The Sensitivity of Belowground Ecosystem to Long-Term Increased Nitrogen Deposition in a Temperate Grassland: Root Productivity, Microbial Biomass, and Biodiversity

Studies exploring ecosystem vulnerability to nitrogen (N) enrichment have mostly focused on aboveground components of ecosystems. However, the sensitivity of the belowground ecosystem to increasing N deposition remains unclear. We estimated responses of belowground net primary productivity (BNPP), soil microbial biomass N (MBN), N mineralization, nitrification, and 16S rRNA gene based bacterial diversity to elevated N inputs. The study was based on a long-term N deposition experiment with monthly N applications at nine rates ranging from 0 to 50 g N m(-2) yr(-1) in a temperate grassland. BNPP, MBN, and microbial diversity showed non-linearities across the N gradient. In both post-hoc test and regression tree model, the significant decrease of BNPP relative to the control started at the 10 g N m(-2) yr(-1) rate (the 53% decrease), whereas in Bayesian regression model, the decrease started at the 5 g N m(-2) yr(-1) rate (the 39% decrease) in year 6. We therefore estimated a critical load range of 5-10 g N m(-2) yr(-1) for BNPP. Regression tree model, Bayesian regression, and post-hoc test consistently suggested that the detrimental effects on MBN might occur above similar to 10 g N m(-2) yr(-1) addition rate. Bacterial diversity and the relative abundance of dominant phyla declined when N addition rate exceeded 5 or 10 g N m(-2) yr(-1). The impacts of N deposition on the root-microbe system strongly depended on the interannual fluctuation in precipitation. The responses of the sensitive belowground indicators are vital to help minimize the detrimental impacts of anthropogenic N inputs.