HR: 0800h
AN: B51C-0959    [Abstracts]
TI: Estimating the Isotope Ratio of Ecosystem Respiration Using the Keeling Plot and the Flux Ratio Method
AU: * Zhang, J
EM: jzhang@umn.edu
AF: Department of Soil, Water, and Climate University of Minnesota, 439 Borlaug Hall 1991 Upper Buford Circle, St Paul, MN 55108 United States
AU: Griffis, T J
EM: tgriffis@umn.edu
AF: Department of Soil, Water, and Climate University of Minnesota, 439 Borlaug Hall 1991 Upper Buford Circle, St Paul, MN 55108 United States
AU: Baker, J M
EM: jbaker@umn.edu
AF: Department of Soil, Water, and Climate University of Minnesota, 439 Borlaug Hall 1991 Upper Buford Circle, St Paul, MN 55108 United States
AU: Baker, J M
EM: jbaker@umn.edu
AF: USDA-ARS, 439 Borlaug Hall 1991 Upper Buford Circle, St Paul, MN 55108 United States
AB: Stable carbon isotope analyses have been used in identifying global carbon sources and sinks and in partitioning ecosystem CO$_{2}$ exchange into component fluxes. The isotope ratio of ecosystem respiration ($\delta^{13}$C$_{r}$) is a critical parameter in applying stable isotope techniques to carbon cycle problems. The commonly used Keeling plot method in estimating $\delta^{13}$C$_{r}$ has limitations related to: 1) insufficient range of CO$_{2}$ mixing ratio; 2) high sensitivity to curve-fitting techniques; and 3) extrapolation of CO$_{2}$ mixing ratio beyond observations. In this study, the Keeling plot method was examined and compared with the flux ratio approach using continuous measurements of the mixing ratios of $^{12}$CO$_{2}$ and $^{13}$CO$_{2}$ over an extensive corn canopy during the 2003 growing season. The seasonal variation of $\delta^{13}$C$_{r}$ estimated from both methods harmonized with the ecosystem phenology. The $\delta^{13}$C$_{r}$ started to increase (became more positive) from mid June and peaked in early August, followed by a decrease into October. The Keeling plot method agreed well with the flux ratio method in the seasonal pattern of $\delta^{13}$C$_{r}$, but tended to give lower values (more negative). The discrepancy between the two approaches was significant in July and August (about 5 per mil) and relatively small in June and September (about 1 to 2 per mil). We examined this discrepancy with respect to wind direction/advection and measurement footprints. In addition, our analysis of high-frequency data (every two minutes) using the flux ratio method indicates that $\delta^{13}$C$_{r}$ may vary significantly at short time-scales (e.g., hourly), which could have significant implications for flux partitioning studies.
DE: 4805 Biogeochemical cycles (1615)
DE: 3322 Land/atmosphere interactions
DE: 3307 Boundary layer processes
DE: 0315 Biosphere/atmosphere interactions
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting