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Nitrogen fixation
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<h2 id="history">History</h2> <p>Biological nitrogen fixation was discovered by <a href="/facts/Jean-Baptiste_Boussingault/UppLh3Lz">Jean-Baptiste Boussingault</a> in 1838.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a><a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> Later, in 1880, the process by which it happens was discovered by German <a href="/facts/Agronomy/8pvE8bmm">agronomist</a> <a href="/facts/Hermann_Hellriegel/YM6CeRFq">Hermann Hellriegel</a> and Hermann Wilfarth [de]<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> and was fully described by Dutch microbiologist <a href="/facts/Martinus_Beijerinck/7UpuX9mR">Martinus Beijerinck</a>.<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> </p><p>"The protracted investigations of the relation of plants to the acquisition of nitrogen begun by <a href="/facts/Nicolas_de_Saussure/lnQpe0CE">de Saussure</a>, <a href="/facts/Georges_Ville/ph3BJrnv">Ville</a>, <a href="/facts/John_Bennet_Lawes/S1mTKxqL">Lawes</a>, <a href="/facts/Joseph_Henry_Gilbert/ezkMvjva">Gilbert</a> and others, and culminated in the discovery of symbiotic fixation by Hellriegel and Wilfarth in 1887."<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> </p><p>"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 <a href="/facts/Soil_science/nJ9SxpEE">soil science</a>."<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> </p><p>In 1901, Beijerinck showed that <i><a href="/facts/Azotobacter_chroococcum/MR9CzapW">Azotobacter chroococcum</a></i> was able to fix atmospheric nitrogen. This was the first species of the <i><a href="/facts/Azotobacter/sL6e2WTw">azotobacter</a></i> genus, so-named by him. It is also the first known <a href="/facts/Diazotroph/HnU5xS7V">diazotroph</a>, species that use <a href="/facts/Diatomic_molecule/hkfKAC7m">diatomic</a> nitrogen as a step in the complete <a href="/facts/Nitrogen_cycle/0X1z74dN">nitrogen cycle</a>.<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> </p> <h2 id="biological">Biological</h2> <p>Biological nitrogen fixation (BNF) occurs when atmospheric nitrogen is converted to ammonia by a <a href="/facts/Nitrogenase/gIhxgCaL">nitrogenase</a> enzyme.<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a> The overall reaction for BNF is: </p><p>N2 + 16ATP + 16H2O + 8e− + 8H+ → 2NH3 +H2 + 16ADP + 16Pi </p><p>The process is coupled to the <a href="/facts/Hydrolysis/JTvwIHPs">hydrolysis</a> of 16 equivalents of <a href="/facts/Adenosine_triphosphate/nB9bwgcU">ATP</a> and is accompanied by the co-formation of one equivalent of H2. The conversion of N2 into ammonia occurs at a <a href="/facts/Metal_cluster/EsuLgxJU">metal cluster</a> called <a href="/facts/FeMoco/qtjpkqT5">FeMoco</a>, an abbreviation for the iron-<a href="/facts/Molybdenum/RX6vp2bF">molybdenum</a> cofactor. The mechanism proceeds via a series of <a href="/facts/Protonation/q5Gf9z0M">protonation</a> and reduction steps wherein the FeMoco <a href="/facts/Active_site/u9ua7rGu">active site</a> <a href="/facts/Hydrogenate/l7EzNSA7">hydrogenates</a> the N2 substrate.<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a> In free-living <a href="/facts/Diazotroph/HnU5xS7V">diazotrophs</a>, nitrogenase-generated ammonia is assimilated into <a href="/facts/Glutamate/7oXbNcvu">glutamate</a> through the <a href="/facts/Glutamine_synthetase/4F28uxzR">glutamine synthetase</a>/glutamate synthase pathway. The microbial <a href="/facts/Nif_gene/s6pIrE7A">nif genes</a> required for nitrogen fixation are widely distributed in diverse environments.<a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a> </p><p>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 <a href="/facts/Anaerobic_organism/bhQnlqCq">anaerobic</a> conditions, respiring to draw down oxygen levels, or binding the oxygen with a <a href="/facts/Protein/Jxmagu3E">protein</a> such as <a href="/facts/Leghemoglobin/5mkEzwlT">leghemoglobin</a>.