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

Abstract

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.