DIC-FCO2 coupled variation modes in a cascade-dammed karst river: Control by carbonate buffering.

Carbonate weathering delivers substantial dissolved inorganic carbon (DIC) to karst rivers, yet strong carbonate buffering decouples this DIC pool from atmospheric CO2 evasion flux (FCO2), eliminating linear scaling between the two. In cascade-dammed karst rivers, reservoir impoundment further reshapes DIC-to-CO2 transformation by modifying water residence time, thermal stratification and water-air gas exchange, hindering mechanistic insights into buffering controls on DIC-FCO2 coupling. Here, we investigated the Xijiang River Basin, China, by integrating seasonal hydrochemical observations, CO2 flux estimates, carbonate-buffering metrics and multivariate statistical analyses. Surface waters were generally supersaturated with CO2 and acted as net atmospheric sources, but FCO2 varied strongly with season and hydrological setting. Natural river reaches showed higher summer emissions, whereas stratified reservoirs suppressed summer evasion and enhanced winter release after destratification. The dominant control on FCO2 changed systematically along the cascade: natural river was mainly regulated by pCO2, stratified reservoir-impacted zone shifted from pCO2 dominance to stronger gas-transfer control from summer to winter, and mixed reservoir-impacted zone remained primarily constrained by k. These spatial and seasonal contrasts revealed three carbonate-buffering-controlled DIC-FCO2 coupling modes: strong-buffering retention or decoupling mode, weak-buffering transmission or release mode, and buffer-equilibrium exchange regulation. Together, the results show that CO2 evasion from cascade-dammed karst rivers is governed not only by carbonate-weathering-derived DIC supply, but also by the capacity of carbonate buffering to retain or transmit DIC signals and by the efficiency of water-air exchange. This framework clarifies why high-DIC karst rivers can display divergent CO2 emission responses under dam regulation and provides a basis for mode-specific emission assessment in carbonate basins.