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[[Image:Nitrogen cycle.jpg|thumb|right|420px|alt=Diagram of a volcano and fields and pollution showing the different stages of nitrogen.|The nitrogen cycle is a complex system caused by natural processes (blue arrows) and activities by humans (red arrows).]]
The '''nitrogen cycle''' is a complex process involving the transformation and movement of [[nitrogen]] into the Earth's [[atmosphere]], by rainfall into the [[soil]], and back again by means of many different chemical processes. It is necessary for life to function since it's a part of [[amino acids]], [[proteins]], and [[nucleic acids]] like [[DNA]] and[[RNA]], and is a necessary element for [[photosynthesis]] in plants.<ref name=twsMAR26b>{{cite news
 |title= The Nitrogen Cycle
 |publisher= Kimball
 |quote= All life requires nitrogen-compounds, e.g., proteins and nucleic acids.* Air, which is 79% nitrogen gas (N2), is the major reservoir of nitrogen.* But most organisms cannot use nitrogen in this form.* Plants must secure their nitrogen in "fixed" form, i.e., incorporated in compounds such as: nitrate ions (NO3−)  ammonia (NH3) urea (NH2)2CO * Animals secure their nitrogen (and all other) compounds from plants (or animals that have fed on plants). 
 |date= 2010-03-26
 |url= http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/N/NitrogenCycle.html
 |accessdate= 2010-03-26
}}</ref> While nitrogen is abundant in [[air]] and prevents oxygen from [[combustion]], the air-born nitrogen is generally not useful in its gaseous state to living beings, and it must be transformed to enable its usefulness. Since nitrogen is [[inert]], it requires considerable [[energy]] to get the nitrogen out of the air.<ref name=twsMAR26c>{{cite news 
 |title= CHM 110 - CHEMISTRY AND ISSUES IN THE ENVIRONMENT
 |publisher= Elmhurst
 |quote=Nitrogen will only react with oxygen in the presence of high temperatures and pressures found near lightning bolts and in combustion reactions in power plants or internal combustion engines. Nitric oxide, NO, and nitrogen dioxide, NO2, are formed under these conditions. Eventually nitrogen dioxide may react with water in rain to form nitric acid, HNO3. The nitrates thus formed may be utilized by plants as a nutrient.
 |date= 2010-03-26
 |url= http://www.elmhurst.edu/~chm/onlcourse/chm110/outlines/nitrogencycle.html
 |accessdate= 2010-03-26
}}</ref> This occurs naturally as well as by human activity. Nitrogen is an important element in the production of [[food]].

On Earth, [[atom]]s of the [[chemical element]], [[nitrogen]] (atomic symbol, N; number of [[proton]]s, 7; [[Atomic mass|standard atomic weight]], 14.0067) &mdash; an essential element for Earth's [[Life|living systems]] &mdash; undergo cyclical movement through the [[atmosphere]], the crust ([[lithosphere]]), [[water]] compartments ([[hydrosphere]]),  living systems, and non-living organic matter, in a process called the <b>nitrogen cycle</b>, or more specifically, the biogeochemical nitrogen cycle.<ref name=nasa1>[http://soil.gsfc.nasa.gov/NFTG/nitrocyc.htm The Nitrogen Cycle: Nitrogen Transformations in Soil, Water, and Air.]</ref> It's a complex process influenced by [[nature|natural reactions]] as well as man-made activity.<ref name=twsMAR26a>{{cite news
 |title= The Nitrogen Cycle: Nitrogen Transformations in Soil, Water, and Air
 |publisher= Soil Science Education Home Page (via NASA)
 |date= 2004
 |url= http://soil.gsfc.nasa.gov/NFTG/nitrocyc.htm
 |accessdate= 2010-03-26
}}</ref>

When animals defecate or when plants decompose, they produce manure, sewage waste, compost, rotting leaves and sticks and other matter, and the result is waste that contains [[organic nitrogen]]. The result is organic soil material called [[humus]]. In addition, [[inorganic nitrogen]] comes from different sources as well; for example, certain [[minerals]] have nitrogen, as well as nitrogen coming from [[precipitation]] such as [[rain]] or [[snow]], and [[fertilizer]]s have nitrogen as well. This added nitrogen in the soil makes it easier for living [[plant (biology)|plants]] to grow healthy. While plants can't use some types of organic nitrogen, it's necessary for [[microbes]] living in the soil to convert the organic nitrogen into inorganic nitrogen which enables the plants to use it for their benefit.<ref name=twsMAR26a/>

