Drivers of Deposited Nitrogen Retention and Its Contribution to Carbon Sequestration in Chinese Forests

Anthropogenic nitrogen (N) deposition alters forest functioning and their capacity to sequester carbon (C), yet its contribution to the forest C sink depends on N retention and allocation between plant biomass and soil pools. Despite high N deposition across China, the magnitude and drivers of N retention and contribution to forest C sequestration remain unclear due to a lack of systematic regional synthesis. Here, we synthesized data from decades of ¹⁵N tracer experiments spanning boreal to tropical regions of China to quantify the retention of deposited N, distribution among plant and soil pools, and contribution to forest C sequestration. On average, Chinese forests retained ~65% of the deposited N, with about two-thirds stored in the soil. Total retention and partitioning of N varied with climate, successional stage, and N forms. Soil organic layer retention declined, while mineral soil retention increased along a north-south gradient in mean annual temperature. Chronically N-saturated and disturbed forests exhibited low plant retention, whereas other forests showed substantial uptake across climate zones. Total ecosystem retention efficiency generally declined from boreal to tropical forests. Across successional stages, retention did not differ significantly; however, primary forests retained more deposited N in mineral soils, whereas plantations favored retention in the organic layer. Stoichiometric scaling indicates that CN response (kg C sequestered per kg deposited N) varies among forest types, ranging from ~7 to 40 kg C kg^-1 N, with boreal forests and plantations exhibiting the strongest C response due to greater N limitation. This suggests that over the past decade, N deposition enhanced C sequestration by approximately 0.11 Pg C year^-1, accounting for 20%-30% of China's forest C sink. Overall, these findings advance understanding of the drivers of deposited N retention and its contribution to C sequestration, with implications for predicting forest N and C dynamics under global change.