The Nitrogen Cycle

The element nitrogen is essential to living organisms. Nitrogen moves through the different ecosystems by the way of the nitrogen cycle. Plants and microorganisms assist nitrogen on its journey through the nitrogen cycle (Gruber and Galloway 2008, 293). In nature a limited number of bacteria species and blue-green algae have the ability to biologically fix nitrogen. These microorganisms transform nitrogen (N)2 to ammonium. Lightning has the ability to fix nitrogen. It converts N2 to nitric oxide (NO). Nitrate is then produced when it rains (Kinzig and Socolow 1994, 24). The Nitrogen Cycle and Human Impact

Humans have impacted biogeochemical cycles with their activities, most notably the carbon and nitrogen cycle. The following sources for reactive nitrogen can lead to an extra influx of fixed nitrogen into the environment: industrial fertilizer, fossil fuel combustion, the increasing human population, and livestock production (Hessen et al. 1997, 321). With the use of nitrogen in the agriculture process to increase crop production mankind has greatly altered the nitrogen cycle (Gruber and Galloway 2008, 293). Nitrogen fertilizers work by fixing atmospheric nitrogen so crops can consume the fixed form (Hessen et al. 1997, 321). The Haber-Bosch process is the common name for this procedure, with the process being first industrially produced in the early twentieth century (Jenkinson 2001, 3). With the world’s population increasing the demand for agriculture products will only continue to keep rising (Hessen et al. 1997, 321).

Agriculture impact. With regards to agriculture, humans have impacted the nitrogen cycle in a few ways. One such way is planting more leguminous crops such as soybeans and alfalfa. Nitrogen fixation rates are increased by these crops (Kinzig and Socolow 1994, 27). In Brazil, soybeans do not normally require nitrogen fertilizer as they do in the United States or Europe. Soybeans in that area have the ability to acquire close to seventy to eighty-five percent of the necessary nitrogen through biological fixation. Another way agriculture and the nitrogen cycle has been impacted by mankind is through deforestation with the slash-and-burn practice. In the tropics, forested land is converted for crop purposes (Filoso et al. 2006, 62-65).

Feed production is a fast growing live-stock production area associated with deforestation. Land is required to grow the crops, like soybeans, to feed the livestock (Steinfeld and Wassenaar 2007, 282). Nitrogen mineralization rates and mobilization in soil have increased with deforestation. Also, increasing the rate of nitrogen being lost to streams (Filoso et al. 2006, 62). Pastures are burned to help maintain grasslands, which is called savannah burning. One type of gas that is emitted during savannah burnings is nitrogen oxides (NOX), which is a greenhouse gas (Steinfeld and Wassenaar 2007, 274). It is believed that agriculture produces a large quantity of NH3 or ammonia gas. NH3 can be trapped or released by plants and soil (Jenkinson 2001, 4)

Livestock production impact. Livestock production by human beings has also played a role in altering the nitrogen cycle. Animals return nitrogen back to the environment when they defecate. Reactive nitrogen has the ability to return to the plant production cycle if it is dropped on a crop or pasture field. Animal manure is also used as a crop fertilizer. When manure is used as fertilizer it impacts the nitrogen cycle in the following ways: N2O is added to the atmosphere, the volatilization and depositing of NH3, and water system eutrophication (Steinfeld and Wassenaar 2007, 278).

Nitrogen movement or loss. Nitrogen moves through the terrestrial nitrogen cycle by the following ways: with assistance from dying plants and microorganisms, by mineralization, by plants taking in inorganic nitrogen or the uptake of inorganic nitrogen by microorganism. Fixed nitrogen has the ability to move through different ecosystems and be lost from a system (Kinzig and Socolow 1994, 25). An ecosystem can lose nitrogen by nitrate leaching, dissolved organic nitrogen, and ammonium (Jenkinson 2001, 8). Nitrogen also has the ability to move through the system through emitted gases or by erosion (Steinfeld and Wassenaar 2007, 278). When nitrification occurs, nitrogen can be lost as NO and N2O. As denitrification occurs, nitrogen can be lost as N2O and N2.

With fertilizers that contain urea and ammonium, nitrogen lose can occur through volatilization. Due to the short time period that ammonia stays in the atmosphere from the moment it is emitted, the majority returns close to the point of origin (Jenkinson 2001, 4). During fossil fuel combustion, nitrogen is fixed and it rises up to the atmosphere. Wet and dry deposits of nitrate and nitric acid can occur because of this (Kinzig and Socolow 1994, 27). In part due to agriculture production, precipitation with nitrate and/or ammonium has increased (Jenkinson 2001, 5).

