RESUMO
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 δ(13)C 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 δ(13)C) was used to derive δ(13)C values of soil-respired CO2. Calibration to the δ(13)C 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 δ(13)C values in soil-respired CO2, the similarity of δ(13)C values obtained for the same test soil with different analytical configurations indicated that an acceptable accuracy of the δ(13)C 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 δ(13)C values for soil-respired CO2 related to the dominance of either C3 (tree, δ(13)C=-27.8 to-31.9 ) or C4 (tropical grass, δ(13)C=-9.8 to-13.6 ) photosynthetic pathways in vegetation at the sampling sites. Standard errors of the Keeling plot intercept δ(13)C values of soil-respired CO2 were typically<0.4 for analysis of soils with high CO2 efflux (>7-9 µmol m(-2) s(-1)).
