Quantification of Carbonate Rock Dissolution Rates and Their Environmental Controls in the Northern Guangdong Karst Region Using the Dissolution Tablet Method

Carbonate dissolution, a key process in the global carbon cycle, sequesters large quantities of atmospheric CO2 and directly regulates inorganic carbon fluxes from terrestrial, river, and ocean ecosystems. Therefore, the accurate quantification of carbonate dissolution rates is essential for refining global carbon cycle models. Although carbonate dissolution rates and their influencing factors have been extensively studied in southwestern karst regions in China, such research in subtropical southern China remains limited, especially regarding the dissolution characteristics, environmental drivers, and dissolution mechanisms under different vegetation types and soil depths. This limits the accuracy of regional and global carbon sink assessments, as the unique environmental conditions in southern China may drive distinct carbonate dissolution dynamics compared with those in the southwestern parts of the country. To address this knowledge gap, we focused on key scientific questions regarding carbonate dissolution characteristics, environmental controlling factors, and dissolution mechanisms under different vegetation types and soil depths in the subtropical karst region of northern Guangdong. We used the dissolution tablet method, a reliable technique for simulating natural rock weathering, to measure the dissolution rates across four burial positions (30 cm above ground, 0 cm at the ground surface, 20 cm below ground, and 50 cm below ground) under two dominant vegetation cover types (evergreen broad-leaved forest and coniferous forest) in the Shimentai National Nature Reserve, a typical karst area in northern Guangdong. The following three critical results were found: (1) The vegetation type significantly affected the carbonate dissolution rate and carbon sink intensity. The average dissolution rate under the evergreen broad-leaved forest (204.4 ± 238.7 g·m-2·a-1) was approximately 7.5 times higher than that under the coniferous forest (27.5 ± 20.1 g·m-2·a-1). Correspondingly, the karst carbon sink intensity under the broad-leaved forest (54.6 ± 63.8 g C·m-2·a-1) was approximately 7.4 times greater than that under the coniferous forest (7.3 ± 5.4 g C·m-2·a-1). (2) The dissolution rate increased significantly with increasing soil depth. The rate at 50 cm below ground (282.4 ± 313.2g·m-2·a-1) was 5 times higher than that at the ground surface (57.5 ± 57.5 g·m-2·a-1) and 2.4 times higher than that at 20 cm below ground (119.5 ± 102.9 g·m-2·a-1), highlighting deep soil as being a hot spot for karst carbon sequestration in southern China. (3) The soil temperature was the core environmental factor regulating the dissolution rate at the small watershed scale, showing a significant negative correlation with this parameter (p < 0.01). The soil temperature decreased with increasing soil depth, and this cooler microenvironment promoted carbonate dissolution, indicating that warming may inhibit rock weathering in the subtropical context of southern China, a pattern distinct from that of temperate regions. This study provides critical in situ data on the carbonate dissolution rate and karst carbon sink intensity in the subtropical karst region of southern China, addressing the information gap in this understudied area. By identifying vegetation type and soil temperature as the key controlling factors and elucidating the mechanism of depth-dependent carbonate dissolution, our findings contribute to the development of region-specific carbonate weathering models for subtropical karst systems. This study also supports more accurate regional carbon sink assessments and provides science-based strategies for enhancing natural carbon sequestration in karst ecosystems of southern China.