Nitrogen cycle



 

The nitrogen cycle is the nitrogen and nitrogen-containing compounds in nature.

photosynthesis and further growth.[1]

Processing, or carbohydrates. Nutrient-poor soils can be planted with legumes to enrich them with nitrogen. A few other plants can form such symbioses.

Other plants get nitrogen from the soil by absorption at their roots in the form of either 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 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 anammox process, which also results in the production of dinitrogen gas.

Processes of the nitrogen cycle

Nitrogen fixation

Main article: Nitrogen fixation

Conversion of N2

The conversion of nitrogen (N2) 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 N2 (atmospheric nitrogen gas) into more chemically reactive forms:[1]

  1. 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 diazotrophs. An example of the free-living bacteria is Azotobacter.
  2. Industrial N-fixation : in the Haber-Bosch process, N2 is converted together with hydrogen gas (H2) into ammonia (NH3) fertilizer and explosives.
  3. Combustion of fossil fuels : automobile engines and thermal power plants, which release NOx.
  4. Other processes : Additionally, the formation of NO from N2 and O2 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.[1] 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 nucleotides 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 article: 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 (NH3) is performed by bacteria such as the Nitrosomonas species, which converts ammonia to nitrites (NO2-). Other bacterial species, such as the Nitrobacter, are responsible for the oxidation of the nitrites into nitrates (NO3-).[1]

Denitrification

Main article: Denitrification

Denitrification is the reduction of nitrites back into the largely inert nitrogen gas (N2), completing the nitrogen cycle. This process is performed by bacterial species such as the Pseudomonas and Clostridium in anaerobic conditions.[1] 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 article: Anammox

In this biological process, 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 atmosphere, and from the land to aquatic systems.

N2O has risen in the atmosphere as a result of agricultural fertilization, biomass burning, cattle and feedlots, and other industrial sources.[3] N2O has deleterious effects in the stratosphere, where it breaks down and acts as a tropospheric (lower atmosphere) ozone production, which contributes to smog, acid rain, and increases nitrogen inputs to ecosystems.[1] Ecosystem processes can increase with nitrogen fertilization, but anthropogenic input can also result in nitrogen saturation, which weakens productivity and can kill plants.[2] Decreases in biodiversity can also result if higher nitrogen availability increases nitrogen-demanding grasses, causing a degradation of nitrogen-poor, species diverse heathlands.[4]

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

  1. ^ a b c d e f Smil, V (2000). Cycles of Life. ScientificAmerican Library, New York. , 2000)
  2. ^ a b c Vitousek, PM; Aber, J; Howarth, RW; Likens, GE; Matson, PA; Schindler, DW; Schlesinger, WH; Tilman, GD (1997). "Human Alteration of the Global Nitrogen Cycle: Causes and Consequences". Issues in Ecology 1: 1-17.
  3. ^ Chapin, S.F. III, Matson, P.A., Mooney H.A. 2002. Principles of Terrestrial Ecosystem Ecology. Springer Publishers:New York
  4. ^ Aerts, R. and F. Berendse. 1988. The effect of increased nutrient availability on vegetation dynamics in wet heathlands. Vegetatio. 76: 63-69

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 cycles
Hydrogen cycle - Nitrogen cycle
Water cycle
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Nitrogen_cycle". A list of authors is available in Wikipedia.