Subsoil carbonate dissolution rates and their determining factors in a karst drainage basin, SW China

Subsoil carbonate dissolution is the drives of atmosphere CO₂ consumption, underground water alkalinity and karst landform. Specifically, the determining factors and corresponding dissolution models for subsoil carbonate dissolution rate (${\text{R}}_{\text{Carb}}$) remain under-researched in various seasons and soil profiles in situ with standard limestone tablets. They are the results of both ecosystem dynamics and hydrological processes. Thus, this study aimed to explore ${\text{R}}_{\text{Carb}}$ under different seasons, soil depths and water drainage conditions in a karst drainage basin and reveal the factors determining them. Six hundred eighty-four standard limestone tablets were exposed to fields in the subsoil of 10 cm, 20 cm, 30 cm and 40 cm deep, sixteen plots, five sites over every 3 to 4 months in a total of 423 days in the basin of the Miaoguang underground river in Southwest China. The determining factors, including soil CO₂ concentration, moisture content, temperature, and pH, were monitored at the same time. The results are summarized in the following statements. ${\text{R}}_{\text{Carb}}$, ranged from -0.39 to 18.15 mg cm⁻² a⁻¹, were in a clear seasonal pattern. Principal component analysis (PCA) of the observed soil factors revealed the mechanisms that in hot and rainy summer, high ${\text{R}}_{\text{Carb}}$ occurred in an open system with continuously supplemented CO₂ (period I) and decreased with rainfall amount (period IV). Low ${\text{R}}_{\text{Carb}}$ occurred in a closed system with inactive biological CO₂ production under cold weather (period II and III). Hence as soil temperature decreased, ${\text{R}}_{\text{Carb}}$ increased. ${\text{R}}_{\text{Carb}}$ increased with soil depths as the elevated soil CO₂, temperature and available acid water. Dissolution occurred in an open system on an annual average scale, and a well-fitting equation of ${\text{R}}_{\text{Carb}}$ and soil factors were given. Local water drainage was critical in carbonate chemical dissolving with implications of formations of local karst landform. Due to no or less groundwater drain and spring seepage, ${\text{R}}_{\text{Carb}}$ measured in site B (a flat saddle with underlying dolomite bedrock) was nearly zero. Conversely, with water retention, ${\text{R}}_{\text{Carb}}$ was higher in gentle limestone hillslope or depression than in steep hillside because of longer water–rock interaction duration. In general, in karst drainage basin areas, ${\text{R}}_{\text{Carb}}$ was determined by multiple factors about seasons, soil depths, and water drainage.