REPOSITORY > RESULTS

Abstract

Global environmental change is one of the most pressing issues in Earth science research, and significant effort has been devoted to understanding the role of the terrestrial reservoir in modulating carbon storage and cycling. Studies dedicated to CO₂ dynamics in terrestrial ecosystems have focused on either surface soils or bedrock cavities. In karstic areas, both systems exist and are interconnected. It is generally assumed that soil CO₂ originates solely from soil respiration. Recent studies, however, have revealed that abiotic terrestrial CO₂ exchange in carbonate-rich areas can significantly influence and, at times, even dominate soil CO₂ dynamics. Likewise, the ventilation of CO₂ from subterranean fissures and caves, which can store large amounts of CO₂, represents a potential major source of soil CO₂.
This doctoral thesis investigates carbon fluxes and their sources in soils developed on geologically different parent materials and assesses the influence of belowground processes. It comprises five investigations in which natural tracers were used to assess the relative contributions of different CO₂ sources to soil CO₂ fluxes and/or to investigate carbon transport through soils in the gas and aqueous phases across different geological settings. Cave air CO₂ dynamics were investigated as a potential source of soil CO₂ in karstic areas. Soil CO₂ fluxes were also studied on the flanks of an active volcano to better understand the physicochemical interactions between deep CO₂ and the shallow subsurface environment. The tracers included stable carbon isotopes (¹²C and ¹³C), radiocarbon (¹⁴C), radon (²²²Rn), and stable oxygen isotopes (¹⁶O and ¹⁸O).
First, CO₂ sources in forest soils developed on different parent materials in Slovenia were investigated. Isotopic analyses revealed that the CO₂ was young and that there was no difference in the age of soil CO₂ above carbonate and non-carbonate bedrock. However, ²²²Rn concentrations in karst soils suggested that the ventilation of subterranean voids significantly affects soil CO₂ dynamics. Second, a study of dissolved inorganic and organic carbon dynamics in forest soils revealed that local lithology, vegetation type, soil texture, and climate are important factors controlling carbon losses in both forms. The δ¹³C values of dissolved inorganic carbon (DIC) revealed that approximately half of the DIC originates from soil CO₂. Consequently, 2–5 mol m⁻² year⁻¹ of carbon is lost through carbonate dissolution in the form of DIC. Dissolved organic carbon losses at a soil depth of 5 cm were lower, at approximately 1 mol m⁻² year⁻¹, and showed no significant differences among the study sites.
Third, soil CO₂ fluxes in forest soil above a known subterranean cave were investigated. The findings revealed that cave ventilation affects soil CO₂ fluxes and can temporarily even dominate them. Fourth, a study of the sources and dynamics of cave air CO₂ in Pisani rov, a gallery of the Postojna Cave system, revealed that outside air temperature has a major influence on cave ventilation. Cave CO₂ originates mainly from soil respiration and enters the cave directly from the soil, whereas CO₂ transported into the cave by dripwater has only a minor effect on cave CO₂ concentrations. Finally, measurements of soil CO₂ efflux and its isotopic composition on the flanks of Mount Etna, Italy, revealed the different pathways through which deep-source CO₂ reaches the surface and demonstrated the influence of volcanic activity on CO₂ effluxes and isotopic composition.
In summary, this thesis demonstrates how natural isotope abundances can provide important insights into CO₂ fluxes and the processes driving them both above and below ground.