Field-based cavity ring-down spectrometry of 13C in soil-respired CO2

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 C-13 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 C-13) was used to derive C-13 values of soil-respired CO2. Calibration to the C-13 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 C-13 values in soil-respired CO2, the similarity of C-13 values obtained for the same test soil with different analytical configurations indicated that an acceptable accuracy of the C-13 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 C-13 values for soil-respired CO2 related to the dominance of either C-3 (tree, C-13=-27.8 to-31.9 parts per thousand) or C-4 (tropical grass, C-13=-9.8 to-13.6 parts per thousand) photosynthetic pathways in vegetation at the sampling sites. Standard errors of the Keeling plot intercept C-13 values of soil-respired CO2 were typically<0.4 parts per thousand for analysis of soils with high CO2 efflux (>7-9molm(-2)s(-1)).