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 paired¹⁵N-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 ( $${20}_{15}^{25}$$201525 %) ( $${{{{{{\rm{mean}}}}}}}_{{{{{{\rm{minimum}}}}}}}^{{{{{{\rm{maximum}}}}}}}$$meanminimummaximum ) than ammonium ( $${12}_{8}^{16}$$12816 %) while soils retain more ammonium ( $${57}_{49}^{65}$$574965 %) than nitrate ( $${46}_{32}^{59}$$463259 %). We estimate that the N deposition-induced C sink in forests in the 2010s is $${0.72}_{0.49}^{0.96}$$0.720.490.96 Pg C yr⁻¹, higher than previous estimates because of a larger role for oxidized N and greater rates of global N deposition.