Abstract. The short-term dynamics of carbon and water fluxes
across the soil–plant–atmosphere continuum are still not fully understood.
One important constraint is the lack of methodologies that enable
simultaneous measurements of soil CO2 concentration and respective
isotopic composition at a high temporal resolution for longer periods of
time. δ13C of soil CO2 can be used to derive
information on the origin and physiological history of carbon, and
δ18O in soil CO2 aids in inferring the interaction
between CO2 and soil water. We established a real-time method for
measuring soil CO2 concentration, δ13C and
δ18O values across a soil profile at higher temporal
resolutions (0.05–0.1 Hz) using an off-axis integrated cavity output
spectroscopy (OA-ICOS). We also developed a
calibration method correcting for the sensitivity of the device against
concentration-dependent shifts in δ13C and δ18O
values under highly varying CO2 concentration. The deviations of
measured data were modelled, and a mathematical correction model was
developed and applied for correcting the shift. By coupling an OA-ICOS with
hydrophobic but gas-permeable membranes placed at different depths in acidic
and calcareous soils, we investigated the contribution of abiotic and biotic
components to total soil CO2 release. We found that in the
calcareous Gleysol, CO2 originating from carbonate dissolution
contributed to the total soil CO2 concentration at detectable
degrees, potentially due to CO2 evasion from groundwater. The
13C-CO2 of topsoil at the calcareous soil site was found
reflect δ13C values of atmospheric CO2, and the
δ13C of topsoil CO2 at the acidic soil site was
representative of the biological respiratory processes. δ18O
values of CO2 in both sites reflected the δ18O of
soil water across most of the depth profile, except for the 80 cm depth at
the calcareous site where a relative enrichment in 18O was
observed.