Carbon benefits of anthropogenic reactive nitrogen offset by nitrous oxide emissions

Additions of reactive nitrogen to terrestrial ecosystems-primarily through fertilizer application and atmospheric deposition-have more than doubled since 1860 owing to human activities(1). Nitrogen additions tend to increase the net uptake of carbon by the terrestrial biosphere, but they also stimulate nitrous oxide release from soils(2). However, given that the magnitude of these effects is uncertain, and that the carbon and nitrogen cycles are tightly coupled, the net climatic impact of anthropogenic nitrogen inputs is unknown(3). Here we use a process-based model of the terrestrial biosphere(4,5) to evaluate the overall impact of anthropogenic nitrogen inputs on terrestrial ecosystem carbon and nitrous oxide fluxes between 1700 and 2005. We show that anthropogenic nitrogen inputs account for about a fifth of the carbon sequestered by terrestrial ecosystems between 1996 and 2005, and for most of the increase in global nitrous oxide emissions in recent decades; the latter is largely due to agricultural intensification. We estimate that carbon sequestration due to nitrogen deposition has reduced current carbon dioxide radiative forcing by 96 +/- 14 mW m(-2). However, this effect has been offset by the increase in radiative forcing resulting from nitrous oxide emissions, which amounts to 125 +/- 20 mW m(-2).