Patterns and drivers of atmospheric nitrogen deposition retention in global forests

Abstract 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 15 N tracer experiments to reveal the variation and underlying mechanisms of 15 N retention in plants and soils. The results showed that the average total ecosystem 15 N 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 15 N, with more allocated to leaves (5.83 ± 0.63%) and roots (5.84 ± 0.44%). In subtropical and tropical forests, 15 N was mainly absorbed by plants and mineral soils, while the organic soil layer in temperate forests retained more 15 N. Additionally, forests retained more than , primarily due to the stronger capacity of the organic soil layer to retain . The mechanisms of 15 N retention varied among ecosystem compartments, with total ecosystem 15 N retention affected by N deposition. Plant 15 N retention was influenced by vegetative and microbial nutrient demands, while soil 15 N 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.