Abstract: Soil exchangeable base cations (Ca²⁺, Mg²⁺, K⁺, and Na⁺) play a pivotal role in buffering soil acidification and supporting plant growth. Clarifying their responses to nitrogen (N) addition and mowing is conducive to accurately assess the changes of grassland ecosystem structure and function in the context of increasing N deposition, and to reveal the regulations of grassland management and utilization measure on N effects. We investigated the responses and underlying mechanisms of soil exchangeable base cation contents and soil acid buffering capacity to five N addition levels (0, 2, 5, 10, 20 g N·m^-2·a^-1) and two mowing treatments (mowing with 10 cm stubble, unmown) in a long-term N addition field experiment in a meadow steppe of Hulunbuir in Inner Mongolia. The results showed that increasing N addition significantly decreased the contents of soil exchangeable Ca²⁺, Mg²⁺, K⁺, Na⁺, total base cations, and soil pH. Under unmown conditions, N addition above 10 g N·m^-2·a^-1 significantly reduced soil acid buffering capacity. Soil base cation content and acid buffering capacity were negatively correlated with soil NH₄⁺, NO₃^- contents, and plant aboveground biomass. This implied that N addition drove soil acidification by stimulating plant growth and base cations uptake, intensifying NH₄⁺ induced displacement of base cations, and increasing NO₃^- leaching-associated loss of base cations. These mechanisms jointly depleted exchangeable base cations and weakened soil acid buffering capacity. Under the high N addition level of 20 g N·m^-2·a^-1, mowing mitigated the decrease of exchangeable base cations and soil pH by weakening the positive effects of N addition on soil inorganic N and plant growth. This study elucidates the ecological processes by which long-term N input leads to the loss of soil base cations and a decline in soil acid buffering capacity in grasslands, and highlights that mowing can mitigate these negative effects. It provides a theoretical basis for scientific grassland management and reasonable N utilization.

Key words: exchangeable base cation, soil acid buffering capacity, plant aboveground biomass, meadow steppe