Storage, pattern and driving factors of soil organic carbon in an ecologically fragile zone of northern China

Knowledge of the spatial pattern of soil organic carbon (SOC) and its influencing factors is important for understanding the carbon cycle. We investigated the SOC density (SOCD) to a depth of 30 cm from ground surface at 644 sites in an agro-pastoral ecotone of northern China that covered 654,564 km(2). SOC storage to a depth of 30 cm was 2360 Tg, and SOCD averaged 3.78 kg.m(-2) for the entire study area. SOCD to a depth of 20 cm and from 20 to 30 cm showed strong and moderate spatial dependence, with a nugget to sill ratio of 0.7% and 43.5%, respectively. Overall, SOCD increased from southwest to northeast, it was more strongly correlated with latitude than with longitude. Forest and sandy land had the highest and lowest SOCD, at 6.28 and 1.76 kg.m(-2), respectively. The SOCD (kg.m(-2)) of grassland decreased with decreasing vegetation cover: high (4.42) > medium (3.62) > low (2.50). For the common soils in the study area, the highest mean SOCD was 9.31 kg.m(-2) in Grey Forest Soils, versus 1.75 kg.m(-2) in Brown Pedocals. In the study area, temperature appears to be the dominant factor that influences SOC, and SOCD decreased with increasing temperature. However, topographic factors (elevation, slope, and aspect) had a weaker effect on SOC. Our results provide a valuable baseline for future research on the long-term evolution of SOCD.