Disentangling the drivers of soil CO₂ ventilation in a Mediterranean dryland using in situ and remote sensing techniques

Abstract. Subterranean CO₂ concentrations are driven by complex interactions between biological and physical processes. In semiarid ecosystems, atmospheric processes can play a relevant role in modulating soil CO₂ storage and release. In the current study, a multi-instrumental dataset, collected in a Mediterranean shrubland in southern Spain, was analyzed, and the main atmospheric drivers controlling soil CO₂ and radon (Rn) dynamics were investigated. Based on a precise methodology, 10 significant ventilation events were detected, and the Spearman correlation coefficients between the soil CO₂ and Rn concentrations and the different atmospheric variables were calculated.

The results identified surface atmospheric pressure as the most consistent and independent driver across the events, exhibiting strong negative correlations with the subterranean CO₂ and Rn concentrations. Surface-level friction velocity (u_*), boundary-layer turbulent kinetic energy dissipation rate (ϵ) and wind shear (sh) showed significant positive correlations. However, their independence was not consistent comparing diluting ventilation events, when u_* was more relevant, with enriching ventilation periods, that were more influenced by boundary-layer ϵ and sh. In contrast, at lower altitudes ϵ, sh, atmospheric boundary layer height and mixing layer height were less strongly correlated with soil CO₂ and Rn concentration changes.

These findings provide new insights into the mechanisms that promote soil-atmosphere transport in drylands, especially those regarding the carbon cycle, and highlight the need to incorporate such mechanisms into Earth system models to improve carbon cycle predictions under future climate scenarios.