<a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a><a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a> </p> <h3>Importance of nitrogen</h3> <p>Atmospheric nitrogen cannot be metabolized by most organisms,<a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a> 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,<a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a> who determined the stoichiometric relationship between C:N:P atoms, The Redfield Ratio, to be 106:16:1.<a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a> </p> <h3>Nitrogenase</h3> <p class="note">Main article: <a href="/facts/Nitrogenase/gIhxgCaL">Nitrogenase</a></p> <p>The protein complex nitrogenase is responsible for <a href="/facts/Catalysis/LPBS7TQC">catalyzing</a> the reduction of nitrogen gas (N2) to ammonia (NH3).<a class="footnote-ref" id="fnref:23" href="#fn:23"><sup>23</sup></a><a class="footnote-ref" id="fnref:24" href="#fn:24"><sup>24</sup></a> In <a href="/facts/Cyanobacteria/S8bKxIU9">cyanobacteria</a>, this <a href="/facts/Enzyme/DSI1Fh7y">enzyme</a> system is housed in a specialized cell called the <a href="/facts/Heterocyst/7nQxaCmQ">heterocyst</a>.<a class="footnote-ref" id="fnref:25" href="#fn:25"><sup>25</sup></a> The production of the <a href="/facts/Nitrogenase/gIhxgCaL">nitrogenase</a> 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).<a class="footnote-ref" id="fnref:26" href="#fn:26"><sup>26</sup></a><a class="footnote-ref" id="fnref:27" href="#fn:27"><sup>27</sup></a><a class="footnote-ref" id="fnref:28" href="#fn:28"><sup>28</sup></a> Additionally, the combined concentrations of both ammonium and nitrate are thought to inhibit NFix, specifically when intracellular concentrations of 2-<a href="/facts/Ketoglutaric_acid/xI9vtCFK">oxoglutarate</a> (2-OG) exceed a critical threshold.<a class="footnote-ref" id="fnref:29" href="#fn:29"><sup>29</sup></a> The specialized heterocyst cell is necessary for the performance of nitrogenase as a result of its sensitivity to ambient oxygen.<a class="footnote-ref" id="fnref:30" href="#fn:30"><sup>30</sup></a> </p><p>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: <a href="/facts/Molybdenum/RX6vp2bF">molybdenum</a>-dependent, <a href="/facts/Vanadium/UvKkKTSm">vanadium</a>-dependent, and <a href="/facts/Iron/QK1JYy8E">iron</a>-only, with all three nitrogenase protein variations containing an iron protein component. Molybdenum-dependent nitrogenase is most common.<a class="footnote-ref" id="fnref:31" href="#fn:31"><sup>31</sup></a> The different types of nitrogenase can be determined by the specific iron protein component.<a class="footnote-ref" id="fnref:32" href="#fn:32"><sup>32</sup></a> Nitrogenase is highly conserved. <a href="/facts/Gene_expression/alCPohLR">Gene expression</a> through <a href="/facts/DNA_sequencing/hsPsNCQW">DNA sequencing</a> can distinguish which protein complex is present in the microorganism and potentially being expressed. Most frequently, the <a href="/facts/Nif_gene/s6pIrE7A"><i>nif</i>H gene</a> is used to identify the presence of molybdenum-dependent nitrogenase, followed by closely related nitrogenase reductases (component II) <i>vnf</i>H and <i>anf</i>H representing vanadium-dependent and iron-only nitrogenase, respectively.<a class="footnote-ref" id="fnref:33" href="#fn:33"><sup>33</sup></a> In studying the ecology and evolution of <a href="/facts/Diazotroph/HnU5xS7V">nitrogen-fixing bacteria</a>, the <i>nifH</i> gene is the <a href="/facts/Biomarker/nk2LgXtq">biomarker</a> most widely used.<a class="footnote-ref" id="fnref:34" href="#fn:34"><sup>34</sup></a> <i>nif</i>H has two similar genes <i>anf</i>H and vnfH that also encode for the nitrogenase reductase component of the nitrogenase complex.<a class="footnote-ref" id="fnref:35" href="#fn:35"><sup>35</sup></a> </p> <h3>Evolution of nitrogenase</h3> <p>Nitrogenase is thought to have evolved sometime between 1.5-2.2 billion years ago (Ga),<a class="footnote-ref" id="fnref:36" href="#fn:36"><sup>36</sup></a><a class="footnote-ref" id="fnref:37" href="#fn:37"><sup>37</sup></a> although some isotopic support showing nitrogenase evolution as early as around 3.2 Ga.