Edaphic Controls Override Climatic and Management Effects on Soil Organic Carbon in Southern African Agroecosystems

Background: Understanding the relative importance of climatic, edaphic, and management controls on soil organic carbon (SOC) is essential for improving SOC prediction and land management in data-scarce agroecosystems. Aim: This study quantified SOC drivers across Southern African agroecosystems using 655 observations covering climatic variables, soil physicochemical properties, and management indicators. Methods: Linear regression, hierarchical modeling, response-scale comparison, variance partitioning, and nonlinear sensitivity analyses were applied to evaluate the robustness of SOC driver attribution. Results: SOC variability was primarily explained by edaphic properties. The edaphic-only model explained 33.2% of SOC variation, closely matching the full additive model including climate, edaphic, and management variables (R² = 0.349; adjusted R² = 0.338). Aluminum oxides, clay content, cation exchange capacity, and pH were the most consistent edaphic predictors, while management indicators were not significant in the full model. Variance partitioning confirmed the dominance of edaphic controls, with a unique edaphic contribution of 29.9%, compared with 1.0% for climate and no independent contribution from management. Nonlinear models improved fit, with the generalized additive model explaining 38.6% of deviance, but did not alter the dominance of edaphic controls. Conclusion: These findings show that SOC variability across Southern African agroecosystems is structured mainly by soil physicochemical conditions rather than by broad climatic gradients or coarse management categories. SOC prediction and management in these systems should therefore prioritize edaphic constraints, particularly mineral reactivity, exchange capacity, and soil chemical conditions.