Nitrogen+Cycle

The **nitrogen cycle** is the process by which [|nitrogen] is converted between its various chemical forms. This transformation can be carried out by both biological and non-biological processes. Important processes in the nitrogen cycle include [|fixation], [|mineralization] , [|nitrification] , and [|denitrification]. The majority of [|Earth's atmosphere] (approximately 78%) is [|nitrogen], [|[1]] making it the largest pool of nitrogen. However, atmospheric nitrogen has limited availability for biological use, leading to a scarcity of usable nitrogen in many types of ecosystems. The nitrogen cycle is of particular interest to [|ecologists] because nitrogen availability can affect the rate of key ecosystem processes, including [|primary production] and [|decomposition]. Human activities such as fossil fuel combustion, use of artificial nitrogen fertilizers, and release of nitrogen in wastewater have dramatically altered the global nitrogen cycle. A 2011 study has cast doubt on the traditional model of the nitrogen cycle described below; nitrogen from rocks may also be a significant source not previously included. [|[2]]

__Nitrogen Cycle__ __(how it works)__

1.) Nitrogen fixing bacteria remove nitrogen gas from the air and convert it to ammonia.

N2---> NH3---H2O--->NH4+

2.) Nitrification occurs, which is the process of converting ammonia in the soil into the nitrate ion.

NH4+---bacteria in soil-->NO2---bacteria in soil-->NO3-

3.)Denitrification occurs, which is when the nitrates are converted back into nitrogen gas and released back into the atmosphere.

NO3bact. in soil-->NO---bact. in soil-->N2O---bact. in soil-->N2



Nitrogen fixation
Main article: [|Nitrogen fixation] Atmospheric nitrogen must be processed, or "fixed" (see page on [|nitrogen fixation] ), to be used by plants. 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 their own [|organic compounds]. Most biological nitrogen fixation occurs by the activity of Mo-nitrogenase, found in a wide variety of bacteria and some [|Archaea]. Mo-nitrogenase is a complex two component [|enzyme] that has multiple metal-containing prosthetic groups. [|[7]] 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 [|symbioses]. Today, about 30% of the total fixed nitrogen is manufactured in [|ammonia] chemical plants. [|[8]]

** Conversion of N2 **
The conversion of nitrogen (N2) from the atmosphere into a form readily available to plants and hence to animals is an important step in the nitrogen cycle, which distributes the supply of this essential nutrient. There are four ways to convert N2 (atmospheric nitrogen gas) into more chemically reactive forms: [|[5]]
 * 1) Biological fixation: some symbiotic bacteria (most often associated with leguminous plants) and some free-living bacteria are able to fix nitrogen as organic nitrogen. An example of mutualistic nitrogen fixing bacteria are the // [|Rhizobium] // bacteria, which live in [|legume] root nodules. These species are [|diazotrophs] . An example of the free-living bacteria is // [|Azotobacter] //.
 * 2) Industrial N-fixation: Under great pressure, at a temperature of 600 C, and with the use of an iron catalyst, hydrogen (usually derived from natural gas or petroleum) and atmospheric nitrogen can be combined to form ammonia (NH3) in the [|Haber-Bosch] process which is used to make fertilizer and explosives.
 * 3) Combustion of fossil fuels: automobile engines and thermal power plants, which release various nitrogen oxides (NOx).
 * 4) Other processes: In addition, the formation of NO from N2 and O2 due to photons and especially lightning, can fix nitrogen.

Assimilation
Plants take nitrogen from the soil, by absorption through 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]. 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. [|[5]] In plants that 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 obtain nitrogen by ingestion of [|amino acids], [|nucleotides] and other small organic molecules.

Environmental impacts
Additional risks posed by increased availability of inorganic nitrogen in aquatic ecosystems include water acidification; [|eutrophication] of fresh and saltwater systems; and toxicity issues for animals, including humans. [|[20]] Eutrophication often leads to lower dissolved oxygen levels in the water column, including hypoxic and anoxic conditions, which can cause cause death of aquatic fauna. Relatively sessile benthos, or bottom-dwelling creatures, are particularly vulnerable because of their lack of mobility, though large fish kills are not uncommon. Oceanic [|dead zones] near the mouth of the Mississippi in the Gulf of Mexico are a well-known examples of algal bloom-induced [|hypoxia]. [|[21]] [|[22]] The New York Adirondack Lakes, [|Catskills], Hudson Highlands, Rensselaer Plateau and parts of Long Island display the impact of nitric acid rain deposition, resulting in the killing of fish and many other aquatic species. [|[23]] Ammonia (NH3) is highly toxic to fish and the level of ammonia discharged from wastewater treatment facilities must be closely monitored. To prevent fish deaths, nitrification via [|aeration] prior to discharge is often desirable. Land application can be an attractive alternative to the aeration.