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

Abstract

Purpose

The objective of this research is to detect abiotic sources of soil CO2 above a subterranean cave in the Slovenian karst region.

Materials and methods

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.

Results and discussion

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.

Conclusions

Ventilation from subterranean cavities is an important source of soil CO2 in karstic areas and should be taken into account during carbon cycling studies.

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Acknowledgments

The study is a part of the PhD thesis of B. Krajnc supported by the Innovative scheme for co-financing of doctoral studies financed by the European Union through the European Social Fund and by the scholarship granted by the World Federation of Scientists. We are grateful to Iztok J. Košir from the Slovenian Institute for Hop Research and Brewing for the pedologic analyses and the Farmland and Forest Fund of the Republic of Slovenia for the permission to do research in the forest. We would also like to thank the Regional Forest Service Postojna of the Slovenian Forest Service and Maksimilijan Gorup for providing information regarding sanitation forest cutting. We appreciate the support of cave guides Stanislav Glažar, Janez Margon and Erik Rebec for their dedication during fieldwork. We would also like to acknowledge the managers of Postojna Jama d.d. and Ministry of Agriculture and Environment of Slovenia for the permission to access and work in the cave.

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Correspondence to Bor Krajnc.

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Responsible editor: Jadran Faganeli

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Krajnc, B., Ferlan, M. & Ogrinc, N. Soil CO2 sources above a subterranean cave—Pisani rov (Postojna Cave, Slovenia). J Soils Sediments 17, 1883–1892 (2017). https://doi.org/10.1007/s11368-016-1543-x

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Keywords

  • Abiotic sources
  • Karst region
  • Soil CO2
  • Stable isotopes
  • Ventilation