18O composition of CO2 and H2O ecosystem pools and fluxes in a tallgrass prairie: Simulations and comparisons to measurements

Abstract

In this paper we describe measurements and modeling of 18O in CO2 and H2O pools and fluxes at a tallgrass prairie site in Oklahoma. We present measurements of the δ18O value of leaf water, depth-resolved soil water, atmospheric water vapor, and Keeling plot δ18O intercepts for net soil-surface CO2 and ecosystem CO2 and H2O fluxes during three periods of the 2000 growing season. Daytime discrimination against C18OO, as calculated from measured above-canopy CO2 and δ18O gradients, is also presented. To interpret the isotope measurements, we applied an integrated land-surface and isotope model (ISOLSM) that simulates ecosystem H218O and C18OO stocks and fluxes. ISOLSM accurately predicted the measured isotopic composition of ecosystem water pools and the δ18O value of net ecosystem CO2 and H2O fluxes. Simulations indicate that incomplete equilibration between CO2 and H2O within C4 plant leaves can have a substantial impact on ecosystem discrimination. Diurnal variations in the δ18O value of above-canopy vapor had a small impact on the predicted δ18O value of ecosystem water pools, although sustained differences had a large impact. Diurnal variations in the δ18O value of above-canopy CO2 substantially affected the predicted ecosystem discrimination. Leaves dominate the ecosystem 18O-isoflux in CO2 during the growing season, while the soil contribution is relatively small and less variable. However, interpreting daytime measurements of ecosystem C18OO fluxes requires accurate predictions of both soil and leaf 18O-isofluxes. Nomenclature Ci = leaf internal CO2 concentration (μmol m-3) CH, CL = CO2 concentrations at the high and low sampling points (μmol m-3) Fal, Fla = CO2 fluxes into and out of the leaf (μmol m-2 s-1) Fg, Fsm, Fs = growth, stem, and net soil-surface COs fluxes (μmol m-2 s-1) I = ecosystem 18O-isoflux (μmol m-2 s-1‰) Ra, Rc, Re, Req = CO218O isotopic ratios in the atmosphere, chloroplast, net ecosystem flux, and in equilibrium with leaf water zo = e-folding depth for soil CO2 source (m) δa, δe, δl, δH, δL, δnl, δsw, δs = δ18O values of atmospheric CO2, net ecosystem CO2 flux, leaf water; CO2 at the high sampling point, CO2 at the low sampling point, net leaf CO2 flux, stem respiration CO2 flux, and soil CO2 flux (‰) Δc = difference in δ18O values of chloroplast and atmospheric CO2 (‰) Δeq = difference in δ18O values of equilibrium chloroplast and atmospheric CO2 (‰) ΔL18, Δs18, Δe18 = net discrimination against C18OO from leaves, soil, and ecosystem (‰) Θeq = disequilibrium between CO2 and leaf water.