A field and laboratory method for monitoring the concentration and isotopic composition of soil CO 2

The stable isotope composition of nmol size gas samples can be determined accurately and precisely using continuous flow isotope ratio mass spectrometry (IRMS). We have developed a technique that exploits this capability in order to measure δ 13 C and δ 18 O values and, simultaneously, the concentration of CO 2 in sub‐mL volume soil air samples. A sampling strategy designed for monitoring CO 2 profiles at particular locations of interest is also described. This combined field and laboratory technique provides several advantages over those previously reported: (1) the small sample size required allows soil air to be sampled at a high spatial resolution, (2) the field setup minimizes sampling times and does not require powered equipment, (3) the analytical method avoids the introduction of air (including O 2 ) into the mass spectrometer thereby extending filament life, and (4) p CO 2 , δ 13 C and δ 18 O are determined simultaneously. The reproducibility of measurements of CO 2 in synthetic tank air using this technique is: ±0.08‰ ( δ 13 C), ±0.10‰ ( δ 18 O), and ±0.7% ( p CO 2 ) at 5550 ppm. The reproducibility for CO 2 in soil air is estimated as: ±0.06‰ ( δ 13 C), ±0.06‰ ( δ 18 O), and ±1.6% ( p CO 2 ). Monitoring soil CO 2 using this technique is applicable to studies concerning soil respiration and ecosystem gas exchange, the effect of elevated atmospheric CO 2 (e.g. free air carbon dioxide enrichment) on soil processes, soil water budgets including partitioning evaporation from transpiration, pedogenesis and weathering, diffuse solid‐earth degassing, and the calibration of speleothem and pedogenic carbonate δ 13 C values as paleoenvironmental proxies. Copyright © 2008 John Wiley & Sons, Ltd.