Assessing magnitudes and directions of CO2 fluxes within a karst landscape

Gas diffusion is a primary driver of carbon dioxide (CO₂) movement through unsaturated soils. In typical soils, high soil concentrations of CO₂ caused by autotrophic and heterotrophic respiration cause the gas to primarily diffuse upward. However, karst landscapes can have subsurface CO₂ sinks, both due to CaCO₃ weathering and losses via underlying caves and fracture networks. In this study our objective was to quantify the magnitude and direction of CO₂ fluxes in a pastured karst system located in Southwest Virginia (James Cave). Our hypotheses were: 1) the zero-flux plane, or location of maximum CO₂ concentration within the soil profile, is located at deeper depths, ≥60 cm depth during warmer months of the year and located at shallower depths, ≤60 cm, during the colder months of the year, 2) the zero-flux plane will exist ˂60 cm depth at the sinkhole location more often than at the upslope locations, and 3) CO₂ fluxes will be primarily upward during the growing season and primarily downward during the colder months of the year. We installed paired CO₂ and soil moisture sensors at 20 cm, 40 cm, and 60 cm depths, with profiles installed in the shoulder, midslope, and bottom (i.e., sinkhole) of a hillslope adjacent to the cave entrance. The sensors recorded hourly data between 7 February 2017 and 13 September 2019. The depth of the zero-flux plane was identified by the depth of maximum CO₂ concentration for each profile, while the measured concentration gradient from 20 to 60 cm was used to estimate CO₂ flux with Fick’s Law. Our findings support our hypotheses that the relative location of the zero-flux plane was located more often at deeper depths during warmer months of the year and located at shallower depths, i.e. ˂60 cm, during colder months of the year. The zero-flux plane was more frequently shallow (i.e., ˂60 cm) at the sinkhole location compared to the upslope profiles. The CO₂ fluxes reflected upward movement during the growing season and downward movement during the colder months of the year. We speculate that these processes reflect the influence of the underlying cave system, which may serve as a CO₂ sink during colder months, when the cave becomes vented via natural convection. Altogether, these findings suggest that downward diffusion may be an important yet oft-overlooked component of carbon fluxes in karst landscapes.