Objectives Unreasonable application of chemical fertilizers leads to acidification of purple soil, however, the effects under different cropping systems are still unclear.

Methods A long-term positioning experiment of purple soil (2012−2023), located in Chongqing City, contained three cropping systems, including dry-dry rotation (maize-bar mustard) system, paddy-dry rotation (rice-bar mustard) system, and single rice cropping (rice-flooding fallow) system. Three fertilization treatments were set up, as: no nitrogen fertilizer, optimized fertilization (OPT), and farmer’s over-fertilization (FP) under each cropping system. After harvest, the maize and rice yields were investigated, and soil samples were collected for measurement of pH, exchangeable acid and cations, base ion and soil pH buffer capacity.

Results Under dry-dry rotation system, the OPT and FP treatments decreased soil pH by 0.39 and 0.82 units (P<0.01), increased exchangeable acid content by 103.41% and 168.41% (P<0.01), and decreased the base saturation by 16.90% and 25.50% (P<0.05), however, FP treatment increased soil pH buffer capacity by 25.87% (P<0.05), compared to no nitrogen fertilizer treatment. Under the paddy-dry rotation system, OPT and FP treatments did not change pH, exchangeable acid content, and soil base saturation significantly, and increased soil pH buffer capacity by 40.20% and 53.98%, respectively (P<0.05). In the rice-flooding fallow system, both OPT and FP did not lead to significant change in soil pH, exchangeable acid content and base saturation, and OPT showed a tendency of increasing soil pH buffer capacity. On average, the annual soil pH decrease rate of the rotation systems were in order of dry-dry rotation (0.033−0.075 unit/a)>paddy-dry rotation (0.016−0.025 unit/a)>rice-flooding fallow system (0.008 unit/a). The correlation analysis showed that the pH buffer capacity under dry-dry rotation was positively correlated with soil exchangeable Al content, while that under paddy-dry rotation and single rice cropping system was positively correlated with soil organic matter content.

Conclusions Under the dry-dry rotation system, both optimal and over application of chemical fertilizers accelerated the acidification process of purple soil, as the increased exchangeable acid and Al content, while under the paddy-dry rotation and single rice cropping system, the application of chemical fertilizers did not show significant acidification effect on purple soil due to the increased soil organic matter content. Therefore, controlling and reducing the amount of chemical fertilizers, and carrying out paddy-dry rotation system are important measures to prevent acidification of purple soil.