Combining a Quantum Cascade Laser Spectrometer with an Automated Closed-Chamber System for δ13C Measurements of Forest Soil, Tree Stem and Tree Root CO2 Fluxes

Recent advances in laser spectroscopy have allowed for real-time measurements of the C-13/C-12 isotopic ratio in CO2, thereby providing new ways to investigate carbon cycling in natural ecosystems. In this study, we combined an Aerodyne quantum cascade laser spectrometer for CO2 isotopes with a LI-COR LI-8100A/8150 automated chamber system to measure the delta C-13 of CO2 during automated closed-chamber measurements. The isotopic composition of the CO2 flux was determined for each chamber measurement by applying the Keeling plot method. We found that the delta C-13 measured by the laser spectrometer was influenced by water vapour and CO2 concentration of the sample air and we developed a method to correct for these effects to yield accurate measurements of delta C-13. Overall, correcting for the CO2 concentration increased the delta C-13 determined from the Keeling plots by 3.4 parts per thousand compared to 2.1 parts per thousand for the water vapour correction. We used the combined system to measure delta C-13 of the CO2 fluxes automatically every two hours from intact soil, trenched soil, tree stems and coarse roots during a two-month campaign in a Danish beech forest. The mean delta C-13 was -29.8 +/- 0.32 parts per thousand for the intact soil plots, which was similar to the mean delta C-13 of -29.8 +/- 1.2 parts per thousand for the trenched soil plots. The lowest delta C-13 was found for the root plots with a mean of -32.6 +/- 0.78 parts per thousand. The mean delta C-13 of the stems was -30.2 +/- 0.74 parts per thousand, similar to the mean delta C-13 of the soil plots. In conclusion, the study showed the potential of using a quantum cascade laser spectrometer to measure delta C-13 of CO2 during automated closed-chamber measurements, thereby allowing for measurements of isotopic ecosystem CO2 fluxes at a high temporal resolution. It also highlighted the importance of proper correction for cross-sensitivity with water vapour and CO2 concentration of the sample air to get accurate measurements of delta C-13.