Using ²²²Rn and carbon isotopes (¹²C, ¹³C and ¹⁴C) to determine CO₂ sources in forest soils developed on contrasting geology in Slovenia

Global carbon estimates have identified abiotic CO₂ as a potentially significant source of atmospheric CO₂, albeit little is known about its origin. The aim of this study was to identify the origin of soil CO₂ using carbon isotopes and ²²²Rn data. The study involved collecting data from seven Slovenian forest soils developed over bedrock with contrasting geology where different origin of soil CO₂ was expected; two sampling sites were located on soils formed above carbonate bedrock, one above metamorphic bedrock and the remainder above clastic sedimentary rocks. Analysis of soil gas including the levels of CO₂, carbon isotope measurements (¹²C, ¹³C and ¹⁴C) and ²²²Rn activity was recorded at a soil depth of 80 cm. Isotopic analysis revealed that the CO₂ was young and there was no difference in the age of soil CO₂ above either carbonate or non-carbonate bedrock. The data also suggest that the ¹³C-enrichment in soil CO₂, above carbonate bedrock was a consequence of the mixing of soil CO₂ with atmospheric CO₂ and/or the ventilation of subterranean CO₂ from pores, fissures and cavities. The latter effect was supported by the high ²²²Rn concentrations observed at these sites. Based on the ${\delta }^{13}{\text{C}}_{{\text{CO}}_2}$ data, photosynthesis prevailed over microbial respiration accounting for the majority (>70 %) of total soil CO₂ over non-carbonate bedrock—at least at the time of sampling. Overall, results from this study could represent useful information for global carbon cycle models used to predict the impacts of climate changes.