Combining a quantum cascade laser spectrometer with an automated closed-chamber system for δ13C measurements of forest soil, tree stem and tree root CO2 fluxess

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