Nitrogen deposition makes a minor contribution to carbon sequestration in temperate forests

Humans have altered global nitrogen cycling such that more atmospheric N-2 is being converted ('fixed') into biologically reactive forms by anthropoge nic activities than by all natural processes combined(1). In particular, ni trogen oxides emitted during fuel combustion and ammonia volatilized as a r esult of intensive agriculture have increased atmospheric nitrogen inputs ( mostly NO3 and NH4) to temperate forests in the Northern Hemisphere(2-4). B ecause tree growth in northern temperate regions is typically nitrogen-limi ted(5), increased nitrogen deposition could have the effect of attenuating rising atmospheric CO2 by stimulating the accumulation of forest biomass. F orest inventories indicate that the carbon contents of northern forests hav e increased concurrently with nitrogen deposition since the 1950s(6-8). In addition, variations in atmospheric CO2 indicate a globally significant car bon sink in northern mid-latitude forest regions(9-12). It is unclear, howe ver, whether elevated nitrogen deposition or other factors are the primary cause of carbon sequestration in northern forests. Here we use evidence fro m N-15-tracer studies in nine forests to show that elevated nitrogen deposi tion is unlikely to be a major contributor to the putative CO2 sink in fore sted northern temperature regions.