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Dictionary:
nitrogen fixation (nī'trə-jən-fĭk'sər) n. |
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| Sci-Tech Encyclopedia: Nitrogen fixation |
The chemical or biological conversion of atmospheric nitrogen (N2) into compounds which can be used by plants, and thus become available to animals and humans. In the 1990s, chemical and biological processes together contributed about 260 million tons (230 million metric tons) of fixed nitrogen per year globally. Industrial production of nitrogen fertilizer accounted for about 85 million tons (80 million metric tons) of nitrogen per year, while spontaneous chemical processes, such as lightning, ultraviolet irradiation, and combustion, leading to the synthesis of nitrogen oxides from O2 and N2, may have accounted for 44 million tons (40 million metric tons) per year. The remainder, roughly half of the global input of newly fixed nitrogen, arose from biological processes. World agriculture, which is very dependent on nitrogen fixation, is increasingly reliant on chemical nitrogen sources.
Chemical fixation
Three chemical processes for fixing atmospheric nitrogen have been developed. All require considerable thermal or electrical energy and yield different products. In arc processes, which are now rarely used, air is passed through an electric arc and about 1% nitric oxide is formed, which can be chemically converted to nitrates. In the cyanamide process, which is now obsolete, heating calcium carbide in nitrogen generates calcium cyanamide, which when moistened hydrolyzes to urea and ammonia. In the widely used Haber process, hydrogen (generated by heating natural gas) is mixed with nitrogen (from air), and burned to yield a nitrogen-hydrogen mixture. The nitrogen-hydrogen mixture is compressed (10–80 megapascals) and heated (200–700°C or 390–1300°F) in the presence of a metal oxide catalyst to give ammonia. The Haber process is the major source of ammonia used for fertilizer. See also Ammonia; Cyanamide; Electrochemical process; Fertilizer;
Biological fixation
Only prokaryotes—bacteria, archaea, and cyanobacteria (earlier called blue-green algae)—fix nitrogen. Nitrogen-fixing microbes, called diazotrophs, fall into two main groups, free-living and symbiotic. See also Archaebacteria; Bacteria; Cyanobacteria; Prokaryotae.
The free-living diazotrophs are subclassified. Aerobic diazotrophs, of which there are over 50 genera, including Azotobacter, methane-oxidizing bacteria, and cyanobacteria, require oxygen for growth and fix nitrogen when oxygen is present. Azotobacter, some related bacteria, and some cyanobacteria fix nitrogen in ordinary air, but most members of this group fix nitrogen only when the oxygen concentration is low. Free-living diazotrophs, which fix nitrogen only when oxygen is absent or vanishingly low, are widespread. The genera Bacillus and Klebsiella include many strains of this type, and representatives of symbiotic diazotrophs behave in this way as well. See also Algae; Bacterial physiology and metabolism.
The best-known symbiotic bacteria belong to the genus Rhizobium. Species of Rhizobium, or related genera, such as Bradyrhizobium and Sinorhizobium, colonize the roots of leguminous plants and stimulate the formation of nodules within which they fix nitrogen microaerobically. Both plants and bacteria show specificity; for example, certain types of plants require special strains of rhizobia. Some types of rhizobium, such as Bradyrhizobium, can fix nitrogen in the absence of plant tissue, but require low oxygen, though most rhizobia fix nitrogen only within the nodules. See also Soil microbiology.
The enzymes responsible for nitrogen fixation are called nitrogenases. The most common nitrogenase consists of two proteins, one large containing molybdenum, iron, and inorganic sulfur (the MoFe-protein or dinitrogenase), the other smaller containing iron and inorganic sulfur (the Fe-protein or dinitrogenase reductase). Nitrogenase reduces one molecule of N2 to two of ammonia (NH3), a reaction which is accompanied by the conversion of 16 molecules of adenosine triphosphate (ATP) to adenosine diphosphate (ADP) and the release of one molecule of H2 as a by-product. Nitrogenase is irreversibly destroyed by air, so all aerobic diazotrophs have developed means of restricting access of oxygen to the active enzyme.
