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Nutrient Pollution

There are several biogeochemical cycles, or biochemical cycles that operate on the global scale, that are of concern: Examples include the water cycle, the carbon cycle, the nitrogen cycle, and the phosphorous cycle.

Nitrogen

The nitrogen cycle may be of particular global concern. Since the advent of the Haber Bosch process, the flow of reactive nitrogen (Nr, as opposed to unreactive N2 that constitutes most of the atmosphere) has greatly increased beyond natural levels.

Sources of reactive nitrogen have greatly increased with artificial sources. Source: Fowler et al. [2]. As observed by Galloway et al. [3], natural sources of Nr have decreased since the Industrial Revolution, but this should offset only a small portion of artificial production.

Much of the excess nitrogen is "denitrified", or converted back to N2 and returned to the atmosphere. The excess flow through the terrestrial environment has an effect on species composition. Excess reactive nitrogen in the ocean can be sequestered and released as the greenhouse gas N2O, the long-term consequences of which are unclear [2].

Locally, nitrogen pollution causes eutrophication, the process whereby nitrogen fertilizers algae growth in a body of water, depleting oxygen and causing death of other organisms, as well as other harmful impacts. Monetized damages from nitrogen pollution in the European Union have been estimated as follows.

Estimated monetized damages of nitrogen pollution, translated to 2020 US Dollars. Source: Sutton et al. [7]. As observed by Keeler et al. [4], damages depend greatly on the site a pollutant is released.

If world damages from nitrogen pollution, per unit nitrogen fertilizer applied [1], are the same as for the EU [7], then world damages are $1 to $5 trillion per year. Most eutrophication damage comes from the food system.

Major sources of eutrophication. Source: Poore and Nemecek [6].

Major sources of non-food eutrophication include inadequately treated wastewater, urban stormwater, and the atmosphere [5].


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References

[1] Food and Agriculture Organization of the United Nations. "FAOSTAT".

[2] Fowler, D., Coyle, M., Skiba, U., Sutton, M., Cape, J., Reis, S., Sheppard, L., Jenkins, A., Grizzetti, B., Galloway, J., Vitousek, P., Leach, A., Bouwman, A., Butterbach-Bahl, K., Dentener, F., Stevenson, D., Amann, M., Voss, M. "The global nitrogen cycle in the twenty-first century". Philosophical Transactions of the Royal Society B 368(1621). July 2013.

[3] Galloway, J., Dentener, F., Capone, D., Boywer, E., Howarth, R., Seitzinger, S., Asner, G., Cleveland, C., Green, P., Holland, E., Karl, D., Michaels, A., Porter, J., Townsend, A., Vöosmarty, C. "Nitrogen cycles: past, present, and future". Biogeochemistry 70, pp. 153-226. 2004.

[4] Keeler, B., Gourevitch, J., Polasky, S., Isbell, F., Tessum, C., Hill, J., Marshall, J. "The social costs of nitrogen". Science Advances 2(10), e1600219. October 2016.

[5] OECD. "Diffuse Pollution, Degraded Waters: Emerging Policy Solutions". March 2017.

[6] Poore, J., Nemecek, T. "Reducing food’s environmental impacts through producers and consumers". Science 360(6392), pp. 987-992. June 2018.

[7] Sutton, M., Howard, C., Erisman, J., et al. (eds);. Costs and benefi ts of nitrogen in the environment. Cambridge University Press, Cambridge, UK pp.513-540. April 2011.