Spatiotemporal variation and driving mechanism of carbonate weathering carbon sink flux in China's north-south transitional zone

Understanding the drivers of the carbonate weathering carbon sink (CWCS) is important for advancing karst carbon cycle research and mitigating climate change. Here, we investigated the spatiotemporal dynamics and controlling factors of carbonate weathering carbon sink flux (CWCSF) in Shaanxi Province, a climatically sensitive region of China, during 2003–2019. Using an integrated framework that combines the Maximum Potential Dissolution (MPD) method, eXtreme Gradient Boosting (XGBoost), and Shapley Additive Explanations (SHAP), we identified CWCSF patterns and their driving mechanisms. Results show a mean CWCSF of 2.22 t C·km⁻²·a⁻¹ with a declining trend of −0.09 t C·km⁻²·a⁻¹. Precipitation was the dominant factor across the study area (contributing 50.9% to the overall CWCSF) and primarily drove the decline in southern CWCSF, whereas reductions in northern CWCSF were linked to vegetation restoration and rising evapotranspiration. Environmental factors, including precipitation, temperature, evapotranspiration, soil moisture, net primary productivity, and human activities, affected CWCSF in nonlinear ways and exhibited distinct threshold effects. For instance, a saturation effect occurred when soil moisture exceeded 0.22 m³/m³, beyond which its positive contribution to CWCSF weakened. Temperature showed a parabolic relationship with CWCSF, with 10–15 °C representing the optimal range for carbonate weathering. Projections for 2026–2060 indicate rising CWCSF and heightened climate sensitivity, suggesting that CWCSF may function as a negative feedback to climate change.