Impact of acid rain on carbonate rock dissolution and karst carbon sink in a karstic soil-carbonate rock system: A case study from Southwest China

Abstract 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 in a karstic soil–carbonate rock system and quantified the relationship between carbon sink and karst carbon sink flux. A subtropical karst spring catchment in southwestern China was selected. 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 was 43.93 mg C/L in the wet season, while the karst carbon sink flux was 11.61 mg C/L. The total carbon sink flux in the spring catchment was approximately 3.8 times greater than the karst carbon sink flux. The dissociation of carbonic acid produces H⁺, which can be exchanged with soil base ions. This process contributed approximately 1.91–3.18 times more DIC to groundwater in the karstic soil–carbonate rock system than direct carbonate rock erosion by H⁺ from carbonic acid dissociation. These results indicate that although acid rain may reduce the efficiency of the karst carbon sink, the presence of karstic soil can mitigate this effect.