Stable isotopes of carbon dioxide (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 C18O16O and C16O2 and their ratio, δa. The TDL system was configured to measure mixing ratios of [C16O2] and [C18O16O] at wavenumber frequencies of 2308.225 and 2308.416 cm-1, respectively. Two-minute precision (1 standard deviation) was 0.0004, 0.09 μmol mol-1, and 0.26‰ for [C18O16O], [C16O2] and δa, respectively. Comparison of TDL and mass spectrometry flask measurements showed relatively good agreement (r2 = 0.94) with a standard deviation of 0.55‰ 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 C18O16O. The isotope signature of net ecosystem CO2 exchange at night ranged from -15 to -7‰ and during the daytime from -40 to -20‰. A simple estimate of canopy-scale photosynthetic discrimination showed significant diurnal variation and averaged 10.5‰ (±8.8‰). 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 C18O16O exchange.
Bibliographical noteFunding Information:
We express our sincere thanks to Bill Breiter (USDA-ARS Biological Field Technician) for his assistance in the field and laboratory. Undergraduate research assistant Ray Anderson was supported with an Undergraduate Research Opportunity fellowship by the College of Agricultural, Food, and Environmental Sciences (COAFES) at the University of Minnesota. Kurt Spokas provided invaluable advice with interfacing the Daqbook and DASYLab software. Iyabo Lawal helped design the flask sampling system and analyzed the flask samples. Mike Dolan (USDA-ARS Technician) provided technical expertise maintaining the isotope ratio mass spectrometer. Dr. Kaycie Billmark and Jianmin Zhang provided comments on an earlier draft of the manuscript. The authors gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model website ( http://www.arl.noaa.gov/ready.html ) used in this manuscript. Financial assistance for this project was provided by a Faculty Development Grant from COAFES (TJG) and was supported by the Office of Science (BER), U.S. Department of Energy, Grant No. DE-FG02-03ER63684 (TJG and JMB) and the U.S. National Science Foundation, Grant No. EAR-0229343 (XL). Finally, we thank Dr. D.R. Bowling and the anonymous reviewers of this manuscript for their helpful comments and criticisms.
- Carbon dioxide
- Ecosystem discrimination
- Flux partitioning
- Oxygen isotopes
- Tunable diode laser (TDL)
- Water vapor