Karst carbon sink: Evidence from long term monitoring of CO2 and CH4 fluxes with stable isotope insights in subtropical Southwest China

Land use transformations significantly influences the balance between soil organic carbon (SOC) sequestration and greenhouse gas emissions. Amidst the escalating global climate crisis, unraveling the impacts of ecological restoration and conservation practices on greenhouse gas dynamics across diverse land uses becomes increasingly urgent, especially within ecologically sensitive karst landscapes. This study conducted monthly monitoring of CO₂ and CH₄ fluxes coupled with stable isotope analysis (δ¹³C_CO₂) from March 2023 to February 2024, encompassing four different land use types (farmland, grassland, shrubland, and forest) in a subtropical karst region of southwest China. For comparison, a non-karst forest land located less than 1 km away was also monitored. Principal findings include: (1) Compared to non-karst areas, karst soils exhibited significantly lower CO₂ emission rates (p < 0.01), showing a sequential deline along vegetation succession gradients: shrubland (134.93±70.5 mg C m⁻² h⁻¹) > forest (131.56±66.75 mg C m⁻² h⁻¹) > farmland (129.91± 81.72mg C m⁻² h⁻¹) > grassland (124.31±54.82 mg C m⁻² h⁻¹). The positive δ¹³C_CO₂ values (1.8‰ to 3.5‰) indicate preferential depletion of the lighter carbon isotope (¹²CO₂) through karst dissolution processes, resulting in relative enrichment of the heavier isotope (¹³CO₂) within the system. (2) Progressive vegetation succession significantly enhanced karst carbon sequestration, with subsurface dissolution rates peaking at 14.31 mg cm^-²·yr^-1 during the shrubland stage. The dissolution rates increased by 4.76-15.94 mg cm^-²·yr^-1 along the grassland-to-shrubland succession sequence, demonstrating the predominant role of pioneer shrub communities in promoting carbon sink potential. (3) Structural equation modeling (SEM) pathway analysis revealed distinct regulatory mechanisms: soil CO₂ fluxes were primarily driven by microbial biomass carbon (MBC) and temperature (R²=0.79), while δ¹³C_CO₂ fractionation was co-regulated by pH, moisture, MBC, and dissolution rates (R²=0.78). These findings demonstrate that karst processes enhance subsurface carbon sequestration through dual mechanisms: reducing net soil CO₂ emissions and  promoting inorganic carbon transfer to groundwater systems via intensified carbon isotope fractionation effects. This study provides the quantitative elucidation of the coupled relationships among vegetation succession, karst processes, and carbon isotope fractionation. It offers critical scientific evidence for optimizing ecological restoration strategies and advancing carbon neutrality objectives in subtropical karst ecosystems.