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. 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.
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). The anthropogenic analogue to BNF is the Haber-Bosch process, in which H2 is reacted with atmospheric N2 at high temperatures and pressures to produce NH3. Lastly, N2 is converted to NO by energy from lightning, which is negligible in current temperate ecosystems, or by fossil fuel combustion.
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