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 Hemisphere^2-4. 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 1950s^6-8. In addition, variations in atmospheric CO₂ indicate a globally significant carbon sink in northern mid-latitude forest regions^9-12. 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 N-15-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.