Short-term CO2(g) exchange between a shallow karstic cavity and the external atmosphere during summer: Role of the surface soil layer

This study tests the hypothesis that the degree of moisture in the soil pore system determines gas exchange processes (ventilation/charge) between the outer atmosphere and the karst-epikarst during the warm, dry period (summer). These processes explain “anomalous” CO2 fluxes measured over this and other ecosystems. Emission of CO2 by ventilation of cavities requires an open double membrane system (host rock and soil) through which air movement can take place (H2Ovapour, CO2, 222Rn, etc.). An experimental study on the behavior of the soil and host rock porous system under changing air humidity conditions, coupled with a broad analytical approach addressing CO2 fluxes using the eddy-covariance technique and monitoring of the cave microclimate serves to define the suitable environmental conditions favoring air transfer between the cave atmosphere and exterior. This study shows the correlation between evapotranspiration, CO2 emissions, and cave ventilation processes due to the daytime opening of the soil membrane. Thus, the role of the soil as a membrane/interface or transfer medium can be observed, and it is directly dependent on weather conditions (temperature, humidity, wind).

Research highlights► Daily episodes of ventilation in caves represent net CO2 emissions to the exterior. ► Emission of CO2 of caves requires an open double membrane system (host rock-soil). ► The soil acts as a barrier limiting gas exchange depending on weather conditions. ► The soil porous system determines the daily cyclic operation of the membrane.