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, with the Sichuan Basin being its most extensive distribution area. Climate warming significantly impacts the carbon balance of agricultural ecosystems worldwide, yet the dynamic changes in topsoil organic carbon (SOC) in this region's purple soil under climate warming remain poorly understood. This study analyzes the SOC changes in purple soil dryland based on 1,087 samples from the Second National Soil Survey in the 1980s and 1,154 soil samples collected in the 2010s. Using geostatistics, analysis of variance (ANOVA), and regression analysis, we explore the changes in SOC content over the past 40 years, the influencing factors, and their response to climate warming. The results show that SOC content increased from 6.97 g·kg⁻¹ to 9.65 g·kg⁻¹, with a growth rate of 38.45%. The growth rates of SOC varied among different geomorphic zones and secondary river basins. There was no significant difference in SOC content among various parent material groups in the 2010s, indicating a decrease in SOC spatial variability caused by parent material over the past 40 years. SOC increments showed a positive parabolic relationship with temperature and a negative parabolic relationship with precipitation, altitude, and slope. Under dry conditions (precipitation <1050 mm), SOC increments were 1.64 times higher than under humid conditions (>1050 mm). The increase in SOC initially rose and then declined with the average annual fertilization rate, crop yield, and their respective growth rates. Given the varied effects of precipitation on the 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 of SOC increments under varying moisture conditions, but 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 through which climate warming affects SOC in purple soil across different geomorphic zones and basins require further investigation. Our findings provide scientific evidence to enhance carbon sequestration capacity and improve cropland quality in the context of climate warming, offering important guidance for optimizing agricultural water management practices in the Sichuan Basin, China.