Field-based cavity ring-down spectrometry of δ¹³C in soil-respired CO₂

Abstract

Measurement of soil-respired CO₂ at high temporal resolution and sample density is necessary to accurately identify sources and quantify effluxes of soil-respired CO₂. A portable sampling device for the analysis of δ¹³C values in the field is described herein.

CO₂ accumulated in a soil chamber was batch sampled sequentially in four gas bags and analysed by Wavelength-Scanned Cavity Ring-down Spectrometry (WS-CRDS). A Keeling plot (1/[CO₂] versus δ¹³C) was used to derive δ¹³C values of soil-respired CO₂. Calibration to the δ¹³C Vienna Peedee Belemnite scale was by analysis of cylinder CO₂ and CO₂ derived from dissolved carbonate standards. The performance of gas-bag analysis was compared to continuous analysis where the WS-CRDS analyser was connected directly to the soil chamber.

Although there are inherent difficulties in obtaining absolute accuracy data for δ¹³C values in soil-respired CO₂, the similarity of δ¹³C values obtained for the same test soil with different analytical configurations indicated that an acceptable accuracy of the δ¹³C data were obtained by the WS-CRDS techniques presented here. Field testing of a variety of tropical soil/vegetation types, using the batch sampling technique yielded δ¹³C values for soil-respired CO₂ related to the dominance of either C₃ (tree, δ¹³C=−27.8 to−31.9 ‰) or C₄ (tropical grass, δ¹³C=−9.8 to−13.6 ‰) photosynthetic pathways in vegetation at the sampling sites. Standard errors of the Keeling plot intercept δ¹³C values of soil-respired CO₂ were typically<0.4 ‰ for analysis of soils with high CO₂ efflux (>7–9 μmol m⁻² s⁻¹).

Keywords:

• carbon-13
• field measurement
• isotope ecology
• isotope measurements
• methods and equipment
• laser spectroscopy
• soil-respired CO₂

Acknowledgements

This research was supported by the Australian Research Council Linkage, Infrastructure and Facilities (LIEF) grant LE110100144 and an Australian Research Council Federation Fellowship (FF0883221) to MIB. Jon Lloyd kindly provided assistance with regression analysis.

Supplementary material

can be viewed online: doi: 10.1080/10256016.2013.750606.