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 ¹³C/¹²C 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 δ¹³C 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 δ¹³C 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 δ¹³C. Overall, correcting for the CO₂ concentration increased the δ¹³C determined from the Keeling plots by 3.4‰ compared to 2.1‰ for the water vapour correction. We used the combined system to measure δ¹³C 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 δ¹³C was −29.8 ± 0.32‰ for the intact soil plots, which was similar to the mean δ¹³C of −29.8 ± 1.2‰ for the trenched soil plots. The lowest δ¹³C was found for the root plots with a mean of −32.6 ± 0.78‰. The mean δ¹³C of the stems was −30.2 ± 0.74‰, similar to the mean δ¹³C of the soil plots. In conclusion, the study showed the potential of using a quantum cascade laser spectrometer to measure δ¹³C 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 δ¹³C. View Full-Text