Unraveling the overlooked hotspots and hot moments of CO2 migration and sequestration in the soil-groundwater system under rainfall-induced water table fluctuation

The fate and transport of carbon dioxide (CO 2 ) in the soil-groundwater system is crucial for understanding terrestrial carbon cycling. However, primary driving mechanisms controlling soil-groundwater CO 2 migration under rainfall-induced water table fluctuation remain unclear. Here, we investigated CO 2 concentration and flux at a representative soil profile in a monsoon-climate catchment through high-frequency monitoring. We explored carbonate dissolution/precipitation potentials and driving factors of CO 2 flux through flux calculation and statistical analysis. The results revealed that CO 2 concentration during the wet season was higher than that during the dry season. Water table fluctuation reshaped soil CO 2 distribution, inducing hotspots (∼1.84 × 10 5 ppm) at 80 cm depth. Water table rise accelerated transitions from carbonate precipitation to dissolution, and triggered negative (downward) soil CO 2 fluxes (minimum: −0.683 μmol/(m 2 ·s)) along depth during the wet season. Water table rise restrained soil CO 2 emissions (minimum: 0.171 μmol/(m 2 ·s)) while simultaneously enhancing CO 2 sequestration, generating hot moments (up to 16.2 μmol/(m 2 ·s)) of subsurface carbon storage, contributing disproportionally larger during the wet season. Subsurface CO 2 sequestration was mainly controlled by water table fluctuation, whereas soil CO 2 emission was driven by the synergy of water table fluctuation and soil microclimate. This study has important implications for refining soil CO 2 modeling and accurate assessment of carbon sequestration in the soil-groundwater system.