<a class="footnote-ref" id="fnref:38" href="#fn:38"><sup>38</sup></a> Nitrogenase appears to have evolved from <a href="/facts/Maturase/ukztC11f">maturase</a>-like proteins, although the function of the preceding protein is currently unknown.<a class="footnote-ref" id="fnref:39" href="#fn:39"><sup>39</sup></a> </p><p>Nitrogenase has three different forms (<i>Nif, Anf, and Vnf</i>) that correspond with the metal found in the active site of the protein (molybdenum, iron, and vanadium respectively).<a class="footnote-ref" id="fnref:40" href="#fn:40"><sup>40</sup></a> Marine metal abundances over Earth's geologic timeline are thought to have driven the relative abundance of which form of nitrogenase was most common.<a class="footnote-ref" id="fnref:41" href="#fn:41"><sup>41</sup></a> Currently, there is no conclusive agreement on which form of nitrogenase arose first. </p> <h3>Microorganisms</h3> <p class="note">Main article: <a href="/facts/Diazotroph/HnU5xS7V">Diazotroph</a></p> <p>Diazotrophs are widespread within domain <a href="/facts/Bacteria/wC8xSCsU">Bacteria</a> including <a href="/facts/Cyanobacteria/S8bKxIU9">cyanobacteria</a> (e.g. the highly significant <i><a href="/facts/Trichodesmium/eeUAb510">Trichodesmium</a></i> and <i><a href="/facts/Cyanothece/tkBWI4vN">Cyanothece</a></i>), <a href="/facts/Green_sulfur_bacteria/rb7cNcX8">green sulfur bacteria</a>, <a href="/facts/Purple_sulfur_bacteria/h3f5dFUn">purple sulfur bacteria</a>, <a href="/facts/Azotobacteraceae/UuAH7Pqf">Azotobacteraceae</a>, <a href="/facts/Rhizobia/xGRcYg5h">rhizobia</a> and <i><a href="/facts/Frankia/uAE60J2C">Frankia</a>.</i><a class="footnote-ref" id="fnref:42" href="#fn:42"><sup>42</sup></a><a class="footnote-ref" id="fnref:43" href="#fn:43"><sup>43</sup></a> Several obligately anaerobic bacteria fix nitrogen including many (but not all) <i><a href="/facts/Clostridium/HRXJyYCn">Clostridium</a></i> spp. Some <a href="/facts/Archaea/Adxh0aHw">archaea</a> such as <i><a href="/facts/Methanosarcina_acetivorans/HGhXmHfR">Methanosarcina acetivorans</a></i> also fix nitrogen,<a class="footnote-ref" id="fnref:44" href="#fn:44"><sup>44</sup></a> and several other <a href="/facts/Methanogen/6tRT6LyH">methanogenic</a> <a href="/facts/Taxa/caQC1e87">taxa</a>, are significant contributors to nitrogen fixation in oxygen-deficient soils.<a class="footnote-ref" id="fnref:45" href="#fn:45"><sup>45</sup></a> </p><p><a href="/facts/Cyanobacteria/S8bKxIU9">Cyanobacteria</a>, commonly known as blue-green algae, inhabit nearly all illuminated environments on Earth and play key roles in the carbon and <a href="/facts/Nitrogen_cycle/0X1z74dN">nitrogen cycle</a> of the <a href="/facts/Biosphere/P1ltA4sb">biosphere</a>. In general, cyanobacteria can use various inorganic and organic sources of combined nitrogen, such as <a href="/facts/Nitrate/5AnVlYnL">nitrate</a>, <a href="/facts/Nitrite/qkz2XScK">nitrite</a>, <a href="/facts/Ammonium/jYm8wc1n">ammonium</a>, <a href="/facts/Urea/6j84AoCG">urea</a>, or some <a href="/facts/Amino_acid/WDxYwBny">amino acids</a>. Several cyanobacteria strains are also capable of diazotrophic growth, an ability that may have been present in their last common ancestor in the <a href="/facts/Archean/PHt8Sm8n">Archean</a> eon.<a class="footnote-ref" id="fnref:46" href="#fn:46"><sup>46</sup></a> Nitrogen fixation not only naturally occurs in soils but also aquatic systems, including both freshwater and marine.<a class="footnote-ref" id="fnref:47" href="#fn:47"><sup>47</sup></a><a class="footnote-ref" id="fnref:48" href="#fn:48"><sup>48</sup></a> Indeed, the amount of nitrogen fixed in the ocean is at least as much as that on land.<a class="footnote-ref" id="fnref:49" href="#fn:49"><sup>49</sup></a> The colonial marine cyanobacterium <i><a href="/facts/Trichodesmium/eeUAb510">Trichodesmium</a></i> is thought to fix nitrogen on such a scale that it accounts for almost half of the nitrogen fixation in marine systems globally.<a class="footnote-ref" id="fnref:50" href="#fn:50"><sup>50</sup></a> Marine surface lichens and non-photosynthetic bacteria belonging in Proteobacteria and Planctomycetes fixate significant atmospheric nitrogen.