Retention of deposited ammonium and nitrate and its impact on the global forest carbon sink

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 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 (${20}_{15}^{25}$%) (${\mathrm{mean}}_{\mathrm{minimum}}^{\mathrm{maximum}}$) 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.

A study using paired ¹⁵N 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.