Through groundwater or surface runoff, nitrogen has the ability to cross over through different ecosystems (Kinzig and Socolow 1994, 25). Inorganic and organic forms of nitrogen can be left in soil by nitrogen based fertilizers. Leaching of nitrate is one possible way that nitrogen can be lost. When nitrates are leached they can pass into groundwater sources and shallow seas (Jenkinson 2001, 3-11). Nitrogen runoff can lead to the acidification of freshwater. It also can cause eutrophication of coastal waters and freshwater sources (Hessen et al. 1997, 323-324). When eutrophication occur the number of algae blooms can increase and decrease the water’s dissolved oxygen content. This can greatly decrease the water quality. Eutrophication caused by nitrogen fertilizer has impacted the water quality for waters such as Chesapeake Bay, the Baltic Sea, the North Sea, and parts of the western Mediterranean Sea. Another consequence to the increased nitrogen fixation may be that different growth of species can occur due to their ability to adapt the extra nitrogen in the environment (Kinzig and Socolow 1994, 24-28).

Fossil fuel impact. Each and every day human beings are affecting the nitrogen cycle by driving an automobile. Livestock and agriculture production are also associated with fossil fuel combustion. Trucks, tractors, and other combustion engines are used for livestock or agriculture purposes. Trucks and tractors are used for the transportation of livestock, feed, and crops that are produced (Steinfeld and Wassenaar 2007, 281). When fossil fuels are combusted, they release nitrogen gases or nitrous oxides to the atmosphere (Gruber and Galloway 2008, 293). As human activity has increased, so has the production of NH3, N20, NO, and NO2 (Jenkinson 2001, 4). Nitrous oxides are environmental concerns due to being greenhouse gases and can lead to acid rain (Kinzig and Socolow 1994, 28). Conclusion

With the use of the Haber-Bosch process to industrialize ammonia for fertilizer, mankind has made a dramatic impact on the global nitrogen cycle. The agriculture industry along with livestock production introduces the largest amount of fixed nitrogen into the environment. This use of fertilizer can lead to water quality changes such as freshwater acidification and eutrophication of coast waters near rivers. Mankind’s uncontrollable rate of fossil fuel combustion also has an impact on the nitrogen cycle with nitrous oxides being released to the atmosphere. These gases are considered greenhouse gases, and they also can lead to acid rain precipitation.

Annotated Bibliography

Jenkinson, D. S. 2001. The impact of humans on the nitrogen cycle, with focus on temperate arable agriculture. Plant and Soil 228:3-15. http://0-link.springer.com.bianca.penlib.du.edu/content/pdf/ (accessed January 6, 2013). Gruber, Nicolas and James N. Galloway. 2008. An Earth-system perspective of the global nitrogen cycle. Nature 451:293-296. http://0-search.proquest.com.bianca.penlib.du.edu/docview/204550397 (accessed January 6, 2013). Knowles, Roger. 2005. Denitrifers associated with methanotrophs and their potential impact on the nitrogen cycle. Ecological Engineering 24:441-446. http://ac.els-cdn.com/S0925857405000170/1-s2.0-S0925857405000170-main.pdf?_tid=fef58212-5939-11e2-b2b2-00000aab0f27&acdnat=1357611820_6300a5bbe3a6a0261387a48e79e4063c (accessed January 6, 2013). Kinzig, Ann P. and Robert H. Socolow. 1994. Human impacts on the nitrogen cycle. Physics Today 47, no. 11 (November): 24-31. http://0-web.ebscohost.com.bianca.penlib.du.edu/ehost/detail?sid=28c1c9aa-3769-428d-85af-0d781df05bb4%40sessionmgr4&vid=1&hid= 17&bdata=JnNpdGU9ZWhvc3QtbGl2ZQ%3d%3d#db=a9h&AN=9411172429 (accessed January 6, 2013). Hessen, Dag O., Arne Henriksen, Atle Hindlar, Jan Mulder, Kjetil Torseth and Nils Vagstad. 1997. Human impacts on the nitrogen cycle: A global problem judged from a local perspective. Ambio 26, no. 5 (August): 321-325. http://0-www.jstor.org.bianca.penlib.du.edu/stable/pdfplus /4314610.pdf?acceptTC=true (accessed January 7, 2013). Filoso, Solange, Luiz Antonio Martinelli, Robert W. Howarth, Elizabeth W. Boyer, and Frank