| Geography Dictionary: nitrogen fixation |
The alteration of atmospheric, molecular nitrogen to nitrogen compounds. The fixation mechanisms responsible are: biological micro-organisms, such as those in the root nodules of leguminous plants, lightning and other natural ionizing processes, and industrial means. These processes are responsible per year, respectively, for 1, 2, and 8 billionths (10-9) of the mass of the reservoir of atmospheric nitrogen. The natural processes of denitrification cannot balance this rate of nitrification, so that the net amount of nitrogen in the atmosphere is, infinitely slowly, diminishing, and could be reduced by 50% in 250 million years.
| Britannica Concise Encyclopedia: nitrogen fixation |
For more information on nitrogen fixation, visit Britannica.com.
| Wikipedia: Nitrogen fixation |
Nitrogen fixation usually refers to the biological process by which nitrogen (N2) in the atmosphere is converted into ammonia.[1] This process is essential for life because fixed nitrogen is required to biosynthesize a basic building block of life, e.g. nucleotides for DNA and amino acids for proteins. Formally, nitrogen fixation also refers to other abiological conversions of nitrogen, such as its conversion to nitrogen dioxide.
Nitrogen fixation is utilized by numerous prokaryotes, including bacteria, actinobacteria, and certain types of anaerobic bacteria. Microorganisms that fix nitrogen are called diazotrophs. Some higher plants, and some animals (termites), have formed associations (symbiosises) with diazotrophs. Nitrogen fixation also occurs as a result of non-biological processes. These include lightning, industrially through the Haber-Bosch Process, and combustion.[2] Biological nitrogen fixation was discovered by the Dutch microbiologist Martinus Beijerinck.
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Biological nitrogen fixation (BNF) occurs when atmospheric nitrogen is converted to ammonia by an enzyme called nitrogenase.[1] The formula for BNF is:
The process is coupled to the hydrolysis of 16 equivalents of ATP and is accompanied by the co-formation of one molecule of H2. In free-living diazotrophs, the nitrogenase-generated ammonium is assimilated into glutamate through the glutamine synthetase/glutamate synthase pathway.
Enzymes responsible for nitrogenase action are very susceptible to destruction by oxygen. (In fact, many bacteria cease production of the enzyme in the presence of oxygen).[1] Many nitrogen-fixing organisms exist only in anaerobic conditions, respiring to draw down oxygen levels, or binding the oxygen with a protein such as Leghemoglobin.[1]
Plants that contribute to nitrogen fixation include the legume family – Fabaceae – with taxa such as clover, soybeans, alfalfa, lupines and peanuts. They contain symbiotic bacteria called Rhizobia within nodules in their root systems, producing nitrogen compounds that help the plant to grow and compete with other plants. When the plant dies, the fixed nitrogen is released, making it available to other plants and this helps to fertilize the soil[1][3] The great majority of legumes have this association, but a few genera (e.g., Styphnolobium) do not. In many traditional and organic farming practices, fields are rotated through various types of crops, which usually includes one consisting mainly or entirely of clover or buckwheat (family Polygonaceae), which were often referred to as "green manure."
Although by far the majority of nitrogen-fixing plants are in the legume family Fabaceae, there are a few non-leguminous plants that can also fix nitrogen. These plants, referred to as "actinorhizal plants", consist of 22 genera of woody shrubs or trees scattered in 8 plant families. The ability to fix nitrogen is not universally present in these families. For instance, of 122 genera in the Rosaceae, only 4 genera are capable of fixing nitrogen.
