Nitrogen cycle
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Reverted edits by [[Special:Contributions/90.203.209.49|90.203.209.49]] ([[User talk:90.203.209.49|talk]]) to last version by Geonarva
[[Image:Nitrogen_Cycle.jpg|thumb|320px|right|Schematic representation of the flow of Nitrogen through the environment. The importance of bacteria in the cycle is immediately recognized as being a key element in the cycle, providing different forms of nitrogen compounds assimilable by higher organisms. See [[Martinus Beijerinck]].]]
The '''nitrogen cycle''' is the [[biogeochemical cycle]] that describes the transformations of [[nitrogen]] and nitrogen-containing compounds in nature. It is a [[Gas|gaseous]] cycle.
[[Earth's atmosphere]] is about 78% [[nitrogen]], making it the largest pool of nitrogen. Nitrogen is essential for many biological processes; and is crucial for any life here on Earth. It is in all [[amino acids]], is incorporated into [[proteins]], and is present in the bases that make up [[nucleic acids]], such as [[DNA]] and [[RNA]]. In [[plants]], much of the nitrogen is used in [[chlorophyll]] molecules which are essential for [[photosynthesis]] and further growth.<ref name=Smil>{{cite book |author=Smil, V |year=2000 |title=Cycles of Life |publisher=ScientificAmerican Library, New York}}, 2000)</ref>
Processing, or [[nitrogen fixation|fixation]], is necessary to convert gaseous nitrogen into forms usable by living organisms. Some fixation occurs in [[lightning]] strikes, but most fixation is done by free-living or [[symbiotic]] [[bacteria]].
These bacteria have the [[nitrogenase]] [[enzyme]] that combines gaseous nitrogen with [[hydrogen]] to produce [[ammonia]], which is then further converted by the bacteria to make its own [[organic compound]]s. Some nitrogen fixing bacteria, such as ''[[Rhizobium]]'', live in the root nodules of [[legumes]] (such as peas or beans). Here they form a [[mutualistic]] relationship with the plant, producing ammonia in exchange for [[carbohydrates]]. Nutrient-poor soils can be planted with legumes to enrich them with nitrogen. A few other plants can form such [[symbiosis|symbioses]]. Nowadays, a very considerable portion of nitrogen is fixated in [[ammonia]] chemical plants.
Other plants get nitrogen from the soil by absorption at their roots in the form of either [[nitrate]] [[ions]] or [[ammonium]] ions. All nitrogen obtained by [[animals]] can be traced back to the eating of plants at some stage of the [[food chain]].
Due to their very high [[solubility]], nitrates can enter groundwater. Elevated nitrate in groundwater is a concern for drinking water use because nitrate can interfere with blood-oxygen levels in infants and cause [[methemoglobinemia]] or blue-baby syndrome.<ref name=Vitousek>{{cite journal |author= Vitousek, PM; Aber, J; Howarth, RW; Likens, GE; Matson, PA; Schindler, DW; Schlesinger, WH; Tilman, GD |year=1997 |title= Human Alteration of the Global Nitrogen Cycle: Causes and Consequences |journal=Issues in Ecology |volume=1 |pages=1–17}}</ref> Where groundwater recharges stream flow, nitrate-enriched groundwater can contribute to [[eutrophication]], a process leading to high [[algal]], especially blue-green algal populations and the death of aquatic life due to excessive demand for oxygen. While not directly toxic to fish life like ammonia, nitrate can have indirect effects on fish if it contributes to this eutrophication. Nitrogen has contributed to severe eutrophication problems in some water bodies. As of 2006, the application of nitrogen [[fertilizer]] is being increasingly controlled in Britain and the United States. This is occurring along the same lines as control of phosphorus fertilizer, restriction of which is normally considered essential to the recovery of eutrophied waterbodies.
Ammonia is highly toxic to fish life and the water discharge level of ammonia from wastewater treatment plants must often be closely monitored. To prevent loss of fish, nitrification prior to discharge is often desirable. Land application can be an attractive alternative to the mechanical [[aeration]] needed for nitrification.
