Soil organic carbon storage, distribution, and influencing factors at different depths in the dryland farming regions of Northeast and North China

The knowledge of the spatial distribution of soil organic carbon (SOC) and of its influencing factors is crucial for understanding the global carbon cycle. Although the influence of climate, soil properties, and soil management on the SOC content has been extensively explored, their relative importance remains unclear, especially under dryland farming. Herein, we investigated the SOC density (SOCD) at different depths (0-10, 10-20, 20-30, and 30-40 cm) across an area of 37.79 x 10(4) km(2) (420 sites) located in the dryland farming regions of Northeast and North China. The total SOC storage (SOCS) was estimated to be 1922.38 Tg, with a mean density of 5.78 kg C m(-2) for the entire area. The three soil groups with the largest SOCS were Fluvo-aquic, Black, and Chernozem soils, accounting for 63.34% of the total SOCS in the study area. Overall, both the SOCD and SOCS increased from southwest to northeast at all investigated depths. A structural equation model was used to distinguish direct from indirect effects of different factors on SOCD. Soil properties (e.g., bulk density and pH) and natural conditions (e. g., mean annual temperature and mean annual precipitation) were found to be the main factors controlling the SOCD variation at depths of 10-20, 20-30, and 30-40 cm. However, the total (direct and indirect) effect of soil management on SOCD at a 0-10 cm depth was greater than that of natural conditions, and smaller than that of soil properties. With increasing soil depth, the total effect of natural conditions on SOCD changed from -0.33 to -0.58, whereas the indirect effect of soil management decreased from 0.47 to 0. These results indicate that compared with environmental factors, soil management practices such as tillage and fertilization had a greater influence on SOCD in the surface soil of dryland. Our study provides a valuable reference for future research on the long-term evolution of SOC in dryland farming regions.