Abstracts

 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 ¹⁵N-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 than ammonium while soils retain more ammonium than nitrate. We estimate that the N deposition-induced C sink in forests in the 2010s  is higher than previous estimates because of a larger role for oxidized N and greater rates of global N deposition.