The impacts of enhanced nitrogen (N) deposition on the global forest carbon (C) sink and other ecosystem services may depend on whether N is deposited in reduced (mainly as ammonium) or oxidized forms (mainly as nitrate) and the subsequent fate of each. However, the fates of the two key reactive N forms and their contributions to forest C sinks are unclear. Here, we analyze results from 13 ecosystem-scale paired N-15-labelling experiments in temperate, subtropical, and tropical forests. Results show that total ecosystem N retention is similar for ammonium and nitrate, but plants take up more labelled nitrate (20(15)(25)%) (mean(minimum)(maximum)) than ammonium (12(8)(16)%) while soils retain more ammonium (57(49)(65)%) than nitrate (46(32)(59)%). We estimate that the N deposition-induced C sink in forests in the 2010s is 0.72(0.49)(0.96) Pg C yr(-1), higher than previous estimates because of a larger role for oxidized N and greater rates of global N deposition.