<a class="footnote-ref" id="fnref:51" href="#fn:51"><sup>51</sup></a> Species of nitrogen fixing cyanobacteria in fresh waters include: <i><a href="/facts/Aphanizomenon/NNBp8s3I">Aphanizomenon</a></i> and <a href="/facts/Dolichospermum_flosaquae/pqArqF6g"><i>Dolichospermum</i></a> (previously Anabaena).<a class="footnote-ref" id="fnref:52" href="#fn:52"><sup>52</sup></a> Such species have specialized cells called <a href="/facts/Heterocyst/7nQxaCmQ">heterocytes</a>, in which nitrogen fixation occurs via the nitrogenase enzyme.<a class="footnote-ref" id="fnref:53" href="#fn:53"><sup>53</sup></a><a class="footnote-ref" id="fnref:54" href="#fn:54"><sup>54</sup></a> </p> <h3>Algae</h3> <p>One type of <a href="/facts/Organelle/vhDFnGbs">organelle</a>, originating from <a href="/facts/Cyanobacteria/S8bKxIU9">cyanobacterial</a> <a href="/facts/Endosymbiont/eWvSS8ID">endosymbionts</a> called <a href="/facts/UCYN-A/czCDXolw">UCYN-A</a>2,<a class="footnote-ref" id="fnref:55" href="#fn:55"><sup>55</sup></a><a class="footnote-ref" id="fnref:56" href="#fn:56"><sup>56</sup></a> can turn nitrogen gas into a biologically available form. This <a href="/facts/Nitroplast/xnaWC2jh">nitroplast</a> was discovered in <a href="/facts/Algae/U1ftVKU3">algae</a>, particularly in the marine algae <a href="/facts/Braarudosphaera_bigelowii/ypsknc0e">Braarudosphaera bigelowii</a>.<a class="footnote-ref" id="fnref:57" href="#fn:57"><sup>57</sup></a> </p><p><a href="/facts/Diatom/x2oEcP1P">Diatoms</a> in the family <i>Rhopalodiaceae</i> also possess <a href="/facts/Cyanobacteria/S8bKxIU9">cyanobacterial</a> <a href="/facts/Endosymbiont/eWvSS8ID">endosymbionts</a> called spheroid bodies or diazoplasts.<a class="footnote-ref" id="fnref:58" href="#fn:58"><sup>58</sup></a> These endosymbionts have lost photosynthetic properties, but have kept the ability to perform nitrogen fixation, allowing these diatoms to fix atmospheric nitrogen.<a class="footnote-ref" id="fnref:59" href="#fn:59"><sup>59</sup></a><a class="footnote-ref" id="fnref:60" href="#fn:60"><sup>60</sup></a> Other diatoms in symbiosis with nitrogen-fixing cyanobacteria are among the genera <i>Hemiaulus</i>, <i>Rhizosolenia</i> and <i>Chaetoceros</i>.<a class="footnote-ref" id="fnref:61" href="#fn:61"><sup>61</sup></a> </p> <h3>Root nodule symbioses</h3> <p class="note">Main article: <a href="/facts/Root_nodule/qUFHMpsq">Root nodule</a></p> <h4>Legume family</h4> <p>Plants that contribute to nitrogen fixation include those of the <a href="/facts/Legume/x6s6gVHY">legume</a> <a href="/facts/Family_(biology)/qTrn6E8E">family</a>—<a href="/facts/Fabaceae/uxuHNdAr">Fabaceae</a>— with <a href="/facts/Taxa/caQC1e87">taxa</a> such as <a href="/facts/Kudzu/fco1pgG9">kudzu</a>, <a href="/facts/Clover/bKr90Vct">clover</a>, <a href="/facts/Soybean/reNwxvo2">soybean</a>, <a href="/facts/Alfalfa/Yesc424s">alfalfa</a>, <a href="/facts/Lupin/qrVAhIMc">lupin</a>, <a href="/facts/Peanut/dMd0XWsu">peanut</a> and <a href="/facts/Rooibos/z3KPqTv0">rooibos</a>.<a class="footnote-ref" id="fnref:62" href="#fn:62"><sup>62</sup></a> They contain <a href="/facts/Symbiosis/MrBArRGa">symbiotic</a> <a href="/facts/Rhizobia/xGRcYg5h">rhizobia</a> bacteria within <a href="/facts/Root_nodule/qUFHMpsq">nodules</a> in their <a href="/facts/Root/x2U60Gdm">root systems</a>, producing nitrogen compounds that help the plant to grow and compete with other plants.<a class="footnote-ref" id="fnref:63" href="#fn:63"><sup>63</sup></a> When the plant dies, the fixed nitrogen is released, making it available to other plants; this helps to fertilize the <a href="/facts/Soil/5Ddpr3QM">soil</a>.<a class="footnote-ref" id="fnref:64" href="#fn:64"><sup>64</sup></a><a class="footnote-ref" id="fnref:65" href="#fn:65"><sup>65</sup></a> The great majority of legumes have this association, but a few <a href="/facts/Genera/m8EFjSms">genera</a> (e.g., <i><a href="/facts/Styphnolobium/g6FIqtyX">Styphnolobium</a></i>) do not. In many traditional farming practices, fields are <a href="/facts/Crop_Rotation/yeqbveKE">rotated</a> through various types of crops, which usually include one consisting mainly or entirely of <a href="/facts/Clover/bKr90Vct">clover</a>. </p><p>Fixation efficiency in soil is dependent on many factors, including the <a href="/facts/Legume/x6s6gVHY">legume</a> and air and soil conditions. For example, nitrogen fixation by red clover can range from 50 to 200 lb/acre (56 to 224 kg/ha).