During [[Anaerobic environment|anaerobic]] (low oxygen) conditions, ''[[denitrification]]'' by bacteria occurs. This results in nitrates being converted to nitrogen gas and returned to the [[Earth's atmosphere|atmosphere]]. [[Nitrate]] can also be reduced to [[nitrite]] and subsequently combine with [[ammonium]] in the [[anammox]] process, which also results in the production of dinitrogen gas.
== Processes of the nitrogen cycle ==
===Nitrogen fixation ===
{{main|Nitrogen fixation}}
====Conversion of N<sub>2</sub>====
The conversion of nitrogen (N<sub>2</sub>) from the atmosphere into a form readily available to plants and hence to animals and humans is an important step in the nitrogen cycle, that determines the supply of this essential nutrient. There are four ways to convert N<sub>2</sub> (atmospheric nitrogen gas) into more chemically reactive forms:<ref name=Smil/>
# Biological fixation: some symbiotic bacteria (most often associated with leguminous plants) and some free-living bacteria are able to fix nitrogen and assimilate it as organic nitrogen. An example of mutualistic nitrogen fixing bacteria are the ''[[Rhizobium]]'' bacteria, which live in [[legume]] root nodules. These species are [[diazotroph]]s. An example of the free-living bacteria is ''[[Azotobacter]]''.
# Industrial N-fixation : in the [[Haber-Bosch]] process, N<sub>2</sub> is converted together with hydrogen gas (H<sub>2</sub>) into ammonia (NH<sub>3</sub>) which is used to make fertilizer and explosives.
# Combustion of fossil fuels : automobile engines and thermal power plants, which release various nitrogen oxides (NO<sub>x</sub>).
# Other processes : Additionally, the formation of NO from N<sub>2</sub> and O<sub>2</sub> due to photons and especially lightning, are important for atmospheric chemistry, but not for terrestrial or aquatic nitrogen turnover.
=== Assimilation ===
Plants can absorb nitrate or ammonium ions from the soil via their root hairs. If nitrate is absorbed, it is first reduced to nitrite ions and then ammonium ions for incorporation into amino acids, nucleic acids, and chlorophyll.<ref name=Smil/> In plants which have a mutualistic relationship with rhizobia, some nitrogen is assimilated in the form of ammonium ions directly from the nodules. Animals, fungi, and other [[heterotrophic]] organisms absorb nitrogen as [[amino acid]]s, [[nucleotide]]s and other small organic molecules.
=== Ammonification ===
When a plant or animal dies, or an animal excretes, the initial form of nitrogen is organic. Bacteria, or in some cases, fungi, converts the organic nitrogen within the remains back into ammonia, a process called ammonification or mineralization.
=== Nitrification ===
{{main|Nitrification}}
The conversion of ammonia to nitrates is performed primarily by soil-living bacteria and other nitrifying bacteria. The primary stage of nitrification, the oxidation of ammonia (NH<sub>3</sub>) is performed by bacteria such as the ''[[Nitrosomonas]]'' species, which converts ammonia to nitrites (NO<sub>2</sub><sup>-</sup>). Other bacterial species, such as the ''[[Nitrobacter]]'', are responsible for the oxidation of the nitrites into nitrates (NO<sub>3</sub><sup>-</sup>).<ref name=Smil/>
=== Denitrification ===
{{main|Denitrification}}
Denitrification is the reduction of nitrites back into the largely inert nitrogen gas (N<sub>2</sub>), completing the nitrogen cycle. This process is performed by bacterial species such as ''[[Pseudomonas]]'' and ''[[Clostridium]]'' in anaerobic conditions.<ref name=Smil/> They use the nitrate as an electron acceptor in the place of oxygen during respiration. These facultatively anaerobic bacteria can also live in aerobic conditions.
=== Anaerobic ammonium oxidation ===
{{main|Anammox}}
In this biological process, [[nitrite]] and [[ammonium]] are converted directly into [[dinitrogen]] gas. This process makes up a major proportion of dinitrogen conversion in the oceans.
