Measuring the 13 C content of soil‐respired CO 2 using a novel open chamber system

Carbon dioxide respired by soils comes from both autotrophic and heterotrophic respiration. 13 C has proved useful in differentiating between these two sources, but requires the collection and analysis of CO 2 efflux from the soil. We have developed a novel, open chamber system which allows for the accurate and precise quantification of the δ 13 C of soil‐respired CO 2 . The chamber was tested using online analyses, by configuring a GasBench II and continuous flow isotope ratio mass spectrometer, to measure the δ 13 C of the chamber air every 120 s. CO 2 of known δ 13 C value was passed through a column of sand and, using the chamber, the CO 2 concentration stabilized rapidly, but 60 min was required before the δ 13 C value was stable and identical to the cylinder gas (−33.3‰). Changing the chamber CO 2 concentration between 200 and 900 µmol.mol −1 did not affect the measured δ 13 C of the efflux. Measuring the δ 13 C of the CO 2 efflux from soil cores in the laboratory gave a spread of ±2‰, attributed to heterogeneity in the soil organic matter and roots. Lateral air movement through dry sand led to a change in the δ 13 C of the surface efflux of up to 8‰. The chamber was used to measure small transient changes (±2‰) in the δ 13 C of soil‐respired CO 2 from a peaty podzol after gradual heating from 12 to 35°C over 12 h. Finally, soil‐respired CO 2 was partitioned in a labelling study and the contribution of autotrophic and heterotrophic respiration to the total efflux determined. Potential applications for the chamber in the study of soil respiration are discussed. Copyright © 2008 John Wiley & Sons, Ltd.