<a class="footnote-ref" id="fnref:66" href="#fn:66"><sup>66</sup></a> </p> <h4>Non-leguminous</h4> <p>The ability to fix nitrogen in nodules is present in <a href="/facts/Actinorhizal_plant/emjoPgye">actinorhizal plants</a> such as <a href="/facts/Alder/KbD9YG8w">alder</a> and <a href="/facts/Bayberry/IRgSoqUS">bayberry</a>, with the help of <i><a href="/facts/Frankia/uAE60J2C">Frankia</a></i> bacteria. They are found in 25 genera in the <a href="/facts/Order_(biology)/GKLJvIpt">orders</a> <a href="/facts/Cucurbitales/YWVCjs3z">Cucurbitales</a>, <a href="/facts/Fagales/o8CjmegD">Fagales</a> and <a href="/facts/Rosales/H73YkSeg">Rosales</a>, which together with the <a href="/facts/Fabales/H0aMr1uL">Fabales</a> form a <i>nitrogen-fixing clade</i> of <a href="/facts/Eurosid/QK2awBU0">eurosids</a>. The ability to fix nitrogen is not universally present in these families. For example, of 122 <a href="/facts/Rosaceae/KYrlFdv6">Rosaceae</a> 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 <a href="/facts/Plesiomorphic/n5K8vSKd">plesiomorphic</a> and subsequently lost in most descendants of the original nitrogen-fixing plant; however, it may be that the basic <a href="/facts/Genetics/bu8wne0A">genetic</a> and <a href="/facts/Physiological/fh25y2pQ">physiological</a> requirements were present in an incipient state in the <a href="/facts/Most_recent_common_ancestor/Cbg1YLCV">most recent common ancestors</a> of all these plants, but only evolved to full function in some of them.<a class="footnote-ref" id="fnref:67" href="#fn:67"><sup>67</sup></a> </p><p>In addition, <i><a href="/facts/Trema_(plant)/qdnoQL6q">Trema</a></i> (<i>Parasponia</i>), a tropical genus in the family <a href="/facts/Cannabaceae/DYEfetvL">Cannabaceae</a>, is unusually able to interact with rhizobia and form nitrogen-fixing nodules.<a class="footnote-ref" id="fnref:68" href="#fn:68"><sup>68</sup></a> </p> Non-legumious nodulating plants<table><tbody><tr><th>Family</th><th>Genera</th><th>Species</th></tr><tr><td><a href="/facts/Betulaceae/Q1G1xg7G">Betulaceae</a></td><td><ul><li><a href="/facts/Alnus/KbD9YG8w">Alnus</a> (alders)</li></ul></td><td>Most or all species</td></tr><tr><td><a href="/facts/Boraginaceae/wGbPMTXF">Boraginaceae</a></td><td><ul><li><a href="/facts/Phacelia/zMPsysFt">Phacelia</a></li></ul></td><td><ul><li><a href="/facts/Phacelia_tanacetifolia/5UQJCXM1">Phacelia tanacetifolia</a></li></ul></td></tr><tr><td><a href="/facts/Cannabaceae/DYEfetvL">Cannabaceae</a></td><td><ul><li><a href="/facts/Trema_(plant)/qdnoQL6q">Trema (Parasponia)</a></li></ul></td><td><ul><li><a href="/facts/Trema_orientale/4bX7vWBv">Trema orientale</a></li><li><a href="/facts/Trema_lamarckiana/zbxgHgS0">Trema lamarckiana</a></li></ul></td></tr><tr><td><a href="/facts/Casuarinaceae/2GLxSV9l">Casuarinaceae</a></td><td><ul><li><a href="/facts/Allocasuarina/1nF9aBB7">Allocasuarina</a></li><li><a href="/facts/Casuarina/YV079CC6">Casuarina</a></li><li><a href="/facts/Ceuthostoma/mNp0gj8d">Ceuthostoma</a></li><li><a href="/facts/Gymnostoma/UkyTUEt3">Gymnostoma</a></li></ul></td><td></td></tr><tr><td><a href="/facts/Coriariaceae/Dun7Bvl8">Coriariaceae</a></td><td><ul><li><a href="/facts/Coriaria/Dun7Bvl8">Coriaria</a></li></ul></td><td><ul><li><a href="/facts/Coriaria_arborea/2wBI6zu0">Coriaria arborea</a></li><li><a href="/facts/Coriaria_myrtifolia/AN7qKWLI">Coriaria myrtifolia</a></li></ul></td></tr><tr><td><a href="/facts/Datiscaceae/orTUNdFr">Datiscaceae</a></td><td><ul><li><a href="/facts/Datisca/orTUNdFr">Datisca</a></li></ul></td><td></td></tr><tr><td><a href="/facts/Elaeagnaceae/5Khza6Dl">Elaeagnaceae</a></td><td><ul><li><a href="/facts/Elaeagnus/x8fvfgeS">Elaeagnus</a> (silverberries)</li><li><a href="/facts/Hippophae/sEqKuzVM">Hippophae</a> (sea-buckthorns)</li><li><a href="/facts/Shepherdia/2v29gYjM">Shepherdia</a> (buffaloberries)</li></ul></td><td></td></tr><tr><td><a