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Human impact on the nitrogen cycle
Human impact on the nitrogen cycle is diverse. Agricultural and industrial nitrogen (N) inputs to the environment currently exceed inputs from natural N fixation. As a consequence of anthropogenic inputs, the global nitrogen cycle (Fig. 1) has been significantly altered over the past century. Global atmospheric nitrous oxide (N2O) mole fractions have increased from a pre-industrial value of ~270 nmol/mol to ~319 nmol/mol in 2005.〔(Alley et al. 2007. IPCC Climate Change 2007: The Physical Science Basis. Contribution of Working Group I in the Third Assessment Report of Intergovernmental Panel on Climate Change. Report Summary for Policy Makers (SPM) ).〕 Human activities account for over one-third of N2O emissions, most of which are due to the agricultural sector.〔 This article is intended to give a brief review of the history of anthropogenic N inputs, and reported impacts of nitrogen inputs on selected terrestrial and aquatic ecosystems.
== History of anthropogenic nitrogen inputs ==
Approximately 78% of earth's atmosphere is N gas (N2), which is an inert compound and biologically unavailable to most organisms. In order to be utilized in most biological processes, N2 must be converted to reactive N (Nr), which includes inorganic reduced forms (NH3 and NH4+), inorganic oxidized forms (NO, NO2, HNO3, N2O, and NO3−), and organic compounds (urea, amines, and proteins).〔 N2 has a strong triple bond, and so a significant amount of energy (226 kcal mol-1) is required to convert N2 to Nr.〔 Prior to industrial processes, the only sources of such energy were solar radiation and electrical discharges.〔 Utilizing a large amount of metabolic energy and the enzyme nitrogenase, some bacteria and cyanobacteria convert atmospheric N2 to NH3, a process known as biological nitrogen fixation (BNF).〔Schlesinger, W. H. 1997. ''Biogeochemistry : An analysis of global change'', San Diego, CA.〕 The anthropogenic analogue to BNF is the Haber-Bosch process, in which fossil fuel H2 is reacted with atmospheric N2 at high temperatures and pressures to produce NH3.〔Smil, V. 2001. ''Enriching the earth : Fritz Haber, Carl Bosch, and the transformation of world food production''. MIT Press, Cambridge, MA.〕 Lastly, N2 is converted to NO by energy from lightning, which is negligible in current temperate ecosystems, or by fossil fuel combustion.〔
Until 1850, natural BNF, cultivation-induced BNF (e.g., planting of leguminous crops), and incorporated organic matter were the only sources of N for agricultural production.〔 Near the turn of the century, Nr from guano and sodium nitrate deposits was harvested and exported from the arid Pacific islands and South American deserts.〔 By the late 1920s, early industrial processes, albeit inefficient, were commonly used to produce NH3.〔 Due to the efforts of Fritz Haber and Carl Bosch, the Haber-Bosch process became the largest source of nitrogenous fertilizer after the 1950s, and replaced BNF as the dominant source of NH3 production.〔 From 1890 to 1990, anthropogenically created Nr increased almost ninefold.〔 During this time, global population more than tripled, partly due to increased food production.
Since the industrial revolution, an additional source of anthropogenic N input has been fossil fuel combustion, which is used to generate energy (e.g., to power automobiles). During combustion of fossil fuels, high temperatures and pressures provide energy to produce NO from N2 oxidation.〔 Additionally, when fossil fuel is extracted and burned, fossil N may become reactive (i.e., NOx emissions).〔 During the 1970s, scientists began to recognize that N inputs were accumulating in the environment and affecting ecosystem functioning.〔
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