| 摘 要： |
Accurate simulation of oxygen isotopic composition (delta O-18(T)) of transpiration (T) and its contribution via isotopic non-steady-state (NSS) to atmospheric water vapor delta O-18 (delta O-18(v)) still faces great challenges. High-frequency in-situ measurements of delta O-18(v) and evapotranspiration (ET) delta O-18 were conducted for two summer days on a subtropical forest plantation. delta O-18 of xylem, leaf, and soil water at 3 or 4-h intervals was analyzed. Leaf water delta O-18 and delta O-18(T) were estimated using the Craig and Gordon (CG), Dongmann and Farquhar-Cernusak models, and evaporation (E) delta O-18 using the CG model. To quantify the effects of delta O-18(T), delta O-18(E), and delta O-18(ET) on delta O-18(v), T, E, and ET isoforcing was calculated as the product of T, E, and ET fluxes, and the deviation of their delta O-18 from delta O-18(v). Results showed that isotopic steady-state assumption (SS) was satisfied between 12:00 and 15:00. NSS was significant, and delta O-18(T) was underestimated by SS before 12:00 and after 18:00. The Peclet effect was less important to delta O-18(T) simulation than NSS at the canopy level. Due to decreasing atmospheric vertical mixing and the appearance of the inversion layer, contribution from positive T isoforcing increased delta O-18(v) in the morning and at night. During the daytime, the contribution from positive T isoforcing increased first and then decreased due to strong vertical mixing and variability in T rate. |
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