Application of a laser-based spectrometer for continuous in situ measurements of stable isotopes of soil CO2 in calcareous and acidic soils

Jobin Joseph, Christoph Külls, Matthias Arend, Marcus Schaub, Frank Hagedorn, Arthur Gessler, Markus Weiler
2019 SOIL  
<p><strong>Abstract.</strong> 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 <span class="inline-formula">CO<sub>2</sub></span> concentration and respective isotopic composition at a high temporal resolution for longer periods of time. <span class="inline-formula"><i>δ</i><sup>13</sup>C</span> of soil <span
more » ... l <span class="inline-formula">CO<sub>2</sub></span> can be used to derive information on the origin and physiological history of carbon, and <span class="inline-formula"><i>δ</i><sup>18</sup>O</span> in soil <span class="inline-formula">CO<sub>2</sub></span> aids in inferring the interaction between <span class="inline-formula">CO<sub>2</sub></span> and soil water. We established a real-time method for measuring soil <span class="inline-formula">CO<sub>2</sub></span> concentration, <span class="inline-formula"><i>δ</i><sup>13</sup>C</span> and <span class="inline-formula"><i>δ</i><sup>18</sup>O</span> values across a soil profile at higher temporal resolutions (0.05–0.1<span class="thinspace"></span>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 <span class="inline-formula"><i>δ</i><sup>13</sup>C</span> and <span class="inline-formula"><i>δ</i><sup>18</sup>O</span> values under highly varying <span class="inline-formula">CO<sub>2</sub></span> 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 <span class="inline-formula">CO<sub>2</sub></span> release. We found that in the calcareous Gleysol, <span class="inline-formula">CO<sub>2</sub></span> originating from carbonate dissolution contributed to the total soil <span class="inline-formula">CO<sub>2</sub></span> concentration at detectable degrees, potentially due to <span class="inline-formula">CO<sub>2</sub></span> evasion from groundwater. The <span class="inline-formula"><sup>13</sup>C</span>-<span class="inline-formula">CO<sub>2</sub></span> of topsoil at the calcareous soil site was found reflect <span class="inline-formula"><i>δ</i><sup>13</sup>C</span> values of atmospheric <span class="inline-formula">CO<sub>2</sub></span>, and the <span class="inline-formula"><i>δ</i><sup>13</sup>C</span> of topsoil <span class="inline-formula">CO<sub>2</sub></span> at the acidic soil site was representative of the biological respiratory processes. <span class="inline-formula"><i>δ</i><sup>18</sup>O</span> values of <span class="inline-formula">CO<sub>2</sub></span> in both sites reflected the <span class="inline-formula"><i>δ</i><sup>18</sup>O</span> of soil water across most of the depth profile, except for the 80<span class="thinspace"></span>cm depth at the calcareous site where a relative enrichment in <span class="inline-formula"><sup>18</sup>O</span> was observed.</p>
doi:10.5194/soil-5-49-2019 fatcat:umja4ofw6vhqdam6o6k45b3ln4