Posidoniaceae]]
|{{plainlist|
* [[Posidonia]] (seagrass)
}}
|
|-
|[[Rhamnaceae]]
|{{plainlist|
* [[Ceanothus]]
* [[Colletia]]
* [[Discaria]]
* [[Kentrothamnus]]
* [[Retanilla]]
* [[Talguenea]]
* [[Trevoa]]
}}
|
|-
|[[Rosaceae]]
|{{plainlist|
* [[Cercocarpus]] (mountain mahoganies)
* [[Chamaebatia]] (mountain miseries)
* [[Dryas (plant)|Dryas]]
* [[Purshia]]/Cowania (bitterbrushes/cliffroses)
}}
|
|}

=== 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]]'' species)
* [[Cycad]]s<ref>{{Cite web|title=Cycad biology, Article 1: Corraloid roots of cycads|url=http://www1.biologie.uni-hamburg.de/b-online/library/cycads/corraloid.htm|access-date=2021-10-14|website=www1.biologie.uni-hamburg.de}}</ref>
* ''[[Gunnera]]''
* ''[[Blasia]]'' ([[liverwort]])
* [[Hornwort]]s<ref>{{Cite journal| vauthors = Rai AN |date=2000|title=Cyanobacterium-plant symbioses|journal=New Phytologist|volume=147|issue=3|pages=449–481|doi=10.1046/j.1469-8137.2000.00720.x|pmid=33862930|doi-access=free}}</ref>

Some symbiotic relationships involving agriculturally-important plants are:<ref>{{cite journal | vauthors = Van Deynze A, Zamora P, Delaux PM, Heitmann C, Jayaraman D, Rajasekar S, Graham D, Maeda J, Gibson D, Schwartz KD, Berry AM, Bhatnagar S, Jospin G, Darling A, Jeannotte R, Lopez J, Weimer BC, Eisen JA, Shapiro HY, Ané JM, Bennett AB | title = Nitrogen fixation in a landrace of maize is supported by a mucilage-associated diazotrophic microbiota | journal = PLOS Biology | volume = 16 | issue = 8 | pages = e2006352 | date = August 2018 | pmid = 30086128 | pmc = 6080747 | doi = 10.1371/journal.pbio.2006352 | doi-access = free }}</ref>
* [[Sugarcane]] and unclear [[endophyte]]s
* [[Foxtail millet]] and ''[[Azospirillum brasilense]]''
* [[Kallar grass]] and ''[[Azoarcus]]'' sp. strain BH72
* [[Rice]] and ''[[Herbaspirillum seropedicae]]''
* [[Wheat]] and ''[[Klebsiella pneumoniae]]''
* [[Maize]] landrace '[[Sierra Mixe corn|Sierra Mixe]]' / 'olotón'<ref>{{cite web | vauthors = Pskowski M |date=July 16, 2019|title=Indigenous Maize: Who Owns the Rights to Mexico's 'Wonder' Plant? |url=https://e360.yale.edu/features/indigenous-maize-who-owns-the-rights-to-mexicos-wonder-plant |website=Yale E360}}</ref> and various [[Bacteroidota]] and [[Pseudomonadota]]

== 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.<ref>{{cite journal| title= The Manufacture of Nitrates from the Atmosphere by the Electric Arc—Birkeland-Eyde Process | vauthors = Eyde S | journal= Journal of the Royal Society of Arts| volume= 57| issue = 2949 | year= 1909| pages= 568–576 | jstor=41338647}}</ref> 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.<ref>{{cite journal |title=Die Umwandlungsgleichung Ba(CN)<sub>2</sub> → BaCN<sub>2</sub> + C im Temperaturgebiet von 500 bis 1000&nbsp;°C |trans-title=The conversion reaction Ba(CN)<sub>2</sub> → BaCN<sub>2</sub> + C in the temperature range from 500 to 1,000&nbsp;°C | vauthors = Heinrich H, Nevbner R | journal = Z. Elektrochem. Angew. Phys. Chem. | volume = 40 | issue = 10 | pages = 693–698 | year = 1934 | url = http://onlinelibrary.wiley.com/doi/10.1002/bbpc.19340401005/abstract |url-access=subscription | access-date = 8 August 2016 | doi = 10.1002/bbpc.19340401005 |s2cid=179115181 | archive-url = https://web.archive.org/web/20160820203326/http://onlinelibrary.wiley.com/doi/10.1002/bbpc.19340401005/abstract | archive-date = 20 August 2016 | url-status = live }}</ref><ref>{{cite book | url = {{google books |plainurl=y |id=87XQAAAAMAAJ}}| title = Fixed nitrogen | vauthors = Curtis HA | year = 1932}}</ref>

