Response of riverine DIC to rainfall in karst regions: controls on carbonate-derived carbon

Understanding the sources of riverine dissolved inorganic carbon (DIC) and their response to rainfall is crucial for elucidating carbon cycling in karst catchments. In the Lijiang River Basin, dual carbon isotope tracing (δ¹³C and Δ¹⁴C), routine monitoring, high frequency rainfall sampling, and a Bayesian isotope mixing model were used to investigate DIC variability and source shifts during storm events. At the basin scale, DIC concentrations and δ¹³C_DIC values exhibited pronounced spatiotemporal variability, with carbonate weathering identified as the dominant carbon source. During rainfall, DIC dynamics differed among hydrological stages, reflecting changes in recharge pathways and hydrochemical conditions. Carbonate rock derived carbon remained the dominant DIC source during Stages A (water level rose slowly) and B (water level rose rapidly), but declined from 69.2% in Stage A to 48.6% in Stage C (water level decreased). In contrast, soil/organic-matter-derived CO₂ increased from 24.9% to 44.2%, whereas atmospheric CO₂ exchange remained relatively low, ranging from 4.2% to 7.2%. The less negative Δ¹⁴C_DIC values indicated increasing modern carbon inputs. These results reveal that rainfall drives a stage dependent reorganization of riverine DIC sources in karst systems, which should be considered in carbon budgeting and carbonate weathering carbon sink assessment.