The main factors controlling spatial variability of soil organic carbon in a small karst watershed, Guizhou Province, China

Soil organic carbon (SOC) is an important component of the global carbon pool. Assessment of SOC in a karst mountainous area is a great challenge due to the high spatial heterogeneity in topography, land use and soil. This study used 2755 soil samples from a karst watershed in southwestern China to quantitatively study the spatial variability in SOC in this small karst watershed. The effects of parent soil material, soil type, land use, slope position, slope orientation and rock exposure rate on the spatial distributions of SOC in the karst watershed were compared by statistical modelling. The results showed that the SOC content in the study area was 25.01 g/kg, and the coefficient of variation was 55.26%, indicating moderate-intensity variation. The nugget coefficient C-0/(C-0 + C) was > 0.70, and the spatial distribution of SOC was mainly affected by structural factors. Parent soil material, soil type, land use, slope position, slope direction and rock exposure rate had significant effects on the SOC (p < 0.001). The independent interpretation of SOC spatial variation by influencing factors is as follows: soil family level (37.40%) > land use (23.10%) > rock exposure rate (22.10%) > soil group level (17.50%) > parent material (10.90%) > slope position (7.20%) > slope direction (5.10%). Soils in the lower soil categories of the soil classification system were better able to reflect SOC spatial distributions. When soil family levels, land uses and rock exposure rates are used as predictors, 56.60% of SOC variability can be explained. Therefore, it is advisable to use a combination of soil family levels, land uses and rock exposure rates as the main controlling factor for predicting or assessing SOC spatial distributions in karst areas.