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

Abstract

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.

Partitioning of canopy and soil CO<sub>2</sub> fluxes in a pine forest at the dry timberline across a 13-year observation period

“…High belowground allocation helps explain the high rate of SOC (soil organic carbon) accumulation observed over the period since afforestation (Grünzweig et al, 2007;Qubaja et al, 2019). Note that, irrespective of the soil carbon accumulation, the abiotic component to the CO 2 flux seems to be significant in dry environments (Table 3) and in particular in the dry seasons, when biological activities drastically decrease (Kowalski et al, 2008;Lopez-Ballesteros et al, 2017;Serrano-Ortiz et al, 2010;Martí-Roura et al, 2019). The results show that considering the abiotic effects on estimating soil respiration and, in turn, on estimating the carbon budget in dry calcareous soils can play an important part in estimating soil and ecosystem respiration fluxes (Angert et al, 2015;Roland et al, 2012).…”

Spectroscopic techniques to analyze stable carbon isotopic compositions of carbon dioxide, methane and volatile organic compounds

“…16,[37][38][39] OA-ICOS techniques have drawn broad interest due to their simple structures and high sensitivities. 1,7,36,[38][39][40][41] Jost et al designed the rst OA-ICOS instrument for determining 13 C/ 12 C and 18 O/ 16 O ratios from CO 2 rotation/vibration ne structures at three different combined bands. Their preliminary results showed that the accuracy of d 13 C and d 18 O at 0.8 Hz is 3.3& and 2.8&, respectively, and the measurements were conducted over a period of 10 hours for pure CO 2 gas samples at 26 h Pa. By averaging 100 spectra over a subset of the data, they achieved an accuracy of 1.6& for d 13 C and 0.8& for d 18 O.…”

“…The optimal accuracy of d 13 C analysis is 0.022& by averaging the data obtained over 256 seconds, and the highest accuracy for d 18 O analysis is ±0.077& at the same time interval. 7 2.2.4. Quantum cascade laser absorption spectrometer (QCLAS).…”

“…1,2 Based on stable carbon isotope analysis, important information such as the source and transformation pathways of carbon-containing gas molecules and organic compounds are obtained. [2][3][4][5][6][7][8][9][10] The measured carbon isotopic ratios are referenced to the international standard material, namely Vienna Pee Dee Belemnite (VPDB), and are expressed in delta notation. 11 So far, the mainstream technique for carbon compound specic isotope analysis (C-CSIA) is isotope ratio mass spectrometry (IRMS).…”

Characterizing Natural Organic Matter Transformations by Microbial Communities in Terrestrial Subsurface Ecosystems: A Critical Review of Analytical Techniques and Challenges

“…Non-destructive approaches employing compound-specific labels (e.g., 13 C/ 15 N-labeled organic compounds) coupled to trace-gas analysis have a long history of use in soil biogeochemistry and microbiology ( Stevens et al, 1993 ; Staddon, 2004 ). The use of natural-abundance isotope, isotopologue, and isotopomer measurements on gaseous fluxes of CO 2 , CH 4 , or N 2 O also provide detailed analysis of potential microbial pathways leading to the transformation and release of those gases ( Wankel et al, 2017 ; Joseph et al, 2019 ). However, coupling soil processes, as informed by trace gas measurements (e.g., respiration, decomposition, denitrification), with the metabolisms underpinning the process remains reliant on destructive approaches.…”