=== Haber process ===
{{Main|Haber process}}
[[File:THC 2003.902.022 D. C. Bardwell Study of Nitrogen Fixation.tif|thumb|right|Equipment for a study of nitrogen fixation by [[alpha ray]]s (Fixed Nitrogen Research Laboratory, 1926)]]
The dominant industrial method for producing ammonia is the [[Haber process]] also known as the Haber-Bosch process.<ref>Smil, V. 2004. Enriching the Earth: Fritz Haber, Carl Bosch, and the Transformation of World Food Production, MIT Press.</ref> Fertilizer production is now the largest source of human-produced fixed nitrogen in the terrestrial [[ecosystem]]. Ammonia is a required precursor to [[fertilizer]]s, [[explosive]]s, and other products. The Haber process requires high pressures (around 200 atm) and high temperatures (at least 400&nbsp;°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<ref>{{Cite journal| vauthors = Glibert PM, Maranger R, Sobota DJ, Bouwman L |author-link1=Patricia Glibert |author-link2=Roxane Maranger |date=2014-10-01|title=The Haber Bosch–harmful algal bloom (HB–HAB) link|journal=Environmental Research Letters|volume=9|issue=10|pages=105001|doi=10.1088/1748-9326/9/10/105001|bibcode=2014ERL.....9j5001G|s2cid=154724892 |issn=1748-9326|doi-access=free}}</ref> 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.<ref>{{Cite journal| vauthors = Erisman JW, Sutton MA, Galloway J, Klimont Z, Winiwarter W |date=October 2008|title=How a century of ammonia synthesis changed the world |journal=Nature Geoscience|language=en|volume=1|issue=10|pages=636–639|doi=10.1038/ngeo325|bibcode=2008NatGe...1..636E|s2cid=94880859 |issn=1752-0908}}</ref>

=== Homogeneous catalysis ===
{{Main|Abiological nitrogen fixation}}
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 complex]]es. The first dinitrogen [[Complex (chemistry)|complex]] to be reported was [[pentaamine(dinitrogen)ruthenium(II) chloride|{{chem|Ru(NH|3|)|5}}({{chem|N|2}})<sup>2+</sup>]].<ref>{{cite journal | title = Nitrogenopentammineruthenium(II) complexes | vauthors = Allen AD, Senoff CV | journal = J. Chem. Soc., Chem. Commun. | issue = 24 | pages = 621–622 | year = 1965 | doi = 10.1039/C19650000621 }}</ref> Some soluble complexes do catalyze nitrogen fixation.<ref name=Peters>{{cite journal | vauthors = Chalkley MJ, Drover MW, Peters JC | title = Catalytic N<sub>2</sub>-to-NH<sub>3</sub> (or -N<sub>2</sub>H<sub>4</sub>) Conversion by Well-Defined Molecular Coordination Complexes | journal = Chemical Reviews | volume = 120 | issue = 12 | pages = 5582–5636 | date = June 2020 | pmid = 32352271 | pmc = 7493999 | doi = 10.1021/acs.chemrev.9b00638}}</ref>

