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

•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.