Root-derived carbon sources supporting photosynthesis have been demonstrated to contribute to plant carbon gain in many laboratory experiments. However, it remains largely unknown whether and to what extent soil dissolved inorganic carbon (DIC) influences leaf photosynthesis in karst habitats characterized by alkaline soils with low water content. We explored this relationship by measuring the concentrations and carbon isotope signals (δ¹³C) of soil DIC, as well as the δ¹³C of water-soluble organic matter (δ_WSOM) in leaves of nine woody species across an altitudinal gradient in karst habitats. The δ_WSOM varied among species by 7.23‰ and deviated from the δ¹³C of photosynthates solely assimilated from atmospheric CO₂ (δ_A) by 0.44–5.26‰, with a mean value of 2.20‰. This systematical discrepancy (δ_A − δ_WSOM) could only be explained by the contribution of soil DIC to leaf total photosynthesis (f_{DIC_soil}). The average values of f_{DIC_soil} considerably varied among the nine species, ranging from 2.48% to 9.99%, and were comparable with or slightly lower than those of previous laboratory experiments. Furthermore, the f_{DIC_soil} of two species significantly increased with altitude, whereas another species exhibited an opposite pattern, suggesting a highly spatial heterogeneity of DIC utilization. The present study improved our understanding of how plants adapt to the alkaline–drought soil conditions of karst habitats and thus acquire additional carbon for growth.