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

• Published in: Agricultural and forest meteorology 2011, Vol.151 (1), p.39-48
• Main Authors: Kammer, Adrian, Tuzson, Béla, Emmenegger, Lukas, Knohl, Alexander, Mohn, Joachim, Hagedorn, Frank
• Format: Article
• Language: English
• Subjects: accuracy; carbon dioxide; equations; Fagus sylvatica subsp. sylvatica; forest soils; forest trees; headspace analysis; laser spectroscopy; mathematical models; measuring devices; methodology; new methods; soil respiration; stable isotopes; temperate forests; temporal variation
• ISSN: 0168-1923; 1873-2240

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 20min 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 10min, 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.