An injection method for measuring the carbon isotope content of soil carbon dioxide and soil respiration with a tunable diode laser absorption spectrometer

We present a novel technique in which the carbon isotope ratio (δ¹³C) of soil CO₂ is measured from small gas samples (<5 mL) injected into a stream of CO₂‐free air flowing into a tunable diode laser absorption spectrometer (TDL). This new method extends the dynamic range of the TDL to measure CO₂ mole fractions ranging from ambient to pure CO₂, reduces the volume of sample required to a few mL, and does not require field deployment of the instrument. The measurement precision of samples stored for up to 60 days was 0.23‰. The new TDL method was applied with a simple gas well sampling technique to obtain and measure gas samples from shallow soil depth increments for CO₂ mole fraction and δ¹³C analysis, and subsequent determination of the δ¹³C of soil‐respired CO₂. The method was tested using an artificial soil system containing a controlled CO₂ source and compared with an independent method using the TDL and an open soil chamber. The profile and chamber estimates of δ¹³C of an artificially produced CO₂ flux were consistent and converged to the δ¹³C of the CO₂ source at steady state, indicating the accuracy of both methods under controlled conditions. The new TDL method, in which a small pulse of sample is measured on a carrier gas stream, is analogous for the TDL technique to the development of continuous‐flow configurations for isotope ratio mass spectrometry. While the applications presented here are focused on soil CO₂, this new TDL method could be applied in a number of situations requiring measurement of δ¹³C of CO₂ in small gas samples with ambient to high CO₂ mole fractions. Copyright © 2010 John Wiley & Sons, Ltd.