Using δ ¹³ C to reveal the importance of different water transport pathways in two nested karst basins, Southwest China

This study used carbon (C) isotope sourcing to determine transport processes of dissolved inorganic carbon (DIC) from the land surface to river catchments in Southwest China. Both nested karst watersheds investigated (Chenqi and Houzhai) are representative of typical karst landform environments (e.g., primary forest, secondary forest, and farmland). We measured DIC concentrations and the δ13C values of rainfall, river water, groundwater, soil, and plants. To do so, we used IsoSource (a Visual Basic program) to determine source partitioning over time (seasonal) and across the two nested watersheds. In 2017, the mean DIC concentration was 0.06 ± 0.03 mmol L−1 and the rainfall δ13CDIC value was −14.4‰ ±1.9‰. We found similar DIC concentrations in the surface and groundwater of both watersheds, ranging from 0.20 to 0.71 mmol L−1 (seasonal) and from −3.7‰ to −9.4‰ (δ13CDIC) in the Chenqi catchment and from 0.33 to 0.60 mmol L−1 (seasonal) and from −10.3‰ to −6‰ (δ13CDIC) in the Houzhai watershed. The average δ13C values of soil and local plants were −24.6 ± 1.4‰ and −28.9 ± 1.2‰ in the Chenqi catchment and −25.8 ± 0.9‰ and −27.2 ± 1.8‰ in Houzhai watershed, respectively. In addition, carbonate bedrock and groundwater were the main sources of surface water in the Chenqi and Houzhai nested watersheds, both being greater than 30%. Source percentages were ∼20% from atmospheric deposition and ∼10% from soil. Furthermore, HCO3− was the predominant form of DIC (pH values > 8), and the contribution rates of dissolved carbonate minerals (HCO3−) were approximately 10.4% and 19.6% in the Chenqi catchment and the Houzhai watershed, respectively.