Application of a quantum cascade laser-based spectrometer in a closed chamber system for real-time delta C-13 and delta O-18 measurements of soil-respired CO2

摘要

Laser spectroscopy is an emerging technique to analyze the stable isotopic composition of soil-respired CO2 (delta C-13(resp) delta O-18(resp)) 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 delta C-13(resp) and delta O-18(resp) In a Swiss beech forest a total of 90 chamber measurements with 20 min sampling time each were performed The Instrument measured the delta C-13 and delta O-18 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 delta C-13(resp) directly from the flux ratio of (CO2)-C-13 and (CO2)-C-12 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 delta O-18-Keeling plots showed a significant bending very likely due to the equilibration of chamber CO2 with the O-18 of surface soil water Therefore we used a quadratic curve fit of the Keeling plots to estimate delta O-18(resp) Our results also revealed that delta C-13(resp) was not constant throughout the CO2 accumulation in the closed soil chambers there were significant but non-systematic variations in delta C-13(resp) for the first 10 min and systematic shifts in delta C-13(resp) of on average 1 9 parts per thousand 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 delta C-13(resp) and delta O-18(resp)