Climatic and topographic controls on soil organic carbon distribution across continents

Soil organic carbon (SOC) plays a crucial role in the global carbon cycle, representing the largest terrestrial organic carbon reservoir. However, the spatial distribution patterns of SOC content and their relationships with environmental factors in the black soil regions of Northeast China (NEC) and the Mississippi River Basin in North America (MNA) remain unclear. This study aims to characterize the spatial distribution of SOC content in these two black soil regions and to investigate its associations with multiple environmental drivers across various spatial scales. We employed a multi-scale random forest modeling framework combined with interpretability techniques, including SHAP values and partial dependence plots, to identify the dominant climatic and topographic drivers of SOC variability. In NEC, SOC is predominantly in the 30–40 g/kg range (27.32 % of the area) with a broader distribution across lower ranges, while MNA shows greater dominance in the same range (35.92 % of the area). Spatial variability differs by scale: NEC exhibits higher heterogeneity at finer scales (5–10 km), whereas MNA shows increased variability at larger scales (20–50 km). At smaller scales (≤20 km), terrain factors such as slope and elevation primarily regulate SOC, while at larger scales (>20 km), climatic factors like temperature and precipitation dominate. Regionally, SOC in NEC shows high sensitivity to temperature, with a sharp decline between 1 °C and 5 °C, whereas SOC in MNA remains relatively stable across lower temperature ranges (−4 °C to 5 °C), exhibiting notable decreases only beyond 8 °C. Geographically, SOC peaks at higher latitudes (>47°N) and elevations (up to 1700 m) in NEC, but at mid-latitudes (43°–45°N) and mid-elevations (400–600 m) in MNA. These findings highlight the necessity of scale-dependent and region-specific strategies for SOC assessment and management in black soil regions.