HR: 11:35h
AN: B41E-06    [PDF]
TI: Constraints on the Use of $^{18}$O in CO$_{2}$ as a Tracer to Partition Gross Carbon Fluxes
AU: * Riley, W J
EM: wjriley@lbl.gov
AF: Lawrence Berkeley National Lab, 90-1106 LBNL, ESD 1 Cyclotron Rd, Berkeley, CA 94720 United States
AU: Still, C J
EM: still@icess.ucsb.edu
AF: U.C. Santa Barbara, Geography Department 3611 Ellison Hall UC Santa Barbara, Santa Barbara, CA 93106 United States
AB: Measurements of $^{18}$O in atmospheric CO$_{2}$ can be used to partition measured net CO$_{2}$ ecosystem fluxes into photosynthesis and respiration. However, uncertainties and temporal variability in the $\delta^{18}$O value of the soil-surface CO$_{2}$ flux ($\delta F_{s}$) and the retro-diffused CO$_{2}$ flux ($\delta F_{r}$) can lead to substantial errors in partitioning estimates. We will discuss an integrated isotope and ecosystem model (ISOLSM) that simulates exchanges of $^{18}$O in H$_{2}$O and CO$_{2}$ in soil and plants, and will apply the model to identify critical factors associated with CO$_{2}$ flux partitioning. Modeling results, regression analysis of model predictions, and an analysis of characteristic times of relevant processes indicate that, in contrast to previous reports, the $\delta^{18}$O value of soil water ($\delta_{sw}$) in the top few cm of soil strongly impacts $\delta F_{s}$. Thus, accurately characterizing near-surface $\delta_{sw}$ is critical to the CO$_{2}$ flux partitioning approach. We also discuss the impact of the soil CO$_{2}$ source distribution within the column, soil temperature, and the $\delta^{18}$O value of atmospheric CO$_{2}$ on predictions of $\delta F_{s}$. Disequilibrium between CO$_{2}$ and leaf water, which may be common in C$_{4}$ grasses, will impact $\delta$ F$_{r}$ and therefore the partitioning of the measured net ecosystem CO$_{2}$ flux. Finally, temporal variability in $\delta$ F$_{r}$, in particular, can lead to errors in flux partitioning when measurements of the $\delta^{18}$O value of leaf water and of $^{18}$O in atmospheric CO$_{2}$ and are not made concurrently. We will present results demonstrating the impact of these factors on partitioning estimates and discuss measurement protocol necessary to accurately partition measured net ecosystem CO$_{2}$ fluxes into their component gross fluxes. We will also briefly discuss the relative merits of $^{18}$O versus $^{13}$C as a tracer for partitioning net fluxes.
DE: 0315 Biosphere/atmosphere interactions
DE: 0322 Constituent sources and sinks
DE: 0400 Biogeosciences
DE: 1615 Biogeochemical processes (4805)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting