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

Humans have altered global nitrogen cycling such that more atmospheric N2 is being converted (‘fixed’) into biologically reactive forms by anthropogenic activities than by all natural processes combined1. In particular, nitrogen oxides emitted during fuel combustion and ammonia volatilized as a result of intensive agriculture have increased atmospheric nitrogen inputs (mostly NO3 and NH4) to temperate forests in the Northern Hemisphere2,3,4. Because tree growth in northern temperate regions is typically nitrogen-limited5, increased nitrogen deposition could have the effect of attenuating rising atmospheric CO2 by stimulating the accumulation of forest biomass. Forest inventories indicate that the carbon contents of northern forests have increased concurrently with nitrogen deposition since the 1950s6,7,8. In addition, variations in atmospheric CO2 indicate a globally significant carbon sink in northern mid-latitude forest regions9,10,11,12. It is unclear, however, whether elevated nitrogen deposition or other factors are the primary cause of carbon sequestration in northern forests. Here we use evidence from 15N-tracer studies in nine forests to show that elevated nitrogen deposition is unlikely to be a major contributor to the putative CO2 sink in forested northern temperature regions.