Objective  The objective was to explore the effects of urea nitrogen (N) addition on acidification indices and the acid buffering mechanisms of exchangeable base cations in a grassland chernozem.

  Method  Based on the urea addition (with N addition rates 0, 2.5, 5, 10, 15 and 20 g m⁻² a⁻¹, respectively) experiment conducted in a meadow grassland chernozem in Erguna, Inner Mongolia, soil acidifying variables were examined after 4 years of N additions.

  Result  Significant decreases in pH and the sum of exchangeable base cations (SEB) of the topsoil (0-10 cm) were found with increasing urea N addition rates. Soil acidifying rate (SAR) increased with the increasing N addition rates and decreasing SEB, and the SAR with N addition was approximately 0.017 ∆pH a⁻¹ g⁻¹ m² a. The best fitting model for soil acid titration curves was the quadratic curve fitting model, which had the greatest determination coefficient (R²). Soil acid neutralizing capacities (ANC) at the reference pH 5.0, 4.5 and 4.0 (ANC_{pH 5.0}, ANC_{pH 4.5}, and ANC_{pH 4.0}) were not significantly different among treatments with N addition rates lower than 10 g m⁻² a⁻¹, while ANC_{pH 5.0}, ANC_{pH 4.5}, and ANC_{pH 4.0} decreased significantly in treatments with N addition rates 15 and 20 g m⁻² a⁻¹.

  Conclusion  Soil acidifying rate (SAR) of the grassland chernozem was sensitive to exogenous N inputs. The higher exchangeable base cations exhibited a better buffering capacity to exogenous H⁺ inputs. The decrease in soil acid neutralizing capacity (ANC) induced by urea N addition was mainly affected by the decreases in initial soil pH and the sum of exchangeable base cations (SEB).