Plants use a variety of inorganic or "fixed" nitrogen, or have their nitrogen incorporated into [[compounds]] such as nitrate ions, such as:
* Soil-based inorganic nitrogen such as [[ammonium]] NH4+
* [[Urea]] (NH2)2CO
* [[Nitrate]] NO3- (or nitrate ions)<ref name=twsMAR26b/>
* [[Nitrite]] or NO3-<ref name=twsMAR26a/><ref name=twsMAR26b>
Animals get their necessary nitrogen from eating plants. Getting nitrogen out of the atmosphere is key for food production. In 1908, Haber pioneered a way to get nitrogen out of the atmosphere to create a better fertilizer, and the innovation was called the Haber-Bosch process. It produces liquid ammonia which is the raw material for nitrogen fertilizer. Today fertilizer factories pour out 100 million tons of nitrogen annually, allowing an estimated two billion more people have food. But this radical change has been a cause for concern.<ref name=twsMAR26d/>

Some nitrates and nitrites can leach out into the soil into the [[groundwater]] and change into nitrogen gases such as N2, NO, N20, and ammonia gas (NH3). When they escape out of the soil, it is called [[volatizize|volatizing]]. It goes into the [[atmosphere]] of the [[Earth (planet)|Earth]].<ref name=twsMAR26a/>

==Different steps in the nitrogen cycle==
* [[Ammonification]] is when nitrogen in organic form is converted by [[microorganisms]] into ammonium or NH4+. It has a positive charge. This lets it become [[adsorption|adsorbed]] and [[fixation (chemistry)|fixated]] to the negatively charged soil particles, or be taken in by plants. When a plant or animal dies or makes waste, the nitrogen is organic and must be changed by either bacteria or [[fungi]] into ammonium (NH4+), a process called ammonification or mineralization. Some microorganisms are not properly called bacteria but are called [[archaea]], which have a genetic makeup similar to humans, and are classified as [[eukaryotes]], and they sometimes have the ability to live in "extreme" environments such as inside the mouths of active volcanoes, or in extreme cold such as [[Antarctica]], or in very acidic or very saline environments (eg the [[Dead Sea]].)

* [[Decay]] is a process of transforming excrement which contains organic nitrogen by microorganisms; the molecules are broken down, so that excretions and dead organisms turn into ammonia.
 
* [[Denitrification]] is when microbes transform nitrogen in both nitrate and nitrite form into gases called [[nitrous oxide]] and [[nitrogen gas]]. For this to happen, it requires soils to be extremely wet, or poorly [[drainage|drained]]. This is why [[wetlands]] are valuable since they contribute an important role in the nitrogen cycle. The resulting gases ''volatize'' into the atmosphere, but then they're no longer available for use by plants. Denitrification involves the reduction of nitrates back into the largely [[inert]] nitrogen gas (N2), completing the nitrogen cycle. This process is performed by bacteria like [[Pseudomonas]] and [[Clostridium]] in anaerobic conditions.

* [[Erosion]] is when the soil is worn away by running [[water]], [[wind]], [[ice]], [[gravity]], human activity such as [[bulldozers]], or by activities such as [[earthquakes]] or [[tsunamis]] or [[tornadoes]] or other geological processes. When the soil is moved by these forces, nitrogen in both organic and inorganic form in the soil are carried along.

* [[Fixation]], sometimes better known as [[Nitrogen fixation]], is when atmospheric nitrogen is changed or ''fixed'' to make it useful by plants. It can happen by:
::#[[Lightning]] strikes, which have enormous energy to break down inert nitrogen molecules, allowing them to combine with oxygen making nitrogen oxides; these can dissolve in rain, forming "nitrates", that are carried by gravity to the earth.
::#[[Symbiotic bacteria]] which have the nitrogenase [[enzyme]] that combines gaseous nitrogen with [[hydrogen]] to produce [[ammonia]], which is then further converted by the bacteria to make their organic compounds needed by the bacterium for internal processes. Some nitrogen fixing bacteria, such as [[Rhizobium]], live in the root nodules of [[legumes]] and work with the plant as a form of [[symbiosis]] and produce [[ammonia]] in exchange for [[carbohydrates]]. If a soil is lacking valuable nutrients, it's possible to plant legumes to add the nitrogen quotient.<ref name=twsMAR26b/> It's possible for nitrogen fixing bacteria to take nitrogen from the air and convert it into ammonia or NH3. It goes into a variety of amino acids and helps make proteins.<ref name=twsMAR26c/>
::#[[Industrial fixation]] is a third way. Under great pressure at 600°C, and using a [[catalyst]], atmospheric nitrogen and hydrogen (usually derived from [[natural gas]] or [[petroleum]]) can be combined to form ammonia (NH3) which can be used directly as fertilizer, but for economic reasons, mostly it's processed further to yield urea and ammonium nitrate (NH4NO3).<ref name=twsMAR26b/>

* [[Haber Process]] is a synthetic process in which nitrogen and hydrogen react under great pressure and temperator in the presence of a catalyst to make ammonia.<ref name=twsMAR26c/> It was named after [[Fritz Haber]], a [[Nobel]] laureate who helped transform food production worldwide.

