Background and aims <p>Soil acidification is a critical indicator of grassland degradation and a growing concern in temperate steppes. However, acidification rates and their biogeochemical drivers remain poorly quantified. This study aimed to assess acidification rates and clarify the dominant acidification mechanisms under both short- and long-term nitrogen (N) addition.</p> Methods <p>A field experiment was conducted in a temperate steppe, with three treatments: natural condition (grazing exclusion and atmospheric N deposition), short-term (2-year), and long-term (15-year) N addition (12&#xa0;g N m⁻²&#xa0;yr⁻¹). Soil acidification rates were calculated using a mass balance approach considering H⁺ production from N cycling, plant uptake, and bicarbonate leaching.</p> Results <p>Under natural conditions, the acidification rate of topsoil (0–10&#xa0;cm) was 0.59 kmolc ha⁻¹&#xa0;yr⁻¹, accompanied by a 0.66-unit pH decline over 15&#xa0;years. Long- and short-term N additions significantly increased the acidification rates to 1.52 and 1.27 kmolc ha⁻¹&#xa0;yr⁻¹, respectively, and reducing topsoil pH by 1.1 and 0.3 units. N cycling was the dominant acidification driver. Acidification was closely associated with Ca²⁺ depletion and Mn²⁺ enrichment. Projections based on observed acidification rates and theoretical soil acid-buffering characteristics indicate that, soil pH would decline to the critical threshold of 4.5 within 42&#xa0;years under current conditions, and within 16–19&#xa0;years under N addition; the combination with mowing may further shorten this period to 8–11&#xa0;years.</p> Conclusion <p>These findings highlight the severity of soil acidification in temperate steppes and the urgent need for mitigation strategies to sustain soil health and ecosystem functions.</p>