HR: 11:05h
AN: B41E-04    [PDF]
TI: Partitioning Net Ecosystem Carbon Exchange Into net Assimilation and Respiration With Canopy-scale Isotopic Measurements: an Error Propagation Analysis With Both $^{13}$C and $^{18}$O Data
AU: * Peylin, P
EM: peylin@lsce.saclay.cea.fr
AF: BiomCo, Inra-grignon, Thiverval Grignon, 78850 France
AU: Ogee, J
EM: ogee@lsce.saclay.cea.fr
AF: LSCE, CEA-Orme des merisiers, Gif sur Yvette, 91191 France
AU: Cuntz, M
EM: cuntz@lsce.saclay.cea.fr
AF: LSCE, CEA-Orme des merisiers, Gif sur Yvette, 91191 France
AU: Bariac, T
EM: thierry.bariac@inra.fr
AF: BiomCo, Inra-grignon, Thiverval Grignon, 78850 France
AU: Ciais, P
EM: ciais@lsce.saclay.cea.fr
AF: LSCE, CEA-Orme des merisiers, Gif sur Yvette, 91191 France
AU: Brunet, Y
EM: yves.brunet@inra.fr
AF: Bioclimatologie, Inra-Bordeaux, Villenave d'ornon, 40000 France
AB: Stable CO$_{2}$ isotope measurements are increasingly used to partition the net CO$_{2}$ exchange between terrestrial ecosystems and the atmosphere in terms of non-foliar respiration ({\it F}$_{R}$) and gross photosynthesis ({\it F}$_{A}$). However the accuracy of the partitioning strongly depends on the isotopic disequilibrium between these two gross fluxes and a rigorous estimation of the errors on {\it F}$_{A}$ and {\it F}$_{R}$ is needed. In this study we account and propagate uncertainties on all terms in the mass balance equations for total and "labeled" CO$_{2}$ in order to get precise estimates of the errors on {\it F}$_{A}$ and {\it F}$_{R}$. We applied our method to a maritime pine forest in the Southwest of France. Using the $\delta^{13}$C-CO$_{2}$ and CO$_{2}$ measurements, we show that the resulting uncertainty associated to the gross fluxes can be as large as 4 æmol m$^{-2}$ s$^{-1}$. In addition, even if we could get more precise estimates of the isoflux and the isotopic signature of {\it F}$_{A}$ we show that this error would not be significantly reduced. This is because the isotopic disequilibrium between {\it F}$_{A}$ and {\it F}$_{R}$ is around 2-3$\permil$, i.e. the order of magnitude of the uncertainty on the isotopic signature of {\it F}$_{R}$ ($\delta_{R}$). With $\delta^{18}$O-CO$_{2}$ and CO$_{2}$ measurements, the uncertainty associated to the gross fluxes lies also around 4 æmol m$^{-2}$ s$^{-1}$. On the other hand, it could be dramatically reduced if we were able to get more precise estimates of the CO$^{18}$O isoflux and the associated discrimination during photosynthesis. This is because the isotopic disequilibrium between {\it F}$_{A}$ and {\it F}$_{R}$ is large, of the order of 10-15$\permil$, i.e. much larger than the uncertainty on $\delta_{R}$. The isotopic disequilibrium between {\it F}$_{A}$ and {\it F}$_{R}$ or the uncertainty on $\delta_{R}$ vary among ecosystems and over the year. Our approach may help to choose the best strategy to study the carbon budget of a given ecosystem using stable isotopes.
DE: 0315 Biosphere/atmosphere interactions
DE: 0330 Geochemical cycles
DE: 0400 Biogeosciences
DE: 0614 Biological effects
DE: 1615 Biogeochemical processes (4805)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting