In recent years considerable effort has been focused on combining micrometeorological and stable isotope techniques to partition net fluxes and to study biosphere-atmosphere exchange processes. While much progress has been achieved over the last decade, some new issues are beginning to emerge as technological advances, such as laser spectroscopy, permit isotopic fluxes to be measured more easily and continuously in the field. Traditional investigations have quantified the isotopic composition of biosphere-atmosphere exchange by using the Keeling two-member mixing model (the classic Keeling plot). An alternative method, based on a new capacity to measure isotopic mixing ratios, is to determine the isotope composition of biosphere-atmosphere exchange from the ratio of flux measurements. The objective of this study was to critically evaluate these methods for quantifying the isotopic composition of ecosystem respiration (δR) over a period of three growing seasons (2003-2005) within a heterogeneous landscape consisting of C3 and C4 species. For C4 canopies, the mixing model approach produced δR values that were 4-6' lower (isotopically lighter) than the flux-gradient method. The analyses presented here strongly suggest that differences between flux and concentration footprint functions are the main factor influencing the inequality between the mixing model and flux-gradient approaches. A mixing model approach, which is based on the concentration footprint, can have a source area influence more than 20-fold greater than the flux footprint. These results highlight the fact that isotopic flux partitioning is susceptible to problems arising from combining signals (concentration and fluxes) that represent very different spatial scales (footprint). This problem is likely to be most pronounced within heterogeneous terrain. However, even under ideal conditions, the mismatch between concentration and flux footprints could have a detrimental impact on isotopic flux partitioning where very small differences in isotopic signals must be resolved.
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Acknowledgements We express our sincere thanks to Bill Breiter (USDA-ARS Biological Field Technician) for his assistance in the field and laboratory. 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 the Office of Science (BER), U.S. Department of Energy, Grant No. DE-FG02-03ER63684 (TJG & JMB). Finally, we recognize the helpful technical support of Steve Sargent at Campbell Scientific Inc.
- Carbon cycling
- Ecosystem respiration
- Heterogeneous terrain
- Keeling plot
- Lagrangian flux and concentration footprints
- Similarity theory
- Stable isotopes