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

摘要

In this paper we describe measurements and modeling of O-18 in CO2 and H2O pools and fluxes at a tallgrass prairie site in Oklahoma. We present measurements of the delta(18)O value of leaf water, depth-resolved soil water, atmospheric water vapor, and Keeling plot delta(18)O intercepts for net soil-surface CO2 and ecosystem CO2 and H2O fluxes during three periods of the 2000 growing season. Daytime discrimination against (COO)-O-18, as calculated from measured above-canopy CO2 and delta(18)O gradients, is also presented. To interpret the isotope measurements, we applied an integrated land-surface and isotope model (ISOLSM) that simulates ecosystem (H2O)-O-18 and (COO)-O-18 stocks and fluxes. ISOLSM accurately predicted the measured isotopic composition of ecosystem water pools and the delta(18)O value of net ecosystem CO2 and H2O fluxes. Simulations indicate that incomplete equilibration between CO2 and H2O within C-4 plant leaves can have a substantial impact on ecosystem discrimination. Diurnal variations in the delta(18)O value of above-canopy vapor had a small impact on the predicted delta(18)O value of ecosystem water pools, although sustained differences had a large impact. Diurnal variations in the delta(18)O value of above-canopy CO2 substantially affected the predicted ecosystem discrimination. Leaves dominate the ecosystem O-18-isoflux in CO2 during the growing season, while the soil contribution is relatively small and less variable. However, interpreting daytime measurements of ecosystem (COO)-O-18 fluxes requires accurate predictions of both soil and leaf O-18-isofluxes.