Vertical distribution and influencing factors of soil organic carbon in the Loess Plateau, China.

Accurate analysis and evaluation of the spatial distribution and the primary factors that affect regional soil organic carbon (SOC) together make an important step in assessing carbon sequestration potential. However, little information is available on distribution of regional SOC in deep soil layers. To analyze the spatial distribution of and factors influencing SOC in a 500 cm soil profile, 1440 soil samples were collected from 90 sites on the Loess Plateau in China. The primary factors dominating the spatial distribution of SOC were quantified using principal component analysis with multiple linear regression (PCA-MLR) analysis. Results showed that the mean SOC of the 500 cm soil profile ranged from 1.20 to 3.37 g kg-1, decreasing with increasing soil depth. The SOC in the deep soil profile decreased across the types of land use in the following order: forestland > cropland > grassland. Based on the factors analyzed in this study, land use accounted for 22% of the variation in SOC and was the dominant factor controlling the spatial distribution of organic carbon in shallow soils (0-100 cm); while soil factors (including soil clay, soil water content, and soil bulk density) were dominant in deep soil layers (200-500 cm), averagely accounting for 44.3%. The SOC stock in the 0-20 cm soil layer was 1.34 kg m-2, accounting for only a small proportion (8%) of the total carbon in the entire 500 cm soil profile. SOC stock in the 200-500 cm layer was 7.62 kg m-2, accounting for 44% of the total carbon in the 0-500 cm soil profile. This study demonstrates that a large amount of organic carbon is stored in deep soil, indicating that a better understanding of the reserves and cycles of deep soil carbon is a critical factor in the effective management of terrestrial ecosystems.