href="/facts/Myricaceae/uamjrpwu">Myricaceae</a></td><td><ul><li><a href="/facts/Comptonia_(plant)/Wuplu9Tv">Comptonia</a> (sweetfern)</li><li><a href="/facts/Myrica/IRgSoqUS">Myrica</a> (babyberries)</li></ul></td><td></td></tr><tr><td><a href="/facts/Posidoniaceae/CW8fCrfr">Posidoniaceae</a></td><td><ul><li><a href="/facts/Posidonia/CW8fCrfr">Posidonia</a> (seagrass)</li></ul></td><td></td></tr><tr><td><a href="/facts/Rhamnaceae/1lAyrg27">Rhamnaceae</a></td><td><ul><li><a href="/facts/Ceanothus/jdF1oqEY">Ceanothus</a></li><li><a href="/facts/Colletia/mEYmTb47">Colletia</a></li><li><a href="/facts/Discaria/lpdIDSWA">Discaria</a></li><li><a href="/facts/Kentrothamnus/6Rmjp7d1">Kentrothamnus</a></li><li><a href="/facts/Retanilla/g0reA58k">Retanilla</a></li><li><a href="/facts/Talguenea/B4XV70uS">Talguenea</a></li><li><a href="/facts/Trevoa/B4XV70uS">Trevoa</a></li></ul></td><td></td></tr><tr><td><a href="/facts/Rosaceae/KYrlFdv6">Rosaceae</a></td><td><ul><li><a href="/facts/Cercocarpus/oh1nX58k">Cercocarpus</a> (mountain mahoganies)</li><li><a href="/facts/Chamaebatia/Q05avaVY">Chamaebatia</a> (mountain miseries)</li><li><a href="/facts/Dryas_(plant)/uWnx0B1q">Dryas</a></li><li><a href="/facts/Purshia/PU0MYwLz">Purshia</a>/Cowania (bitterbrushes/cliffroses)</li></ul></td><td></td></tr></tbody></table> <h3>Other plant symbionts</h3> <p>Some other plants live in association with a <a href="/facts/Cyanobiont/H1QuF2NQ">cyanobiont</a> (cyanobacteria such as <i><a href="/facts/Nostoc/WoDSJKYR">Nostoc</a></i>) which fix nitrogen for them: </p> <ul><li>Some lichens such as <i><a href="/facts/Lobaria/NxtkKnBq">Lobaria</a></i> and <i><a href="/facts/Peltigera/vGxs9F3q">Peltigera</a></i></li> <li><a href="/facts/Mosquito_fern/2pgFpADh">Mosquito fern</a> (<i><a href="/facts/Azolla/2pgFpADh">Azolla</a></i> species)</li> <li><a href="/facts/Cycad/Kc2JDExr">Cycads</a><a class="footnote-ref" id="fnref:69" href="#fn:69"><sup>69</sup></a></li> <li><i><a href="/facts/Gunnera/ekbTvF2T">Gunnera</a></i></li> <li><i><a href="/facts/Blasia/5LQPYIQX">Blasia</a></i> (<a href="/facts/Liverwort/2lmpEV4A">liverwort</a>)</li> <li><a href="/facts/Hornwort/NLDvbs9G">Hornworts</a><a class="footnote-ref" id="fnref:70" href="#fn:70"><sup>70</sup></a></li></ul> <p>Some symbiotic relationships involving agriculturally-important plants are:<a class="footnote-ref" id="fnref:71" href="#fn:71"><sup>71</sup></a> </p> <ul><li><a href="/facts/Sugarcane/925qszH4">Sugarcane</a> and unclear <a href="/facts/Endophyte/895a4l1C">endophytes</a></li> <li><a href="/facts/Foxtail_millet/eslp0b0o">Foxtail millet</a> and <i><a href="/facts/Azospirillum_brasilense/DFyQxg6P">Azospirillum brasilense</a></i></li> <li><a href="/facts/Kallar_grass/E6jm2gHD">Kallar grass</a> and <i><a href="/facts/Azoarcus/RpA8RbGa">Azoarcus</a></i> sp. strain BH72</li> <li><a href="/facts/Rice/wvL1hBCz">Rice</a> and <i><a href="/facts/Herbaspirillum_seropedicae/vNcadxqV">Herbaspirillum seropedicae</a></i></li> <li><a href="/facts/Wheat/lrk6HdV3">Wheat</a> and <i><a href="/facts/Klebsiella_pneumoniae/edtyTP4E">Klebsiella pneumoniae</a></i></li> <li><a href="/facts/Maize/lJ6c5xMH">Maize</a> landrace '<a href="/facts/Sierra_Mixe_corn/fx4Rm4f2">Sierra Mixe</a>' / 'olotón'<a class="footnote-ref" id="fnref:72" href="#fn:72"><sup>72</sup></a> and various <a href="/facts/Bacteroidota/D8vX8BHh">Bacteroidota</a> and <a href="/facts/Pseudomonadota/ggjeK6II">Pseudomonadota</a></li></ul> <h2 id="industrial-processes">Industrial processes</h2> <h3>Historical</h3> <p>A method for nitrogen fixation was first described by <a href="/facts/Henry_Cavendish/UsIDwP3F">Henry Cavendish</a> in 1784 using electric arcs reacting nitrogen and oxygen in air. This method was implemented in the <a href="/facts/Birkeland%25E2%2580%2593Eyde_process/KlYdcX0b">Birkeland–Eyde process</a> of 1903.