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

摘要

We present a novel technique in which the carbon isotope ratio (δ~(13)C) of soil CO_2is measured fromsmall gas samples (<5 mL) injected into a stream of CO_2-free air flowing into a tunable diode laserabsorption spectrometer (TDL). This new method extends the dynamic range of the TDL to measureCO_2mole fractions ranging from ambient to pure CO_2, reduces the volume of sample required afew mL, and does not require field deployment of the instrument. The measurement precision ofsamples stored for up to 60 days was 0.23%o. The new TDL method was applied with a simple gas wellsampling technique to obtain and measure gas samples from shallow soil depth increments for CO_2mole fraction and S13C analysis, and subsequent determination of theδ~(13)C of soil-respired CO_2. Themethod was tested using an artificial soil system containing a controlled CO2 source and comparedwith an independent method using the TDL and an open soil chamber. The profile and chamberestimates of δ~(C of an artificially produced CO2 flux were consistent and converged to the 313c of theCO_2source at steady state, indicating the accuracy of both methods under controlled conditions. Thenew TDL method, in which a small pulse of sample is measured on a carrier gas stream, is analogousfor the TDL technique to the development of continuous-flow configurations for isotope ratio massspectrometry. While the applications presented here are focused on soil CO2, this new TDL methodcould be applied in a number of situations requiring measurement ofδ~(C of CO_2in small gassamples with ambient to high CO_2mole fractions.