Regional spatial variability of soil organic carbon in 0–5 m depth and its dominant factors

[Display omitted] •A critical depth was detected (180 cm), above which SOC differed significantly among land uses.•Low SOC content in the central area of the northern Chinese Loess Plateau corresponded to the aeolian sand area of Ordos.•Soil texture and temperature controlled SOC throughout the 0–500 cm soil profile.•The spatial variations and forcing factors of SOC were all depth-dependent. Soil organic carbon (SOC) stored in deeper soils is becoming increasingly significant in carbon management and carbon cycle especially in the face of climate and land use alterations. However, spatial variability of deep SOC at a regional scale and its related dominant factors are rarely investigated. To ascertain the vertical distribution and regional spatial variations in the SOC content of deep soil, 1648 soil samples were collected from 67 sites across the Chinese Loess Plateau (CLP) at a depth of 500 cm. Results showed that the mean SOC content of three land use types were recorded as follows: cropland (2.93 ± 0.29 g kg−1) > forest (2.56 ± 0.24 g kg−1) > grassland (2.39 ± 0.29 g kg−1), and the vertical distribution of soil profile from 0 to 500 cm was similar. Furthermore, a significant alteration in SOC content was observed in the soil layer of 0–180 cm among the three land use types (p