Feasibility of quantifying ecosystem-atmosphere C18O16O exchange using laser spectroscopy and the flux-gradient method

Stable isotopes of carbon dioxide (CO,) can be used as natural tracers to better understand carbon cycle processes and exchange pathways between the biosphere and atmosphere. In this study, we used a tunable diode laser (TDL) technique for continuous fast measurement of the stable isotopomers (COO)-O-18-O-16 and (CO)-O-16, and their ratio, Sa. The TDL system was configured to measure mixing ratios of [(CO2)-O-16] and [(COO)-O-18-O-16] at wavenumber frequencies of 2308.225 and 2308.416 cm(-1), respectively. Two-minute precision (I standard deviation) was 0.0004, 0.09 mu mol mol(-1), and 0.26 parts per thousand for [(COO)-O-18-O-16], [(CO)-O-16] and S., respectively. Comparison of TDL and mass spectrometry flask measurements showed relatively good agreement (r(2) = 0.94) with a standard deviation of 0.55 parts per thousand for the residual values. Estimates of the isotope signature of ecosystem flux components over a soybean (Glycine max) field were examined. These data represent one of the first continuous flux measurements of (COO)-O-18-O-16. The isotope signature of net ecosystem CO2 exchange at night ranged from -15 to -7 parts per thousand and during the daytime from -40 to -20 parts per thousand. A simple estimate of canopy-scale photosynthetic discrimination showed significant diurnal variation and averaged 10.5 parts per thousand ( +/- 8.8 parts per thousand). The large difference between the isotope signature of respiration and midday canopy photosynthesis represented significant isotopic disequilibrium. Coupled with recent advances in measuring water vapor isotopomers with the TDL technique, a new opportunity is emerging to better understand the dynamics, complex interactions, and discrimination mechanisms controlling land-atmosphere (COO)-O-18-O-16 exchange. (c) 2005 Elsevier B.V. All rights reserved.