Global Nitrogen Deposition Promotes Carbon Sink Formation in Terrestrial Ecosystems

Nitrogen (N) deposition has been shown to alleviate N limitations on ecosystem productivity, increasing the terrestrial carbon (C) sink. Accurately quantifying the fate of deposited N is essential for reliably estimating its contribution to the global C sink. We conducted an extensive analysis and modeling of 829 observational data points to elucidate the retention patterns of reduced N (NH_x) and oxidized N (NO_y) deposited in ecosystems and evaluate their contributions to terrestrial C sinks via stoichiometric scaling. Our results indicate that, on average, 36% of deposited N is retained in terrestrial ecosystems. Globally, the retention of deposited N amounted to 39.15 Tg N yr^-1, with the retention rates of NH_x and NO_y being 25.02 and 14.13 Tg N yr^-1, respectively. Notably, a smaller amount of deposited N absorbed by trees was retained in woody tissues than in nonwoody tissues. Consequently, the estimated N-induced C sink in terrestrial ecosystems was 0.88 Pg C yr^-1, accounting for 25.48% of the global terrestrial C sink. This study highlights the global distribution patterns of deposited N retention and the induced biome-specific C sink, underscoring the importance of incorporating pool-specific C:N stoichiometry for accurately quantifying ecosystem-scale carbon sequestration in global modeling.