Nitrogen fertilization induces greater loss of base cations and accumulation of exchangeable acids in acidic soils than in neutral soils

• The application of nitrogen fertilizer leads to the loss of soil base cations and the increase of exchangeable acids. • Initial soil pH significantly regulates the degree of nitrogen-induced acidification. • Soils with organic matter content greater than 25.0g/kg have a significant alleviating effect on acidification. • Acidic soils exhibit greater losses of base cations and larger increases in exchangeable acids compared to neutral soils. Agricultural soil acidification affects 40–70% of croplands worldwide and is intensifying with increasing nitrogen fertilization. Although nitrogen fertilizer-induced acidification is well-documented, the quantitative relationships and thresholds between base cations and exchangeable acids across different soil pH levels remain poorly understood. This study conducted a meta-analysis of 2,348 field trials from 157 sites worldwide, quantifying the dynamics of base cations (K + , Ca 2+ , Mg 2+ ) and exchangeable acids in acidic and neutral soils under long-term nitrogen fertilization. It aims to: (1) quantify nitrogen effects on base cations and exchangeable acids; (2) evaluate the relationship between base cation loss and pH decline in soils with different pH values; and (3) explore how organic matter alleviates exchangeable acids increase. The results showed that under nitrogen fertilization, the exchangeable K + , Ca 2+ , and Mg 2+ decreased significantly in acidic soils by 20.3%, 48.6%, and 43.3%, respectively, but showed relative lower decreases in neutral soils by 17.9%, 14.5%, and 6.3%, respectively. In addition, the exchangeable acid content in acidic soils increased significantly by 116.5%, which was much higher than that in neutral soils (35.7%). Subgroup analyses revealed that the accumulation of exchangeable acids could be effectively inhibited when the soil organic matter content exceeded 25.0 g/kg; moreover, the addition of calcium was of great significance for the retention of soil organic matter. This study emphasizes that acidic soils are more sensitive to nitrogen fertilization than neutral soils and more susceptible to soil acidification. These findings are crucial for understanding the acidification threshold of nitrogen fertilizer application, accumulative effects of fertilization time, and dynamic equilibrium between base cations and exchangeable acids in soils with different pH values. They also provide scientific evidence for quantifying the threshold of organic matter content for acidification relief and mitigating soil acidification caused by nitrogen fertilization.