Retention fraction of 15N-labelled deposited ammonium and nitrate in forests

The impacts of enhanced nitrogen (N) deposition on 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 its contribution to forest C sink is unclear. We conducted ecosystem-scale paired 15N-labelling experiments in nine forests across China to quantify N retention fractions for both deposited ammonium and nitrate, including tropical and sub-tropical forests for the first time. By combining these results with four previous experiments from temperate Europe and North America, here we show that total ecosystem N retention is similar for ammonium and nitrate, but plants consistently take up more of the labelled nitrate than ammonium while soils retain more ammonium than nitrate. Nitrogen retention in plants and soils across sites is predicted by a combination of tree (NPP and woody biomass) and soil (organic layer mass and soil C/N ratios) variables. Greater proportions of deposited N are retained in N-limited ecosystems with low soil N availability and high soil C/N ratios. We estimate that N deposition-induced C sink in forests contributes more than 20% of the total terrestrial C sink. Although less N is deposited in oxidized than reduced state, their total contributions to the global forest C sink are approximately equal due to more efficient use by trees of the oxidized than the reduced form. Our study demonstrates differential fates of reduced and oxidized N deposition that improves current understanding of the C-N interaction in forests and indicates a greater C sink attributable to atmospheric N deposition than previous estimates.