Global patterns of nitrogen saturation in forests

Abstract

Abstract Since the industrial revolution, accelerated atmospheric nitrogen (N) deposition by human activities have increased N availability in forest ecosystems close to human settlements, potentially causing many nitrogen-limited forests to become nitrogen-saturated, with significant effects on productivity, biodiversity, and biogeochemical cycles. Four decades after recognizing the N saturation problem, however, global patterns of N saturation in forests still remain uncertain. In N-saturated forests, oversupply of N leads to higher N losses including those in form of N2O as compared to N-limited forests, suggesting that the sensitivity of soil N2O emission to N deposition (sN) might be used as an indicator of N saturation. In this study, we modeled the sN of global forests using data from N addition experiments. Testing with field observations on N saturation status, the global patterns of N-limited and N-saturated forests indicated by sN show an accuracy above 70% on global and geographic-regional scales. Our results suggest that 43% of global forests are N-saturated, and the proportions of forests being N-saturated are particularly high in East Asia and Western Europe (over 60%). The produced global map of N-saturated forests sheds light on the spatially varying N availability in forests, which founds a basis for predicting the influence of changing N deposition on forest greenhouse gas emissions and productivity, facilitating optimized environmental management practices for different regions.