Soil organic carbon density as affected by topography and physical protection factors in the secondary forest area of Zhangguangcai Mountains, northeast China

In the hilly area of western Zhangguangcai Mountains, a typical secondary forest region of northeast China, a total of 36 sample plots were established symmetrically in accordance with the position and aspect of slope, and soil samples representing different genetic horizons were collected within a profile depth of 100 cm. The effects of topographical factors (the position, aspect, and gradient of slope) on soil organic carbon (SOC) content and density were investigated, and the relations between SOC content and physical protection factors (clay, aggregate) analyzed. Stepwise regression analysis was employed to quantify the relative influence of each factor. The SOC density within 1 m profile ranged from 8.9-31.3 kg/m, of which 55.2% was concentrated in the humus horizon (A1 horizon) on average. SOC distribution was significantly influenced by slope position and aspect. Downslope sites hold 83% more SOC in the A1 horizon and 67% more SOC in the 1 m profile than upslope sites, while ubac sites hold 37% more SOC in the A1 horizon and 17% more SOC in the 1 m profile than adret sites, respectively. No significant relation was found between the amount of SOC and the gradient of slope, when the down- or upslope sites were examined alone. No significant relation between SOC and any of the two physical protection factors could be ascertained if taking out some statistical disturbances (i.e. slope position and buried horizon), which leads us to the recognition that clay and aggregate protections are not controlling factors for SOC accumulation. Stepwise regression analysis suggests that slope position is the dominating factor, which alone could explain 57.4% of the spatial variability of SOC density for the A1 horizon and 63.2% for the 1 m profile, while clay and aggregate are eliminated from the equation due to smaller contribution. The results of this study have implications for accurate estimation of SOC stock in hilly areas, and for site selection of carbon sink forests.