Reactive soil inputs during high-flow events decouple carbon chemistry and CO2 evasion in a granitic headwater stream

Christina Martina Schubert , Robert van Geldern, Harald Maid, Christian Placht, Marcus Speck, Johannes A. C. Barth, Anna-Neva Visser

Quantifying CO2 dynamics in low-alkalinity headwater streams is challenging because thermodynamically based evaluations of pCO2 and CO2 fluxes (FCO2) assume coherent DIC-alkalinity-pH coupling. This study examines how hydrologically driven disturbances violate this assumption in a forested, granitic headwater stream using 15 months of calculated pCO2 and FCO2, alongside measurements of pH, alkalinity, dissolved inorganic and organic carbon (DIC, DOC, d13CDIC, d13CDOC), major ion chemistry, redox-sensitive metals, and natural organic matter characterization (SUVA254, 13C-NMR). During baseflow, pCO2 declined sharply from the spring (~11 088 µatm) to downstream sites, approaching atmospheric levels. In contrast, high-flow events produced large increases in pCO2 and FCO2 (up to 4.5-fold downstream), accompanied by decreases in d13CDIC (2-4‰), pH (up to 0.9 units), and alkalinity (up to 0.01 mmol L1), elevated DOC, and enhanced mobilization of Fe, Al, and Mn, while measured DIC showed no proportional increase. Charge-balance errors exceeding 20 % during these events indicate contributions of unquantified proton-active solutes. Soil mineralogical analyses and NOM extractions revealed organic functional groups and fine minerals consistent with these perturbations. Together, these results demonstrate that high-flow mobilization of soil-derived organic and inorganic species decouples DIC-alkalinity-pH dynamics, systematically biasing thermodynamically based pCO₂ and FCO2 evaluations in weakly buffered granitic streams.