Measuring field-scale isotopic CO2 fluxes with tunable diode laser absorption spectroscopy and micrometeorological techniques

The combination of micrometeorological and stable isotope techniques offers a relatively new approach for elucidating ecosystem-scale processes. Here we combined a micrometeorological gradient technique with tunable diode laser absorption spectroscopy (TDLAS) using the Trace Gas Analyzer (TGA 100, Campbell Scientific, Inc., Utah, USA) to measure field-scale isotopic CO2 mixing ratios and fluxes of (CO2)-C-12 and (CO2)-C-13. The experiment was conducted in a recently harvested soybean (Glycine max) field that had been in corn (Zea mays) production the previous 4 years. Measurements were made over a period of 26 days from October 25 to November 19, 2002. Weather conditions were unusually cold and dry during the experiment. Isotopic gradients were small and averaged -0.153 and -0.0018 mumol mol(-1) m(-1) for (CO2)-C-12 and (CO2)-C-13, respectively for U. > 0.1 M s(-1). The average (CO2)-C-12) and (CO2)-C-13, flux for the period was 1.0 and 0.012 mumol m(-2) s(-1,) respectively. The isotope ratio of respired carbon (delta(13)C(R)) obtained from the linear intercept of a Keeling plot was -27.93parts per thousand (+/-0.32parts per thousand) for the experimental period. The Keeling plot technique was compared to a new flux ratio methodology that estimates delta(13)C(R) from the slope of a linear plot of (CO2)-C-13, versus (CO2)-C-12, flux. This method eliminates a number of potential limitations associated with the Keeling plot and provides a delta(13)C(R) value that can be directly related to the flux footprint. In this initial comparison, our analysis showed that the flux ratio method produced a similar delta(13)C(R) value (-28.67parts per thousand), but with greater uncertainty (+/-2.1parts per thousand). Better results are expected during growing season conditions when fluxes are substantially larger and the signal to noise ratio is improved. The isotope ratio of respired carbon was consistent with C-3 agricultural systems indicating that soybean decomposition was the dominant substrate for respiration. The observed increase in ecosystem respiration (RE) and decrease in delta(13)C(R) following tillage indicated that the incorporation of fresh soybean residue provided the major source for decomposition and further illustrates that the combination of micrometeorological and stable isotope techniques can be used to better interpret changes in carbon cycle processes. Long-term and continuous measurements Of isotopic CO2 exchange using tunable diode laser absorption spectroscopy and micrometeorological techniques offers a new opportunity to study carbon cycle processes at the field-scale. (C) 2004 Elsevier B.V. All rights reserved.