==Human influences on the nitrogen cycle==
As a result of extensive cultivation of legumes (particularly soy, alfalfa, and clover), growing use of the [[Haber-Bosch process]] in the creation of chemical fertilizers, and pollution emitted by vehicles and industrial plants, human beings have more than doubled the annual transfer of nitrogen into biologically available forms.<ref name=Vitousek/> In addition, humans have significantly contributed to the transfer of nitrogen trace gases from [[Earth]] to the [[Earth's atmosphere|atmosphere]], and from the land to aquatic systems.
N<sub>2</sub>O has risen in the atmosphere as a result of agricultural fertilization, biomass burning, cattle and feedlots, and other industrial sources.<ref name=Chapin> Chapin, S.F. III, Matson, P.A., Mooney H.A. 2002. ''Principles of Terrestrial Ecosystem Ecology.'' Springer Publishers:New York</ref> N<sub>2</sub>O has deleterious effects in the [[stratosphere]], where it breaks down and acts as a [[catalyst]] in the destruction of atmospheric [[ozone]]. [[Ammonia]] (NH<sub>3</sub>) in the atmosphere has tripled as the result of human activities. It is a reactant in the atmosphere, where it acts as an [[aerosol]], decreasing air quality and clinging on to [[water]] droplets, eventually resulting in [[acid rain]]. [[Fossil fuel]] [[combustion]] has contributed to a 6 or 7 fold increase in NOx flux to the atmosphere. NOx actively alters [[atmospheric chemistry]], and is a precursor of [[troposphere|tropospheric]] (lower atmosphere) ozone production, which contributes to [[smog]], acid rain, and increases nitrogen inputs to ecosystems.<ref name=Smil/> [[Ecosystem]] processes can increase with nitrogen [[fertilization]], but [[anthropogenic]] input can also result in nitrogen saturation, which weakens productivity and can kill plants.<ref name=Vitousek/> Decreases in [[biodiversity]] can also result if higher nitrogen availability increases nitrogen-demanding grasses, causing a degradation of nitrogen-poor, species diverse heathlands.<ref>Aerts, R. and F. Berendse. 1988. The effect of increased nutrient availability on vegetation dynamics in wet heathlands. ''Vegetatio.'' 76: 63-69</ref>
===Wastewater===
[[Onsite sewage facilities]] such as septic tanks and holding tanks release large amounts of nitrogen into the environment by discharging through a [[drainfield]] into the ground. Microbial activity consumes the nitrogen and other contaminants in the wastewater. However, in certain areas the soil is unsuitable to handle some or all of the wastewater, and as a result, the wastewater with the contaminants enters the [[aquifers]]. These contaminants accumulate and eventually end up in drinking water. One of the contaminants concerned about the most is [[nitrogen]] in the form of [[nitrates]]. A nitrate concentration of 10 ppm or 10 milligrams per liter is the current EPA limit for drinking water and typical household wastewater can produce a range of 20-85 ppm (milligrams per liter).
The health risk associated with drinking >10 ppm nitrogen water is the development of [[methemoglobinemia]] and has been found to cause [[blue baby syndrome]]. Several states have now started programs to introduce [[advanced wastewater treatment systems]] to the typical onsite sewage facilities. The result of these systems is an overall reduction of nitrogen, as well as other contaminants in the wastewater.
==References==
{{reflist}}
==Bibliography==
*The Nitrogen Cycle, and New Tank Syndrome http://www.aquariumdomain.com/guideTheNitrogenCycle.asp, accessed 2006-07-16.
*Raven, P.H. and G.B. Johnson. 1996. ''Biology.'' Wm. C. Brown Publishers.
{{Biogeochemical cycle}}
[[Category:Nitrogen|Cycle]]
[[Category:Ecological cycles]]
[[Category:Soil biology]]
[[Category:Nitrogen metabolism]]
[[Category:Metabolism]]
[[Category:Biogeography]]
[[Category:Fishkeeping]]
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