<a class="footnote-ref" id="fnref:73" href="#fn:73"><sup>73</sup></a> The fixation of nitrogen by lightning is a very similar natural occurring process. </p><p>The possibility that atmospheric nitrogen reacts with certain chemicals was first observed by Desfosses in 1828. He observed that mixtures of <a href="/facts/Alkali_metal/IkWbCLk2">alkali metal</a> oxides and carbon react with nitrogen at high temperatures. With the use of <a href="/facts/Barium_carbonate/IGjS0E0G">barium carbonate</a> as starting material, the first commercial process became available in the 1860s, developed by Margueritte and Sourdeval. The resulting <a href="/facts/Barium_cyanide/ufebmyn8">barium cyanide</a> reacts with steam, yielding ammonia. In 1898 <a href="/facts/Adolph_Frank/0vPJnVlL">Frank</a> and <a href="/facts/Nikodem_Caro/tJ2S3oM0">Caro</a> developed what is known as the <a href="/facts/Frank%25E2%2580%2593Caro_process/IGjLkykH">Frank–Caro process</a> to fix nitrogen in the form of <a href="/facts/Calcium_cyanamide/IRcdYh9K">calcium cyanamide</a>. The process was eclipsed by the <a href="/facts/Haber_process/DMlVT2WP">Haber process</a>, which was discovered in 1909.<a class="footnote-ref" id="fnref:74" href="#fn:74"><sup>74</sup></a><a class="footnote-ref" id="fnref:75" href="#fn:75"><sup>75</sup></a> </p> <h3>Haber process</h3> <p class="note">Main article: <a href="/facts/Haber_process/DMlVT2WP">Haber process</a></p> <p>The dominant industrial method for producing ammonia is the <a href="/facts/Haber_process/DMlVT2WP">Haber process</a> also known as the Haber-Bosch process.<a class="footnote-ref" id="fnref:76" href="#fn:76"><sup>76</sup></a> Fertilizer production is now the largest source of human-produced fixed nitrogen in the terrestrial <a href="/facts/Ecosystem/kVE8mSmR">ecosystem</a>. Ammonia is a required precursor to <a href="/facts/Fertilizer/ysh3r6NJ">fertilizers</a>, <a href="/facts/Explosive/sUme5Pp8">explosives</a>, 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<a class="footnote-ref" id="fnref:77" href="#fn:77"><sup>77</sup></a> 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.<a class="footnote-ref" id="fnref:78" href="#fn:78"><sup>78</sup></a> </p> <h3>Homogeneous catalysis</h3> <p class="note">Main article: <a href="/facts/Abiological_nitrogen_fixation/m8q36uBL">Abiological nitrogen fixation</a></p> <p>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 <a href="/facts/Dinitrogen_complex/H0pTKfwd">dinitrogen complexes</a>. The first dinitrogen <a href="/facts/Complex_(chemistry)/1tCyeQUm">complex</a> to be reported was <a href="/facts/Pentaamine(dinitrogen)ruthenium(II)_chloride/grg63Uou">Ru(NH3)5(N2)2+</a>.<a class="footnote-ref" id="fnref:79" href="#fn:79"><sup>79</sup></a> Some soluble complexes do catalyze nitrogen fixation.<a class="footnote-ref" id="fnref:80" href="#fn:80"><sup>80</sup></a> </p> <h2 id="lightning">Lightning</h2> <p>Nitrogen can be fixed by <a href="/facts/Lightning/dLTuxz8n">lightning</a> converting nitrogen gas (N2) and oxygen gas (O2) in the atmosphere into NO<i>x</i> (<a href="/facts/Nitrogen_oxides/ved7hE6X">nitrogen oxides</a>). The N2 molecule is highly stable and nonreactive due to the <a href="/facts/Triple_bond/poDWRayX">triple bond</a> between the nitrogen atoms.<a class="footnote-ref" id="fnref:81" href="#fn:81"><sup>81</sup></a> Lightning produces enough energy and heat to break this bond<a class="footnote-ref" id="fnref:82" href="#fn:82"><sup>82</sup></a> allowing nitrogen atoms to react with oxygen, forming NOx. These compounds cannot be used by plants, but as this molecule cools, it reacts with oxygen to form NO2,<a class="footnote-ref" id="fnref:83" href="#fn:83"><sup>83</sup></a> which in turn reacts with water to produce HNO2 (<a href="/facts/Nitrous_acid/xaVd1w2D">nitrous acid</a>) or HNO3 (<a href="/facts/Nitric_acid/UDbwGp6s">nitric acid</a>). When these acids seep into the soil, they make <a href="/facts/Nitrate/5AnVlYnL">NO3− (nitrate)</a>, which is of use to plants.