Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/18/2005
Publication Date: 12/14/2005
Citation: Griffis, T.J., Lee, X., Baker, J.M., King, J.Y., Sargent, S.D. 2005. Feasibility of quantifying ecosystem-atmosphere C18O16O exchange using laser spectroscopy and the flux-gradient method. Agricultural and Forest Meteorology. 135:44-60. Interpretive Summary: Stable isotopes of CO2 can be used as natural tracers to better understand carbon cycle processes and exchange pathways between the biosphere and atmosphere. However, their concentrations are difficult to measure; to this point the only viable procedure has been to collect flask samples for subsequent laboratory analysis on a mass spectrometer. We tested a newly available portable instrument that uses a tunable diode laser and is capable of measuring the CO2 isotopomers continuously in a sampled stream of air. We compared it to a mass spectrometer for measuring the individual concentrations of C18O16O and C16O2 above a bare soil surface and above a soybean canopy. The instrument produced results that were comparable to the mass spectrometer, with the additional advantage of providing continuous data. The data were used to determine the extent to which soybeans discriminate against the heavier isotope during photosynthesis, and to compute the isotope ratio of respired carbon. This research benefits other scientists by providing parameters that are needed to constrain models used to estimate continental-scale carbon exchange processes, which are critical components in predicting the magnitude and impact of potential climate change.
Technical Abstract: Stable isotopes of CO2 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 C18 O 16 O and C16 O 2 and their ratio, delta. The TDL system was configured to measure mixing ratios of C16 O 2 and C18 O 16 O at wavenumber frequencies of 2308.225 and 2308.416 cm-1, respectively. Half-hourly precision (1 standard deviation) was 0.0002 µmol mol-1, 0.06 µmol mol-1, and 0.07‰ for C18 O 16 O, C16 O 2 and delta, respectively. Comparison of TDL and mass spectrometry flask measurements showed relatively good agreement (r 2 = 0.94) and were not statistically different at the 0.01 signficance level. Estimates of the isotope ratio of ecosystem flux components over a bare soil surface and a soybean (Glycine max) canopy were examined using the traditional Keeling mixing model and the isotopomer flux ratio method. These data represent one of the first direct flux measurements of C18 O 16 O. The isotope ratio of nighttime respired CO2 ranged from -15‰ to -7‰ and daytime net ecosystem CO2 exchange ranged from -40 to -20‰. Canopy-scale photosynthetic discrimination showed significant diurnal variation and averaged 10.5‰ (±8.8‰). The large difference between the isotope ratio 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 18 O exchange.