* [[Immobilization]] is when inorganic forms of nitrogen are converted into organic forms as microbes, and when plants die. It's the opposite of [[mineralization]]. When nitrogen enters the soil in mineral form -- that is when it's not from a plant or animal source -- it's inorganic.

* [[Inorganic nitrogen]] is added to the soil by rain or snowfall or as fertilizers. This is why it's better to have actual rain water be the water source for indoor plants, since they get a generous helping of nitrogen as a result; using tap water usually doesn't bring this added benefit. In the soil, microorganisms convert organic nitrogen into inorganic forms, and this process is known as [[biological nitrogen fixation]]. Afterwards, the inorganic nitrogen is ready to be used by plants.

* [[Leaching]] is when nitrogen, when it's in the form of nitrate, is negatively charged. As a result, it's not attracted to the negatively charged [[clay]] and [[humus]] in the soil, that is, it's repelled, and the nitrogen won't be absobed by the clay. Rather, it moves down through the soil into the groundwater where streams and drinking water can become [[contamination|contaminated]]. Ammonia is highly [[toxicity|toxic]] to [[fish]] and, as a result, it's important to monitor the amount of ammonia in [[sewage treatment]]. One way to prevent this is to conduct [[nitrification]] before discharging materials downstream.

* [[Mineralization]] can happen to nitrogen, like it can happen to any element, when it's converted from an organic to an inorganic form by microbes, and can be used by plants for growing.

* [[Nitrification]] happens under certain conditions. Microbes in the soil use [[ammonium nitrate]] in the soil for energy and therefore [[oxidation|oxidize]] ammonium nitrogen first into Nitrite Nitrogen (NO2-) and then into Nitrate Nitrogen (NO3-). It's primarily done by bacteria in the soil such as [[Nitrosomonas]]. It's very important for nitrites to be converted to nitrates because the former are toxic to plants; if nitrites keep building up, it will eventually harm plant life. Because nitrates are highly soluble, it's easy for them to enter the groundwater, but it can affect drinking water; it's more dangerous for young human babies because it can interfere with their blood-oxygen levels and cause a condition known as [[methemoglobinemia]]. Also, when there is too much nitrates in streams, it can lead to excessive growth of [[algae]] which can lead to the death of aquatic life such as fishes because of the algae's excessive demand for oxygen.

* [[Organic nitrogen]] can't be used by plants. But it's formed when living matter dies, such as plant roots, leaves, sticks, animals including humans and insects. It's also formed when animals excrete and make manure, compost, and sewage sludge. The process of [[decomposition]] is when organic nitrogen is broken down into inorganic nitrogen by microbes.
[[Image:Nitrogen Cycle by T Sulcer.jpg|thumb|left|420px|alt=Diagram.|The nitrogen cycle is essential to living beings.]]

* [[Runoff]] happens when soil becomes so wet that it can no longer hold water; the water ''runs off'' the surface of the soil. If the soil has a protective layer of grasses or trees with strong roots, then the runoff will be mostly water; but if there isn't a protective covering, the runoff can [[erosion|erode]] substantial amounts of soil. This process can take both forms of nitrogen along with it -- both inorganic and organic. Sometimes this causes pollution and ends up in streams, lakes, rivers, reservoirs, and bays, and it can cause harm to [[aquatic]] life such as [[fish|fishes]].

* [[Uptake]] is the opposite of runoff. It's sometimes called [[assimilation (ecology)|assimilation]]. It happens when plant roots absorb inorganic nitrogen and other ingredients in the soil, and this helps the plants grow. Some plants get nitrogen from the soil, and by absorption of their roots in the form of either nitrate ions or ammonium ions. When animals eat plants, they get nitrogen via this pathway. Some plants can absorb nitrate or ammonium ions from the soil via their root hairs.

* [[Volatilization]] is when nitrogen is moved out of the soil and into the Earth's atmosphere. It becomes nitrogen in the form of a gas such as NH3, NO, N2O, or N2.<ref name=twsMAR26a/>

