Full-Profile Accounting of Model-Derived Soil Organic Carbon Reveals Regional Density-Stock Mismatches Across Mainland China

National soil-carbon assessments often emphasize carbon density without fully integrating soil depth and spatial extent. Here, we quantified the soil organic carbon (SOC) stock represented by the complete 1 km CSDLv2 raster domain across mainland China using six depth layers spanning 0–5, 5–15, 15–30, 30–60, 60–100, and 100–200 cm. Across 13,810,230 valid grid cells, the 0–200 cm soil profile stored 215.82 Pg C, of which 57.72 Pg C occurred in the upper 0–30 cm and 158.09 Pg C in the 30–200 cm subsoil. Although 73.2% of the total stock occurred below 30 cm, this large contribution primarily reflected the greater cumulative thickness of the subsoil interval; thickness-normalized SOC storage was substantially higher in the surface layer than in the subsoil (20.34 vs. 9.83 kg C m−2 m−1). Northwest China contained the largest integrated SOC stock (90.79 Pg C), whereas Northeast China exhibited the highest mean profile carbon density (33.17 kg C m−2), revealing a clear regional reversal between carbon density and total stock. Surface–subsoil decoupling and relative subsoil-carbon distance (CSD) further characterized regional differences in vertical SOC organization. Cross-dataset evaluation against an independent 2010–2024 SOC compilation comprising 17,990 depth intervals from 5870 profiles yielded an original-scale RMSE of 4.73 kg C m−2, a mean bias of 0.44 kg C m−2, and a Pearson correlation of 0.44. Replacing the 1 km layers with 90 m layers changed the national stock estimate by only 0.02%, while alternative DI thresholds and CSD reference quantiles affected screening magnitudes but preserved the broad regional patterns. These results highlight pronounced regional differences in the amount and vertical organization of soil carbon and support differentiated management priorities, including deep-carbon conservation in Northeast China, profile-structure diagnosis in North China and the Loess Plateau, water–soil coordination in Southern China, context-dependent water–carbon management in Northwest China, and disturbance avoidance and natural recovery in Southwest China and Tibet.