| Family: Genera
Betulaceae (Birch): Alnus (Alder) Casuarinaceae (she-oaks):
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Elaeagnaceae (oleaster):
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There are also several nitrogen-fixing symbiotic associations that involve cyanobacteria (such as Nostoc). These include some lichens such as Lobaria and Peltigera:
Cyanobacteria inhabit nearly all illuminated environments on Earth and play key roles in the carbon and nitrogen cycle of the biosphere. Generally, cyanobacteria are able to utilize a variety of inorganic and organic sources of combined nitrogen, like nitrate, nitrite, ammonium, urea or some amino acids. Several cyanobacterial strains are also capable of diazotrophic growth. Genome sequencing has provided a large amount of information on the genetic basis of nitrogen metabolism and its control in different cyanobacteria. Comparative genomics, together with functional studies, has led to a significant advance in this field over the past years. 2-oxoglutarate has turned out to be the central signalling molecule reflecting the carbon/nitrogen balance of cyanobacteria. Central players of nitrogen control are the global transcriptional factor NtcA, which controls the expression of many genes involved in nitrogen metabolism, as well as the PII signalling protein, which fine-tunes cellular activities in response to changing C/N conditions. These two proteins are sensors of the cellular 2-oxoglutarate level and have been conserved in all cyanobacteria. In contrast, the adaptation to nitrogen starvation involves heterogeneous responses in different strains.[4] Nitrogen fixation by cyanobacteria in coral reefs can fix twice the amount of nitrogen than on land – around 1.8kg of nitrogen is fixed per hectare per day.
Nitrogen can also be artificially fixed. for use in fertilizers, explosives, or in other products. The most common method is the Haber process. Artificial fertilizer production is now the largest source of fixed nitrogen in the Earth's ecosystem.[5]
The Haber process requires high pressures (around 200 atm) and high temperatures (at least 400 °C), routine conditions for industrial catalysis. This highly efficient process uses natural gas as a hydrogen source and air as a nitrogen source.
Much research has been conducted on the discovery of catalysts for nitrogen fixation, often with the goal of reducing the energy required for this conversion. Such research has thus far failed to even approach the efficiency and ease of the Haber process, however. Many compounds react with atmospheric nitrogen under ambient conditions. For example, lithium metal converts to lithium nitride under an atmosphere of nitrogen. Treatment of the resulting nitride gives ammonia.
The first dinitrogen complex was reported in 1965 based on ammonia coordinated to ruthenium ([Ru(NH3)5(N2)]2+),[6] research in chemical fixation focused on transition metal complexes. Since then a large number of transition metal compounds that contain dinitrogen as a ligand have been discovered. The dinitrogen ligand can be bound either to a single metal or bridge two (or more) metals. The coordination chemistry of dinitrogen is complex and currently under intense investigation. This research may lead to new ways of using dinitrogen in synthesis and on an industrial scale.
The first example of homolytic cleavage of dinitrogen under mild conditions was published in 1995. Two equivalents of a molybdenum complex reacted with one equivalent of dinitrogen, creating a triple bonded MoN complex[7]. Since this triple bounded complex has been used to make nitriles [8]. The first catalytic system converting nitrogen to ammonia at room temperature and pressure was discovered in 2003 and is based on another molybdenum compound, a proton source and a strong reducing agent.[9][10][11] Unfortunately, the catalytic reduction only fixes a few nitrogen molecules.
In contrast to the graphic shown above, the major product of this reaction is ammonia (NH3) and not an ammonium salt ([NH4][X]). In fact, approximately 75% of the ammonia produced can be distilled away from the reaction vessel (suggesting the ammonia is not protonated) into a vessel containing HCl as a trap. This method of trapping the NH3 was doubtlessly chosen because it makes the product easier to handle. Also, note that because only 1 equiv of Cl anion is available under catalytic conditions (via reduction of the precatalyst molybdenum chloride, shown) therefore it is unlikely that the product ammonium salt would always have this counterion.
Note also that although the dinitrogen complex is shown in brackets this species can be isolated and characterized. Here the brackets do not indicate that the intermediate is not observed.
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
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 | Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved. Read more |
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| Sci-Tech Encyclopedia. McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved. Read more |
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| Geography Dictionary. A Dictionary of Geography. Copyright © Susan Mayhew 1992, 1997, 2004. All rights reserved. Read more |
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| Britannica Concise Encyclopedia. Britannica Concise Encyclopedia. © 2006 Encyclopædia Britannica, Inc. All rights reserved. Read more |
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| Wikipedia. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article "Nitrogen fixation". Read more |
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