CO2 Dynamics and Transport Mechanisms Across Atmosphere–Soil–Cave Interfaces in Karst Critical Zones

Cave systems serve as key interfaces connecting surface and underground carbon cycles, and research on their carbon dynamics provides a unique perspective for revealing the mechanisms of carbon transport and transformation in karst critical zones. In this study, we established a multi-factor monitoring framework spanning the atmosphere–soil–cave continuum and associated meteorological conditions, continuously recorded cave microclimate parameters (temperature, relative humidity, atmospheric pressure, and cave winds) and CO2 concentrations across atmospheric–soil–cave interfaces, and employed stable carbon isotope (δ13C) tracing in Mahuang Cave, a typical karst cave in southwestern China, from 2019 to 2023. The results show that the seasonal amplitude of atmospheric CO2 and its δ13C is small, while soil–cave CO2 and δ13C fluctuate synchronously, exhibiting “high concentration-light isotope” signatures during the rainy season and the opposite pattern during the dry season. Cave CO2 concentrations drop by about 29.8% every November. Soil CO2 production rates are jointly controlled by soil temperature and volumetric water content, showing a threshold effect. The δ13C response exhibits nonlinear behavior due to the combined effects of land-use type, vegetation cover, and soil texture. Quantitative analysis establishes atmospheric CO2 as the dominant source in cave systems (66%), significantly exceeding soil-derived contributions (34%). At diurnal, seasonal, and annual scales, carbon-source composition, temperature and precipitation patterns, ventilation effects, and cave structure interact to control the rhythmic dynamics and spatial gradients of cave microclimate, CO2 levels, and δ13C signals. Our findings enhance the understanding of carbon transfer processes across the karst critical zone.