== Lightning ==
[[File:Lightning Pritzerbe 01 (MK).jpg|thumb|[[Lightning]] heats the air around it in a high-temperature [[Plasma (physics)|plasma]], breaking the bonds of {{chem|N|2}}, starting the formation of [[nitrous acid]] ({{chem|HNO|2}}).]]
Nitrogen can be fixed by [[lightning]] converting nitrogen gas ({{chem|N|2}}) and oxygen gas ({{chem|O|2}}) in the atmosphere into {{NOx}} ([[nitrogen oxides]]). The {{chem|N|2}} molecule is highly stable and nonreactive due to the [[triple bond]] between the nitrogen atoms.<ref name="Tuck-1976">{{Cite journal| vauthors = Tuck AF |date=October 1976 |title=Production of nitrogen oxides by lightning discharges|journal=Quarterly Journal of the Royal Meteorological Society|volume=102|issue=434|pages=749–755|doi=10.1002/qj.49710243404|issn=0035-9009|bibcode=1976QJRMS.102..749T}}</ref> Lightning produces enough energy and heat to break this bond<ref name="Tuck-1976" /> allowing nitrogen atoms to react with oxygen, forming {{chem|NO|x}}. These compounds cannot be used by plants, but as this molecule cools, it reacts with oxygen to form {{chem|NO|2}},<ref>{{cite journal| vauthors = Hill RD |date=August 1979|title=Atmospheric Nitrogen Fixation by Lightning|journal=Journal of the Atmospheric Sciences|volume=37|pages=179–192|doi=10.1175/1520-0469(1980)037<0179:ANFBL>2.0.CO;2|issn=1520-0469|bibcode=1980JAtS...37..179H|doi-access=free}}</ref> which in turn reacts with water to produce {{chem|HNO|2}} ([[nitrous acid]]) or {{chem|HNO|3}} ([[nitric acid]]). When these acids seep into the soil, they make [[nitrate|NO<sub>3</sub><sup>−</sup> (nitrate)]], which is of use to plants.<ref>{{Cite web|url=https://journals.ohiolink.edu/pg_99?126555292207822::NO::P99_ENTITY_ID,P99_ENTITY_TYPE:20567211,MAIN_FILE&cs=38gV8XNFDWVznjRjSa1erAIidpqPJBYgnOix4OM5wvpjv8vJAbG2NGhNYwWAVItkNehLIgXVYuozMCrUrENyuYA|title=Tropospheric Sources of NOx: Lightning And Biology | vauthors = Levin JS |date=1984|access-date=2018-11-29}}</ref><ref name="Tuck-1976" />

== See also ==
* [[Birkeland–Eyde process]]: an industrial fertilizer production process
* [[Carbon fixation]]
* [[Denitrification]]: an organic process of nitrogen release
* [[George Washington Carver]]: an American botanist
* [[Heterocyst]]
* [[Nitrification]]: biological production of nitrogen
* [[Nitrogen cycle]]: the flow and transformation of nitrogen through the environment
* [[Nitrogen deficiency]]
* [[Nitrogen fixation package]] for quantitative measurement of nitrogen fixation by plants
* [[Nitrogenase]]: enzymes used by organisms to fix nitrogen
* [[Ostwald process]]: a chemical process for making nitric acid ({{chem|HNO|3}})
* [[Electrification of catalytic processes]]: electrochemical reduction of N<sub>2</sub>

== References ==
{{Reflist}}

== External links ==
* {{cite web|url=http://www.mcdb.ucla.edu/Research/Hirsch/imagesb/HistoryDiscoveryN2fixingOrganisms.pdf|title=A Brief History of the Discovery of Nitrogen-fixing Organisms | vauthors = Hirsch AM |date=2009|publisher=[[University of California, Los Angeles]]}}
* {{cite web|url=http://dornsife.usc.edu/labs/capone|title=Marine Nitrogen Fixation laboratory|publisher=[[University of Southern California]]}}
* {{Cite web|url=https://digital.sciencehistory.org/collections/gm80hv42t|title=Travis P. Hignett Collection of Fixed Nitrogen Research Laboratory Photographs // Science History Institute Digital Collections|website=digital.sciencehistory.org|access-date=2019-08-16}} [[Science History Institute]] Digital Collections (Photographs depicting numerous stages of the nitrogen fixation process and the various equipment and apparatus used in the production of atmospheric nitrogen, including generators, compressors, filters, thermostats, and vacuum and blast furnaces).
* "[https://books.google.com/books?id=p4o9AQAAIAAJ Proposed Process for the Fixation of Atmospheric Nitrogen]", historical perspective, [[Scientific American]], 13 July 1878, p.&nbsp;21
* [https://naturemicrobiologycommunity.nature.com/posts/a-global-ocean-snapshot-of-nitrogen-fixers-by-matching-sequences-to-cells-in-the-tara-ocean A global ocean snapshot of nitrogen fixers by matching sequences to cells in the Tara Ocean]

{{Authority control}}

[[Category:Nitrogen cycle]]
[[Category:Metabolism]]
[[Category:Plant physiology]]
[[Category:Soil biology]]