<a class="footnote-ref" id="fnref:84" href="#fn:84"><sup>84</sup></a><a class="footnote-ref" id="fnref:85" href="#fn:85"><sup>85</sup></a> </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Birkeland%25E2%2580%2593Eyde_process/KlYdcX0b">Birkeland–Eyde process</a>: an industrial fertilizer production process</li> <li><a href="/facts/Carbon_fixation/JqvfmCdH">Carbon fixation</a></li> <li><a href="/facts/Denitrification/cBKgWJ3D">Denitrification</a>: an organic process of nitrogen release</li> <li><a href="/facts/George_Washington_Carver/vY7TrHzd">George Washington Carver</a>: an American botanist</li> <li><a href="/facts/Heterocyst/7nQxaCmQ">Heterocyst</a></li> <li><a href="/facts/Nitrification/XVmbAlyd">Nitrification</a>: biological production of nitrogen</li> <li><a href="/facts/Nitrogen_cycle/0X1z74dN">Nitrogen cycle</a>: the flow and transformation of nitrogen through the environment</li> <li><a href="/facts/Nitrogen_deficiency/YoCAyNQf">Nitrogen deficiency</a></li> <li><a href="/facts/Nitrogen_fixation_package/zhjLXQwa">Nitrogen fixation package</a> for quantitative measurement of nitrogen fixation by plants</li> <li><a href="/facts/Nitrogenase/gIhxgCaL">Nitrogenase</a>: enzymes used by organisms to fix nitrogen</li> <li><a href="/facts/Ostwald_process/lczT8kXb">Ostwald process</a>: a chemical process for making nitric acid (HNO3)</li> <li><a href="/facts/Electrification_of_catalytic_processes/UXwuCNNg">Electrification of catalytic processes</a>: electrochemical reduction of N2</li></ul> <h2 id="external-links">External links</h2> <ul><li>Hirsch AM (2009). <a href="http://www.mcdb.ucla.edu/Research/Hirsch/imagesb/HistoryDiscoveryN2fixingOrganisms.pdf">"A Brief History of the Discovery of Nitrogen-fixing Organisms"</a> (PDF). <a href="/facts/University_of_California%2c_Los_Angeles/KwPz7AK1">University of California, Los Angeles</a>.</li> <li><a href="http://dornsife.usc.edu/labs/capone">"Marine Nitrogen Fixation laboratory"</a>. <a href="/facts/University_of_Southern_California/N0v9R5gs">University of Southern California</a>.</li> <li><a href="https://digital.sciencehistory.org/collections/gm80hv42t">"Travis P. Hignett Collection of Fixed Nitrogen Research Laboratory Photographs // Science History Institute Digital Collections"</a>. <i>digital.sciencehistory.org</i>. Retrieved 16 August 2019. <a href="/facts/Science_History_Institute/qC4b6qoQ">Science History Institute</a> 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).</li> <li>"<a href="https://books.google.com/books?id=p4o9AQAAIAAJ">Proposed Process for the Fixation of Atmospheric Nitrogen</a>", historical perspective, <a href="/facts/Scientific_American/jkl8bhUN">Scientific American</a>, 13 July 1878, p. 21</li> <li><a href="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</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Einsle O, Rees DC (2020). "Structural Enzymology of Nitrogenase Enzymes". Chemical Reviews. 120 (12): 4969–5004. doi:10.1021/acs.chemrev.0c00067. PMC 8606229. PMID 32538623. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8606229" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8606229</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Burris RH, Wilson PW (June 1945). "Biological Nitrogen Fixation". Annual Review of Biochemistry. 14 (1): 685–708. doi:10.1146/annurev.bi.14.070145.003345. ISSN 0066-4154. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Wagner SC (2011). "Biological Nitrogen Fixation". Nature Education Knowledge. 3 (10): 15. Archived from the original on 13 September 2018. 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IOP Conference Series: Earth and Environmental Science. 1259 (1): 012110. Bibcode:2023E&ES.1259a2110A. doi:10.1088/1755-1315/1259/1/012110. ISSN 1755-1307. <a href="https://doi.org/10.1088%2F1755-1315%2F1259%2F1%2F012110" target="_blank">https://doi.org/10.1088%2F1755-1315%2F1259%2F1%2F012110</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></p></li> <li id="fn:15"><p>Einsle O, Rees DC (2020). "Structural Enzymology of Nitrogenase Enzymes". Chemical Reviews. 120 (12): 4969–5004. doi:10.1021/acs.chemrev.0c00067. PMC 8606229. PMID 32538623. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8606229" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8606229</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></p></li> <li id="fn:16"><p>Einsle O, Rees DC (2020). "Structural Enzymology of Nitrogenase Enzymes". Chemical Reviews. 120 (12): 4969–5004. doi:10.1021/acs.chemrev.0c00067. PMC 8606229. 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