{| class="wikitable sortable"
|-
|+ '''Nitrogen cycle processes (N=nitrogen)'''
|-
!'''This process...'''
!'''turns these things...'''
!'''into these things'''
|-
|Ammonification<!---process--->
|Organic N<!---before--->
|Ammonium N<!---after--->
|-
|Denitrification<!---process--->
|Nitrate N or nitrite N<!---before--->
|Nitrous oxides and nitrogen gas<!---after--->
|-
|Erosion<!---process--->
|Organic or inorganic N (on soil particles)<!---before--->
|Surface water or other parts of the landscape<!---after--->
|-
|Immobilization<!---process--->
|Inorganic N (ammonium N, nitrate N)<!---before--->
|Organic N<!---after--->
|-
|Leaching<!---process--->
|Nitrate N<!---before--->
|Groundwater<!---after--->
|-
|Mineralization<!---process--->
|Organic N<!---before--->
|Inorganic N (ammonium N, nitrate N)<!---after--->
|-
|Nitrification<!---process--->
|Ammonium N<!---before--->
|Nitrite N and then to nitrate N<!---after--->
|-
|Runoff<!---process--->
|Nitrate N or ammonium N<!---before--->
|Surface water<!---after--->
|-
|Uptake<!---process--->
|Nitrate N or ammonium N<!---before--->
|Plant roots<!---after--->
|-
|Volatilization<!---process--->
|Ammonium N<!---before--->
|Ammonia N (gas form)<!---after--->
|-
|}
<small>Source: The Nitrogen Cycle: Nitrogen Transformations in Soil, Water, and Air by Soil Science Education Home Page (via NASA)<ref name=twsMAR26a/></small>

==Impact of humans==
There is concern that a number of activities by humans have influenced the nitrogen cycle, and it is not clear what ramifications these changes have had or will continue to have. Of interest is the growing of certain types of plants such as [[soy]],<ref name=twsMAR26b/> [[alfalfa]]<ref name=twsMAR26b/> and [[clover]], the use of chemical fertilizers in soil to enhance crop yields, and pollution emitted by cars and trucks and planes and industrial activity. There is speculation that N2O, or [[nitrous oxide]] can have negative affects in the atmosphere; this is believed to be exacerbated by agricultural use of fertilizers, the burning of biomass and cattle and feedlots, and industrial output. There is concern that nitrous oxide can have a negative affect on the atmospheric layer known as [[ozone]], since the ozone layer is believed to shield the earth from dangerous rays from the [[sun]]. Further, changes in the cycle may have resulted in more ammonia in the air which can decrease air quality and cling to water droplets, possibly resulting in [[acid rain]]. Last, there is concern that changes in the cycle have led to contamination by certain areas of nitrates, with speculation about the safety of drinking water, that is, they pollute drinking water.<ref name=twsMAR26d>{{cite news
 |author= Dan Charles
 |title= The Tragedy of Fritz Haber: Nobel Laureate Transformed World Food Production, War 
 |publisher= NPR
 |quote= A young, high-strung German chemist named Fritz Haber rose to the challenge. Around 1908, he discovered a way to tap into the atmosphere's vast reservoir of nitrogen gas and convert it into compounds plants can use. The innovation, called the Haber-Bosch process, produces liquid ammonia, the raw material for making nitrogen fertilizer.
 |date= July 11, 2002
 |url= http://www.npr.org/programs/morning/features/2002/jul/fritzhaber/
 |accessdate= 2010-03-26
}}</ref>

There is a movement among [[biogeochemists]] to see the nitrogen cycle as it relates to the cycles of other compounds in the earth, such as the [[water cycle]], or the cycling of other chemical compounds.<ref name=twsMAR26e>{{cite news
 |title= Earth's Cycles, Once in Concert, Falling Out of Sync
 |publisher= US News
 |date= August 5, 2009
 |url= http://www.usnews.com/science/articles/2009/08/05/earths-biogeochemical-cycles-once-in-concert-falling-out-of-sync.html
 |accessdate= 2010-03-26
}}</ref> A reporter explained:
<blockquote>A biogeochemical cycle is a pathway by which a chemical element, such as carbon, or compound, like water, moves through Earth's biosphere, atmosphere, hydrosphere and lithosphere. In effect, the element is "recycled," although in some cycles the element is accumulated or held for long periods of time. Chemical compounds are passed from one organism to another, and from one part of the biosphere to another, through biogeochemical cycles. Water, for example, can go through three phases (liquid, solid, gas) as it cycles through the Earth system. It evaporates from plants as well as land and ocean surfaces into the atmosphere and, after condensing in clouds, returns to Earth as rain and snow. Researchers are discovering that biogeochemical cycles--whether the water cycle, the nitrogen cycle, the [[carbon cycle]], or others--happen in concert with one another. Biogeochemical cycles are "coupled" to each other and to Earth's physical features.<ref name=twsMAR26e/></blockquote>

==Notes==
''View images of nitrogen cycle by right-clicking these links [http://soil.gsfc.nasa.gov/NFTG/nitrocyc.htm link1] [http://www.britannica.com/EBchecked/topic-art/416271/118424/The-nitrogen-cycle link2] & selecting 'open in new tab'.''

==See also==
* [[Water cycle]]
* [[Carbon cycle]]

==References==
{{reflist}}