AbstractStudies 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 ∼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.