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

This study characterizes the processes involved in seasonal CO 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 CO (g) and δCO in the caves, soils and outside atmospheric air was periodically conducted. The isotopic ratio of the source contribution enhancing the CO concentration was calculated using the Keeling model. We compared the isotopic ratio of the source in the soil (δC–soil) with that in the soil-underground system (δC–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 CO and the δCO of the sources of CO in the soil (δC–soil) and the system (δC–system). The roots respiration and soil organic matter degradation are the main source of CO in underground environments, and the inlet of the gas is mainly driven by diffusion and advection. Drier and warmer conditions enhance soil-exterior CO interchange, reducing the CO concentration and increasing the δCO of the soil air. Moreover, the isotopic ratio of the source of CO in both the soil and the system tends to heavier values throughout the dry and warm season. We conclude that seasonal variations of soil CO concentration and its C/C isotopic ratio are mainly regulated by thermo-hygrometric conditions. In cold and wet seasons, the increase of soil moisture reduces soil diffusivity and allows the storage of CO in the subsoil. During dry and warm seasons, the evaporation of soil water favours diffusive and advective transport of soil-derived CO to the atmosphere. The soil CO diffusion is enough important during this season to modify the isotopic ratio of soil produced CO (3–6‰ heavier). Drought induces release of CO with an isotopic ratio heavier than produced by organic sources. Consequently, climatic conditions drive abiotic processes that turn regulate a seasonal storage of soil-produced CO within soil and underground systems. The results here obtained imply that abiotic emissions of soil-produced CO must be an inherent consequence of droughts, which intensification has been forecasted at global scale in the next 100 years.
This research was funded by the Spanish Ministry of Economy and Competitiveness projects CGL2016-78318-C2-1R and CGL2016-78318-C2-2R AEI/FEDER/UE and its programme Torres Quevedo (PTQ 13-06296). Funding was also provided by the People Programme (Marie Curie Actions-Intra-European Fellowships, call 2013) of the European Union's Seventh Framework Programme (FP7/2007-2013) under the REA grant agreement nº 624204.