Soil CO2 sources above a subterranean cave—Pisani rov (Postojna Cave, Slovenia)

Abstract

The objective of this research is to detect abiotic sources of soil CO2 above a subterranean cave in the Slovenian karst region. The research was performed in the forest above Pisani rov (Postojna Cave) near the town of Postojna (SW Slovenia) and also in the cave. Soil gas, atmospheric air and cave air carbon stable isotope composition (δ13CCO2) and CO2 concentration were measured. Sampling and measurements were performed bi-monthly at the test and control sites above the cave. The abiotic source of soil CO2 was estimated using a stable isotope mass balance calculation. Similar seasonal patterns of soil CO2 and δ13CCO2 values were observed at both the test and control sites until spring, with higher levels of CO2 observed in summer and lower in winter. The δ13CCO2 showed the opposite trend, i.e. lower values (−26 to −20 ‰) in summer and higher values (up to −17 ‰) in winter and early spring. In spring, the soil CO2 concentration decreases and the δ13CCO2 value increases only at the control site. A time series of a modelled “isotopically light” endmember revealed large shifts in the data values, due to the presence of an abiotic CO2 source. Results suggest that the subterranean CO2 pool and its ventilation is the main source of soil CO2, accounting for up to 80 % of the soil gas during cold periods. Ventilation from subterranean cavities is an important source of soil CO2 in karstic areas and should be taken into account during carbon cycling studies.