Divergent Dynamics and Drivers of Soil Organic Carbon in Mulberry Plantations Across Contrasting Calcisols and Chromic Luvisols in a Karst Region of Southwest China

The fragile Karst landscapes of southwest China face persistent challenges of soil degradation and rocky desertification. While sustainable land use such as mulberry plantation can support ecological restoration, the dynamics of soil organic carbon (SOC) and its driving mechanisms across contrasting soil types remain poorly understood, limiting the development of targeted pedogenically aware carbon management strategies. A comparative field study was conducted in central Guizhou, China, over an eight-month mulberry growing season (April to November). We monitored SOC, physicochemical properties, GRSP, and enzyme activities in plantations established on two contrasting limestone-derived soils (Calcisols and Chromic Luvisols). This study aimed to clarify the relationships between SOC and key soil parameters within each soil type and to identify their dominant driving factors. Soil type significantly influenced SOC concentration, dynamics, and its regulatory mechanisms. SOC was significantly higher and exhibited greater seasonal variability in Calcisols (31.51–39.71 g·kg−1) than in Chromic Luvisols (22.50–28.51 g·kg−1), with Calcisols maintaining 1.28–1.57 times the SOC concentration of Chromic Luvisols. Regression analysis revealed that SOC was significantly positively correlated with TN, AN, AK, and GRSP, but significantly negatively correlated with AP. Random forest modeling further identified distinct key correlated factors in each soil type as follows: TN, T-GRSP, and Urease were primary in Calcisols, whereas TN, T-GRSP, and pH dominated in Chromic Luvisols. Partial least squares path modeling confirmed that soil type does not directly associate with SOC but exerts an indirect effect by modulating core biochemical mediators specifically (Alkaline protease, T-GRSP, and TN); The model also indicated that pH and TN exert direct positive effects on SOC accumulation. In Karst mulberry systems, pedogenically distinct soils (Calcisols vs. Chromic Luvisols) shape SOC storage, stability, and regulatory mechanisms through divergent biogeochemical pathways. SOC management should therefore be soil-type-specific, prioritizing nitrogen synergy in Calcisols and pH-mediated stabilization in Chromic Luvisols, rather than applying uniform strategies. This study thereby establishes a mechanistic framework for understanding and managing SOC in heterogeneous Karst landscapes, providing a critical foundation for developing targeted, soil-specific carbon sequestration practices in ecologically vulnerable regions.