Diurnal and phenological variations of O3 and CO2 fluxes of rice canopy exposed to different O3 concentrations

Abstract

A dynamic chamber system was designed to measure simultaneously the diurnal and phenological canopy ozone (O3) and carbon dioxide (CO2) fluxes in the paddy field under different O3 concentrations (0, 40, 80 and 120 nmol mol−1). On the diurnal timescale, a decreasing trend of canopy O3 flux was observed from morning to evening and the O3 flux increased with increasing O3 concentration, while canopy CO2 flux generally followed the track of photosynthetic active radiation, with higher values at noon except at the end of the growing season when rice was senescent. The constant CO2 flux among different O3 treatments in this experiment suggested that the photosynthesis of the rice canopy was not affected by short-duration (ca. 10 min) O3 exposure of elevated concentration. The daily mean O3 and CO2 fluxes increased with rice growth until the dough stage and the late jointing stage, respectively, then decreased with rice aging. The peak values of O3 flux appeared later than those of CO2 flux because the latter was closely synchronized with the leaf area index of the rice canopy. Diurnal mean canopy O3 flux varied from 18.7 to 43.3 nmol m−2 s−1, and nocturnal mean canopy O3 flux varied from 2.7 to 17.8 nmol m−2 s−1 and from 7.0 to 25.4 nmol m−2 s−1 for the 40 and 80 nmol mol−1 O3 treatments, respectively. The considerable amount of nocturnal O3 flux indicated a significant contribution of non-stomatal factors to canopy O3 uptake. The adjusted Jarvis multiplicative models were used and well parameterized to fit the measured O3 and CO2 fluxes of our rice cultivar from environmental variables. Although more validation work is needed, the present results suggest that the models can be considered as a tool for canopy flux predictions in the paddy field.