1 Marine Biological Laboratory, Woods Hole, USA
2 Institute of Terrestrial Ecology, Bangor, Wales, UK
3 Danish Forest and Landscape Research Institute, Hørsholm, Denmark
4 Norwegian Forest Research Institute, Ås, Norway
5 University of Amsterdam, The Netherlands
6 Swiss Federal Institute for Forest, Snow and Landscape Research, Birmensdorf, Switzerland
7 Norwegian Institute for Water Research, Oslo, Norway
Humans have altered global nitrogen (N) cycling such that more atmospheric N2 is being converted ("fixed") into biologically reactive forms by anthropogenic activities than by all natural processes combined. In particular, N oxides emitted during fuel combustion and ammonia volatilized due to intensive agriculture have increased atmospheric N inputs (mostly NO3 and NH4) to temperate forests in the Northern Hemisphere. Because tree growth in north temperate regions is typically N limited, increased N deposition could function to attenuate rising atmospheric carbon dioxide (CO2) by stimulating forest biomass accumulation. Forest inventories suggest that the C contents of northern forests have increased concurrently with N deposition since the 1950s. Also, atmospheric CO2 variations point to a globally significant C sink in north mid-latitude forest regions. It is unclear, however, whether elevated N deposition or other factors are the primary cause of C sequestration in northern forests. Here we use evidence from 15N tracer studies in 9 forests to show that elevated N deposition is not likely a major contributor to the putative CO2 sink in forested north temperate regions.