Jianmin Zhang1, Timothy J. Griffis1 and John M. Baker2 P1.11 Using Continuous Stable Isotope Measurements to Partition Net Ecosystem CO2 Exchange Jianmin Zhang1, Timothy J. Griffis1 and John M. Baker2 1Department of Soil, Water, and Climate, University of Minnesota, St. Paul, Minnesota, USA 2USDA-ARS, Department of Soil, Water and Climate, University of Minnesota, St. Paul, Minnesota, USA OBJECTIVES Partition the net ecosystem exchange of CO2 (NEE) by combining stable isotope measurements and micrometeorological observations Examine the temporal variation of the critical isotope characteristics of the ecosystem RESULTS Climate and phenology Precipitation: 400mm (May.~Sep.), 26% less than normal Soil water content (θ) : between 0.15 and 0.35 cm3cm-3 LAI: maximum 4.5 (Aug. 7) Flux partitioning The daytime R tended to be lower than Re The peak of Re was skewed towards afternoon, while R peaked around noon δ13Cp remained relatively constant for the whole season The values of δ13Cp compared well to those from plant isotope analyses Diurnal variation of isotopomers Isotope ratio changed with CO2 mixing ratio Air enriched in 13C during the day and depleted at night More deviation at night and in early morning than in daytime MEASUREMENTS Site : 17 ha homogeneous agricultural field at the Rosemount Research and Outreach Center at U of M Vegetation: corn (C4 pathway) in 2003, soybean (C3 pathway) in previous year Isotopomers: mixing ratios of 12CO2 and 13CO2 using tunable diode laser (TDL) technique Fluxes: energy flux components and NEE measured by eddy covariance technique Other measurements: leaf area index (LAI), precipitation, soil water content, temperature, and humidity Plant isotope analyses: Fisons Optima continuous flow model mass spectrometer Fig.1 Climate and phenology in 2003. Fig. 2 Ensemble diurnal variation of CO2 mixing ratio and isotope ratio during July 29 to August 5, 2003. Fig.5 Ensemble diurnal variations of photosynthesis and respiration from isotopic partitioning method and regression model. Fig.6 Ensemble diurnal variations of δ13Cz, δ13Cb, δ13Cp, and δ13C. Variations of δ 13Cb and δ13Cr Half-hour δ13Cb using eq.1~2 and half-hour 12CO2 and 13CO2 at two heights (real-time method) Half-hour δ13Cb from the slope of the plot of two-minute 12CO2 and 13CO2 (linear-fit method) δ13Cr of early season varied from -10‰ to -28‰, in correspondence with the phenology CONCLUSIONS The isotopic method provides real-time partitioning of NEE and is ecophysiologically based The isotopic method differed from the regression partitioning method in diurnal pattern The partitioning was most sensitive to δ13Cb. METHODOLOGY Estimation of δ13Cb and δ13Cr The isotope ratios of NEE (δ13Cb) and ecosystem respiration (δ13Cr, nighttime δ13Cb ) estimated from two heights: REFERENCES Bowling D.R. et al. (2001) Partitioning net ecosystem carbon exchange with isotopic fluxes of CO2. Glob. Chang. Biol. 7, 127-145. Evans J.R. et al. (1986) Carbon isotope discrimination measured concurrently with gas exchange to investigate CO2 diffusion in leaves of higher plants. Aust. J. Plant. Physiol. 13, 281-292. Farquhar G.D. et al. (1989) Carbon isotope discrimination and photosynthesis. Annu. Rev. Plant. Physiol. Plant Mol. Biol. 40, 503-537. Griffis T.J. et al. (2004) Measuring field-scale isotopic CO2 fluxes with tunable diode laser absorption spectroscopy and micrometeorological techniques. Agric. For. Meteorol. 124,15-29. (2) (1) Flux partitioning Isotope partitioning (eq.3~5) and nighttime regression partitioning (eq.6): (3) (4) ACKNOWLEDGEMENTS This research was supported by the Office of Science (BER), U.S. Department of Energy, grant No. DE-FG02-03ER63684, and the University of Minnesota, Grant-in-Aid-of-Research, Artistry and Scholarship Program (TJG). (5) (6) Fig. 3 Comparison of real-time and linear-fit method in estimating δ13Cb during Aug.3~Aug.10, 2003. Fig. 4 Seasonal variation of δ13Cr in 2003