Application of a laser-based spectrometer for continuous in situ measurements of stable isotopes of soil CO 2 in calcareous and acidic soils

Joseph, Jobin; Külls, Christoph; Arend, Matthias; Schaub, Marcus; Hagedorn, Frank; Gessler, Arthur; Weiler, Markus

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 inline-formulaCO2 concentration and respective isotopic composition at a high temporal resolution for longer periods of time. inline-formulaδ13C of soil inline-formulaCO2 can be used to derive information on the origin and physiological history of carbon, and inline-formulaδ18O in soil inline-formulaCO2 aids in inferring the interaction between inline-formulaCO2 and soil water. We established a real-time method for measuring soil inline-formulaCO2 concentration, inline-formulaδ13C and inline-formulaδ18O 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 inline-formulaδ13C and inline-formulaδ18O values under highly varying inline-formulaCO2 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 inline-formulaCO2 release. We found that in the calcareous Gleysol, inline-formulaCO2 originating from carbonate dissolution contributed to the total soil inline-formulaCO2 concentration at detectable degrees, potentially due to inline-formulaCO2 evasion from groundwater. The inline-formula13C-inline-formulaCO2 of topsoil at the calcareous soil site was found reflect inline-formulaδ13C values of atmospheric inline-formulaCO2, and the inline-formulaδ13C of topsoil inline-formulaCO2 at the acidic soil site was representative of the biological respiratory processes. inline-formulaδ18O values of inline-formulaCO2 in both sites reflected the inline-formulaδ18O of soil water across most of the depth profile, except for the 80 cm depth at the calcareous site where a relative enrichment in inline-formula18O was observed.

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Joseph, Jobin / Külls, Christoph / Arend, Matthias / et al: Application of a laser-based spectrometer for continuous in situ measurements of stable isotopes of soil CO2 in calcareous and acidic soils. 2019. Copernicus Publications.

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