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

Abstract

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 δ¹³CO₂ 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 (δ¹³C_s–soil) with that in the soil-underground system (δ¹³C_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 δ¹³CO₂ of the sources of CO₂ in the soil (δ¹³C_s–soil) and the system (δ¹³C_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 δ¹³CO₂ 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.

We conclude that seasonal variations of soil CO₂ concentration and its ¹³C/¹²C 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 CO₂ in the subsoil. During dry and warm seasons, the evaporation of soil water favours diffusive and advective transport of soil-derived CO₂ to the atmosphere. The soil CO₂ diffusion is enough important during this season to modify the isotopic ratio of soil produced CO₂ (3–6‰ heavier). Drought induces release of CO₂ 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 CO₂ within soil and underground systems. The results here obtained imply that abiotic emissions of soil-produced CO₂ must be an inherent consequence of droughts, which intensification has been forecasted at global scale in the next 100 years.

Keywords: Vadose zone, CO₂ exchange, δ¹³CO₂, Climatic control, Soil CO₂ diffusion