<p>Acid rain can dissolve carbonate rocks and affect karst carbon sinks. This study investigated the effect of acid rain on the dissolution of carbonate rocks within a karstic soil–carbonate rock system and quantified the relationship between the carbon sink and karst carbon sink flux. A subtropical karst spring catchment in southwestern China was chosen as the study area. The hydrochemistry of acid rain and spring water, along with the δ¹³C of dissolved inorganic carbon (DIC), was systematically monitored. NH₄⁺, H₂SO₄, and HNO₃ from precipitation contributed 3.57%, 3.49%, and 1.57% to carbonate rock dissolution, and 1.74%, 1.70%, and 0.77% to groundwater DIC, respectively. These acidic ions reduced the karst carbon sink flux by approximately 17.37%. The carbon sink flux reached 43.93&#xa0;mg C/L during the wet season, whereas the karst carbon sink flux was 11.61&#xa0;mg C/L. Overall, the total carbon sink flux in the spring catchment was about 3.8 times higher than the karst carbon sink flux. The dissociation of carbonic acid produces H⁺, which can be exchanged with soil base ions. This process contributed more DIC to groundwater in the Yaji karstic soil–carbonate rock system than direct carbonate rock erosion by H⁺ from carbonic acid dissociation. While this study demonstrates that karstic soil processes significantly buffer acid rain and strengthen the carbon sink effect, their wider applicability may be limited by site-specific factors such as soil composition, hydrological conditions, and land use.</p>