Karst landscapes serve as vital carbon reservoirs, yet the long-term dynamics of their multiple carbon pools and the underlying driving mechanisms remain insufficiently understood. This study integrated land use, environmental remote sensing, and socioeconomic data to assess changes in carbon stocks in the Southwest China Karst region from 1985 to 2023, using the InVEST (Integrated Valuation of Ecosystem Services and Trade-offs) and optimal parameters-based geographical detector models. The results showed that: (a) forestland expanded by 7,048.37 km2 while grassland decreased by 11,297.18 km2, with prominent transitions between cropland and forestland; (b) the total carbon stocks increased by 20.47 Tg, with notable gains in southern Guizhou, eastern Yunnan, and central Guangxi. Critically, however, belowground carbon pools declined continuously by 58.55 Tg, which diverged from other pools; (c) forestland contributed 74% of the total carbon storage. Transitions from cropland and grassland to forestland added 559.59 Tg of carbon, whereas transitions from forestland to cropland or construction land caused a loss of 468.98 Tg; and (d) normalized difference vegetation index (NDVI), slope, and land use intensity were primary drivers, with the interaction between NDVI and slope showing the strongest influence on total carbon. Notably, belowground carbon was uniquely sensitive to the interaction between NDVI and gross domestic product. This study clarifies the asynchronous changes among different carbon pools in karst areas, providing a scientific basis for formulating targeted carbon sink management measures.