<Table> <Tr> <Td> </Td> <Td> This article may be too technical for most readers to understand . Please help improve it to make it understandable to non-experts, without removing the technical details . The talk page may contain suggestions . (July 2013) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article may be too technical for most readers to understand . Please help improve it to make it understandable to non-experts, without removing the technical details . The talk page may contain suggestions . (July 2013) (Learn how and when to remove this template message) </Td> </Tr> <P> 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 (N O) mole fractions have increased from a pre-industrial value of ~ 270 nmol / mol to ~ 319 nmol / mol in 2005 . Human activities account for over one - third of N O 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 . </P> <P> Approximately 78% of earth's atmosphere is N gas (N), which is an inert compound and biologically unavailable to most organisms . In order to be utilized in most biological processes, N must be converted to reactive N (Nr), which includes inorganic reduced forms (NH and NH), inorganic oxidized forms (NO, NO, HNO, N O, and NO), and organic compounds (urea, amines, and proteins). N has a strong triple bond, and so a significant amount of energy (226 kcal mol - 1) is required to convert N 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 N to NH, a process known as biological nitrogen fixation (BNF). The anthropogenic analogue to BNF is the Haber - Bosch process, in which H is reacted with atmospheric N at high temperatures and pressures to produce NH . Lastly, N is converted to NO by energy from lightning, which is negligible in current temperate ecosystems, or by fossil fuel combustion . </P>

Human interaction with the nitrogen cycle is primarily due to