Soil inorganic carbon (SIC) accounts for more than 80 % of total soil carbon stocks in arid regions, yet its formation pathways remain poorly understood, particularly the role of soil organic carbon (SOC) mineralization in producing CO2 that precipitates as pedogenic inorganic carbon (PIC). Using a delta 13C end-member mixing approach in riparian soils of the lower Heihe River, Northwest China, the contribution of SOC-derived CO2 to PIC formation was quantitatively resolved across soil textures and depths. On average, 41 % of the CO2 contributing to PIC formation originated from SOC mineralization, with biogenic contributions declining with depth and remaining consistently higher in loam and loamy sand than in sandy soils. The estimated amount of SOC-derived CO2 associated with PIC formation (mCO2, defined here as the CO2 input required for carbonate precipitation rather than the stoichiometric CO2 content of PIC) exhibited strong positive relationships with SIC content and electrical conductivity. This pattern highlights a previously underrecognized coupling among soil ionic conditions, SOC availability, and carbonate precipitation processes. These findings provide direct isotopic evidence for the mechanistic role of SOC mineralization in facilitating PIC formation and underscore the importance of incorporating biogenic CO2 pathways into assessments of dryland carbon budgets and carbon-management strategies.