Assessing Landscape and Seasonal Controls on Soil CO2 Fluxes in a Karst Sinkhole

Carbon dioxide (CO2) gas diffusion is an important component of carbon cycling in soils. This process is particularly relevant in karst landscapes, which con-tain easily weathered rock, subsurface fractures, and cave networks. We instrumented three soil profiles- the shoulder, back slope, and toe slope of a sinkhole- above a karst cave in Virginia. Each profile had solid -state CO2 sensors and soil water content/temperature sensors at 20 and 60 cm depth that collected hourly measurements from 2017 to 2019. We calculated CO2 fluxes using Fick's first law along with measured soil and assumed atmospheric CO2 concentrations. With this approach, we identified occasional near-surface zero-flux planes, in which CO2 likely diffused both upward and downward. All profiles had upward CO2 fluxes during warm-season months, with maximum fluxes of 1.2 lmol CO2 m-2 s-1 in the shoulder and back slope versus 2.0 lmol CO2 m-2 s-1 in the toe slope. During cool-season months, upward CO2 fluxes were smaller (0-0.3 lmol CO2 m-2 s-1) and were often counteracted by downward fluxes in the toe slope, possi-bly driven by ventilation into the underlying cave. The toe slope had a cumulative annual efflux of 14.5 mol CO2 m-2, which was .3 times greater than the other profiles. Fluxes were sensitive to soil porosity, with an order-of-magnitude difference when porosity was assumed to be 0.40 versus 0.56 cm3 cm -3. The results of this study offer new insight into short-term and seasonal varia-tions in diffusive CO2 gas transport in karst soils, and they may inform other investigations of non-uniform diffusion processes.