Patterns and drivers of atmospheric nitrogen deposition retention in global forests

Forests are the largest carbon sink in terrestrial ecosystems, and the impact of nitrogen (N) deposition on this carbon sink depends on the fate of external N inputs. However, the patterns and driving factors of N retention in different forest compartments remain elusive. In this study, we synthesized 408 observations from global forest ¹⁵N tracer experiments to reveal the variation and underlying mechanisms of ¹⁵N retention in plants and soils. The results showed that the average total ecosystem ¹⁵N retention in global forests was 63.04 ± 1.23%, with the soil pool being the main N sink (45.76 ± 1.29%). Plants absorbed 17.28 ± 0.83% of ¹⁵N, with more allocated to leaves (5.83 ± 0.63%) and roots (5.84 ± 0.44%). In subtropical and tropical forests, ¹⁵N was mainly absorbed by plants and mineral soils, while the organic soil layer in temperate forests retained more ¹⁵N. Additionally, forests retained more ${}^{15}N{H}_4^{\mathrm{+}}$ than ${}^{15}N{O}_3^{\mathrm{-}}$ , primarily due to the stronger capacity of the organic soil layer to retain ${}^{15}N{H}_4^{\mathrm{+}}$ . The mechanisms of ¹⁵N retention varied among ecosystem compartments, with total ecosystem ¹⁵N retention affected by N deposition. Plant ¹⁵N retention was influenced by vegetative and microbial nutrient demands, while soil ¹⁵N retention was regulated by climate factors and soil nutrient supply. Overall, this study emphasizes the importance of climate and nutrient supply and demand in regulating forest N retention and provides data to further explore the impacts of N deposition on forest carbon sequestration.