Nitrogen cycle

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.

The nitrogen cycle is the process by which nitrogen is converted between into ammonia acids. They make proteins used by producers. This transformation can be carried out via 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 79%) is nitrogen,[1] making it the largest pool of nitrogen. However, atmospheric nitrogen is unavailable 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.

Contents

Human influences on the nitrogen cycle

As a result of extensive cultivation of legumes (bean plants), 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.[2] In addition, humans have significantly contributed to the transfer of nitrogen trace gases from Earth to the atmosphere, and from the land to aquatic systems. Human alterations to the global nitrogen cycle are most intense in developed countries and in Asia, where vehicle emissions and industrial agriculture are highest.[3]

N2O (nitrous oxide) has risen in the atmosphere as a result of agricultural fertilization, biomass burning, cattle and feedlots, and other industrial sources.[4] N2O has deleterious effects in the stratosphere, where it breaks down and acts as a catalyst in the destruction of atmospheric ozone. N2O in the atmosphere is a greenhouse gas, currently the third largest contributor to global warming, after carbon dioxide and methane. While not as abundant in the atmosphere as carbon dioxide, for an equivalent mass, nitrous oxide is nearly 300 times more potent in its ability to warm the planet.[5]

Ammonia (NH3) 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. NO2 actively alters atmospheric chemistry, and is a precursor of tropospheric (lower atmosphere) ozone production, which contributes to smog, acid rain, damages plants and increases nitrogen inputs to ecosystems.[6] 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.[7]

Wastewater treatment

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 (parts per million) 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.

The health risk associated with drinking water (with >10 ppm nitrate) 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.

Environmental damage

Additional risks posed by increases in fixed nitrogen in aquatic systems include spurring the creation and growth of eutrophic lakes and oceanic dead zones through algal bloom-induced hypoxia.[8][9]

The extent and effects of the anthropogenically-induced doubling of biologically available nitrogen in the soils, waters, and air of the earth during the past century are still poorly understood.[10]

References

  1. ↑ Steven B. Carroll; Steven D. Salt (2004). Ecology for gardeners. Timber Press. p. 93. ISBN 9780881926118. http://books.google.com/books?id=aM4W9e5nmsoC&pg=PA93. 
  2. ↑ 2.0 2.1 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. ↑ Holland, Elisabeth A.; Dentener, Frank J.; Braswell, Bobby H.; Sulzman, James M. (1999). "Contemporary and pre-industrial global reactive nitrogen budgets". Biogeochemistry 46: 7. doi:10.1007/BF01007572. 
  4. ↑ Chapin, S.F. III, Matson, P.A., Mooney H.A. 2002. Principles of Terrestrial Ecosystem Ecology. Springer, New York 2002 ISBN 0387954430, p.345
  5. ↑ Proceedings of the Scientific Committee on Problems of the Environment (SCOPE) International Biofuels Project Rapid Assessment, 22–25 September 2008, Gummersbach, Germany, R.W. Howarth and S. Bringezu, editors. 2009 Executive Summary, p. 3
  6. ↑ Smil, V (2004). Cycles Of Life: Civilization And The Biosphere. Diane Pub Co. ISBN 0756773504. 
  7. ↑ Aerts, Rien and Berendse, Frank (1988). "The Effect of Increased Nutrient Availability on Vegetation Dynamics in Wet Heathlands". Vegetatio 76 (1/2): 63. http://www.jstor.org/pss/20038308. 
  8. ↑ Rabalais, Nancy N., R. Eugene Turner, and William J. Wiseman, Jr. (2002). "Gulf of Mexico Hypoxia, aka "The Dead Zone"". Ann. Rev. Ecol. Sys. 33: 235–63. doi:10.1146/annurev.ecolsys.33.010802.150513. http://www.jstor.org/pss/3069262. 
  9. ↑ Dybas, Cheryl Lyn. (2005). "Dead Zones Spreading in World Oceans". BioScience 55 (7): 552–557. doi:10.1641/0006-3568(2005)055[0552:DZSIWO]2.0.CO;2. 
  10. ↑ Hager, Thomas. 2008. The Alchemy of Air: A Jewish Genius, a Doomed Tycoon, and the Scientific Discovery that Fed the World but Fueled the Rise of Hitler. Harmony Books, New York ISBN 0307351793
Biogeochemical cycles
Carbon cycle - Hydrogen cycle - Nitrogen cycle - Oxygen cycle - Phosphorus cycle - Sulfur cycle - Water cycle - Mercury cycle