Effects of atmospheric nitrogen deposition on carbon allocation and vegetation carbon turnover time of forest ecosystems in China

Nitrogen deposition has important effects on the forest carbon cycle, especially for forest ecosystems in China which experienced remarkable atmospheric nitrogen enrichment. However, the effects of nitrogen deposition on carbon allocation and carbon turnover time in China's forests on the national scale have not been well understood. In this study, we investigated how carbon allocation and vegetation carbon turnover time (tau veg) change in China's forests during 1990 to 2015 and attribution to nitrogen deposition using the Community Land Model version 4.5. Results indicate that net primary productivity (NPP) allocated to leaves, fine roots, and wood, and tau veg show increasing trends, despite a decrease (-0.03% yr-1) in the fraction of NPP allocated to leaves and fine roots and an increase (0.06% yr- 1) in the fraction of NPP allocated to wood. Modeled NPP allocated to three components and associated allocation coefficients were significantly correlated to N deposition. N deposition accounted for 46% and 40% of the trends of allocation coefficients of leaves or fine roots, and wood, respectively. N deposition promoted the increasing rate of tau veg in forests by 0.009 yr yr-1. Forest ecosystems with a significant increasing trend of tau veg induced by N deposition covered about 76% of the forest area. The trends of tau veg induced by nitrogen deposition in deciduous forests were larger than that in evergreen forests. The increase in cumulative NPP (19.11%) induced by N deposition was larger than the carbon loss (14.70%), which suggested that more carbon was stored in the vegetation. Our results highlight the importance of the effects of nitrogen deposition on carbon allocation in promoting the carbon sink of forest, and recommend to enhance the long-term vegetation biomass observation, especially the fine-roots biomass and root litterfall to improve the model ability in predicting responses of carbon allocation and vegetation turnover processes to environmental changes.