Application of a quantum cascade laser-based spectrometer in a closed chamber system for real-time δ13C and δ18O measurements of soil-respired CO2

Laser spectroscopy is an emerging technique to analyze the stable isotopic composition of soil-respired CO2 (δ13Cresp, δ18Oresp) in situ and at high temporal resolution. Here we present the first application of a quantum cascade laser-based spectrometer (QCLS) in a closed soil-chamber system to determine simultaneously δ13Cresp and δ18Oresp. In a Swiss beech forest, a total of 90 chamber measurements with 20 min sampling time each were performed. The instrument measured the δ13C and δ18O of the CO2 in the chamber headspace at every second with a precision of 0.25‰, resulting in Keeling plots with 1200 data points. In addition, we calculated δ13Cresp directly from the flux ratio of 13CO2 and 12CO2. The flux-ratio values were 0.8‰ lower than the Keeling plot intercepts when the flux rates were derived from quadratic curve fits of the CO2 increase. The δ18O-Keeling plots showed a significant bending very likely due to the equilibration of chamber CO2 with the 18O of surface soil water. Therefore, we used a quadratic curve fit of the Keeling plots to estimate δ18Oresp. Our results also revealed that δ13Cresp was not constant throughout the CO2 accumulation in the closed soil chambers: there were significant but non-systematic variations in δ13Cresp for the first 10 min, and systematic shifts in δ13Cresp of on average 1.9 ‰ in the second part of the 20-min measurements. These biases were probably caused by non-steady-state conditions in the soil-chamber system. Our study illustrates that the high temporal resolution of QCLS measurements allows the detection of non-linearities in the isotopic effluxes of CO2 from the soil due to soil-chamber feedbacks. This information can be used to improve the estimates for δ13Cresp and δ18Oresp.

Research highlights▶ First application of a QCL-based spectrometer in a closed soil-chamber system. ▶ Non-linearities in high-resolution 13C- and 18O-Keeling plots. ▶ Comparison of different approaches to estimate δ13C and δ18O of soil respiration.