Characteristics and influencing factors of topsoil organic carbon content changes in purple soils of the Sichuan basin

Purple soil is a unique soil type in China and is most extensively distributed in the Sichuan Basin. Climate warming significantly affects the carbon balance of agricultural ecosystems worldwide; however, dynamic changes in topsoil organic carbon (SOC) in the purple soil of this region remain poorly understood. This study analyzed SOC changes in purple soil drylands based on 1087 samples from the Second National Soil Survey collected in the 1980s and 1154 soil samples collected in the 2010s. Using geostatistics, analysis of variance (ANOVA), and regression analysis, we explored the changes in SOC content over the past 40 years, influencing factors, and their response to climate warming. The results show that the SOC content increased from 6.97 to 9.65 g∙kg⁻¹, with a growth rate of 38.45%. The growth rates of SOC varied among the different geomorphic zones and secondary river basins. There was no significant difference in SOC content among the various parent material groups in the 2010s, indicating a decrease in SOC spatial variability caused by the parent material over the past 40 years. The SOC increments demonstrated a positive parabolic relationship with temperature and a negative parabolic relationship with precipitation, altitude, and slope. Under dry conditions (precipitation <1 050 mm), the SOC increments were 1.64 times higher than those under humid conditions (>1 050 mm). The SOC increment exhibited a parabolic variation, initially increasing and then decreasing, with corresponding indicators of fertilization and crop productivity. Given the varied effects of precipitation on dynamic changes in SOC in purple soil, we further examined the relationship between SOC changes and climate warming under different average annual precipitation levels. The results revealed similar parabolic trends in SOC increments under varying moisture conditions. However, SOC under dry conditions was more sensitive to temperature changes. This indicates that, while SOC in purple soil is influenced by both temperature and precipitation, the inherent characteristics of purple soil may buffer against some of the fluctuations caused by climate warming and hydrological conditions. The specific mechanisms by which climate warming affects the SOC in purple soils across different geomorphic zones and basins require further investigation. Our findings provide scientific evidence for enhancing carbon sequestration capacity and improving cropland quality in the context of climate warming, offering important guidance for optimizing agricultural water management practices in the Sichuan Basin, China.