Characteristics and quantifications of soil acidification under different land uses and depths in northern subtropical China

Extensive nitrogen (N) fertilization and intensified agriculture have greatly accelerated soil acidification in China. However, research has focused more on non-calcareous soils with pH < 6.5. Until now, knowledge concerning the characteristics and rate of acidification of calcareous soils in northern China is still lacking. In this study, soil samples (n = 139) under different land uses (upland, paddy fields, and forests) and at different soil depths (0–20 cm, 20–40 cm) were collected from a typical northern subtropical agricultural region. The characteristics and drivers of soil acidification were evaluated based on the measurable results and quantification of the proton (H⁺) budget. Compared to historical data (n = 143) from the 1980s, the pH of the topsoil was decreased by 2.88 and 1.88 units in upland and paddy fields, respectively. Meanwhile, due to the selective uptake of cations over anions by crops, subsoil pH in upland and paddy fields also reduced significantly by 1.30 and 1.12 units, respectively. In contrast, no significant change in the soil pH was observed in the forests. In the study area, the soil acidification rates were 17.7 ± 3.2 and 13.5 ± 2.7 kmol ha⁻¹ yr⁻¹ for upland and paddy fields, respectively. N transformation was the dominant driver of soil acidification (63–77 %), followed by excess uptake of cations by crops (23–37 %), whereas the contribution of H⁺ deposition and HCO₃^- process was negligible (<1 %). Quantitative results showed that soil acidification was more severe in upland than in paddy fields, with a faster decline in soil pH, a higher soil exchangeable acidity, a greater proportion of strongly acidic soils, and a 30 % higher soil acidification rate. This might be interpreted by the differences in N fertilization rate, water management, topographical distribution, and crop yields. Thus, our data suggest that strategies such as reducing the N fertilization rate, implementing appropriate water management strategies, practicing crop rotation, and adequate straw return could effectively mitigate soil acidification in northern subtropical China.