A mechanistic model of H218O and C18OO fluxes between ecosystems and the atmosphere:: Model description and sensitivity analyses -: art. no. 1095

[1] The concentration of O-18 in atmospheric CO2 and H2O is a potentially powerful tracer of ecosystem carbon and water fluxes. In this paper we describe the development of an isotope model (ISOLSM) that simulates the O-18 content of canopy water vapor, leaf water, and vertically resolved soil water; leaf photosynthetic (OCO)-O-18-O-16 (hereinafter (COO)-O-18) fluxes; CO2 oxygen isotope exchanges with soil and leaf water; soil CO2 and (COO)-O-18 diffusive fluxes (including abiotic soil exchange); and ecosystem exchange of (H2O)-O-18 and (COO)-O-18 with the atmosphere. The isotope model is integrated into the land surface model LSM, but coupling with other models should be straightforward. We describe ISOLSM and apply it to evaluate (1) simplified methods of predicting the (COO)-O-18 soil-surface flux; (2) the impacts on the (COO)-O-18 soil-surface flux of the soil-gas diffusion coefficient formulation, soil CO2 source distribution, and rooting distribution; (3) the impacts on the (COO)-O-18 fluxes of carbonic anhydrase (CA) activity in soil and leaves; and (4) the sensitivity of model predictions to the delta(18)O value of atmospheric water vapor and CO2. Previously published simplified models are unable to capture the seasonal and diurnal variations in the (COO)-O-18 soil-surface fluxes simulated by ISOLSM. Differences in the assumed soil CO2 production and rooting depth profiles, carbonic anhydrase activity in soil and leaves, and the delta(18)O value of atmospheric water vapor have substantial impacts on the ecosystem CO2 flux isotopic composition. We conclude that accurate prediction of (COO)-O-18 ecosystem fluxes requires careful representation of (H2O)-O-18 and (COO)-O-18 exchanges and transport in soils and plants.