Abiotic and seasonal control of soil-produced CO2 efflux in karstic ecosystems located in Oceanic and Mediterranean climates

摘要

This study characterizes the processes involved in seasonal CO2 exchange between soils and shallow underground systems and explores the contribution of the different biotic and abiotic sources as a function of changing weather conditions. We spatially and temporally investigated five karstic caves across the Iberian Peninsula, which presented different microclimatic, geologic and geomorphologic features. The locations present Mediterranean and Oceanic climates. Spot air sampling of CO2 (g) and delta(CO2)-C-13 in the caves, soils and outside atmospheric air was periodically conducted. The isotopic ratio of the source contribution enhancing the CO2 concentration was calculated using the Keeling model. We compared the isotopic ratio of the source in the soil (delta C-13(s)-soil) with that in the soil-underground system (delta C-13(s)-system). Although the studied field sites have different features, we found common seasonal trends in their values, which suggests a climatic control over the soil air CO2 and the delta(CO2)-C-13 of the sources of CO2 in the soil (delta C-13(s)-soil) and the system (delta C-13(s)-system). The roots respiration and soil organic matter degradation are the main source of CO2 in underground environments, and the inlet of the gas is mainly driven by diffusion and advection. Drier and warmer conditions enhance soil-exterior CO2 interchange, reducing the CO2 concentration and increasing the delta(CO2)-C-13 of the soil air. Moreover, the isotopic ratio of the source of CO2 in both the soil and the system tends to heavier values throughout the dry and warm season.