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ABSTRACT

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 15 N-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 ( $${20}_{15}^{25}$$ 20 15 25 %) ( $${{{{{{\rm{mean}}}}}}}_{{{{{{\rm{minimum}}}}}}}^{{{{{{\rm{maximum}}}}}}}$$ mean minimum maximum ) than ammonium ( $${12}_{8}^{16}$$ 12 8 16 %) while soils retain more ammonium ( $${57}_{49}^{65}$$ 57 49 65 %) than nitrate ( $${46}_{32}^{59}$$ 46 32 59 %). We estimate that the N deposition-induced C sink in forests in the 2010s  is $${0.72}_{0.49}^{0.96}$$ 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. A study using paired 15 N tracers shows atmospheric N deposited in oxidized form is more likely retained by trees, while the reduced form is retained in soil. The authors argue that this is a greater contribution of deposited N to the global forest C sink than previously reported.

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