Effects of elevated ozone on the water and heat flux of winter wheat in farmland

Tropospheric ozone (O₃) has often formed in the emission of volatile organic compounds and nitrogen oxides from human activities. The elevated O₃ concentration can seriously reduce the photosynthetic rate of wheat in the late grain-filling stage. The resulting loss of wheat yield can then threaten the food security and the carbon-water cycle of farmland ecosystems. Particularly, there is a closely coupled relationship between grain yield and field water use of wheat. Taking the Nongmai 88 variety as the research object, this study aims to investigate the impacts of elevated O₃ on farmland water and heat flux in the wheat field. The response of wheat field transpiration to elevated O₃ concentrations was also determined using free air ozone concentration enrichment system (O₃-FACE) simulation. The field microclimate was continuously monitored on AA (ambient air) and E-O₃ (1.5 × ambient O₃ concentration) in O₃-FACE. There was a great variation in the water and heat flux at different stages of wheat growth under elevated O₃, according to the residual energy balance. The results show that latent heat flux was an important energy consumption item of wheat farmland ecosystem, accounting for 61.65%(AA) and 63.77%(E-O₃) of energy expenditure, respectively. Sensible heat flux and soil heat flux accounted for 24.50%(AA), 21.07%(E-O₃) and 13.95% (AA), 15.20%(E-O₃), respectively.In terms of diurnal variation, net radiation, soil heat flux, both sensible and latent heat flux shared the "n" type unimodal trends, reaching the maximum at noon. The net radiation increased gradually during the wheat growing season. Whereas, the latent heat flux showed a trend of first increasing and then decreasing. The sensible heat flux was opposite to the latent one, indicating the trend of first decreasing and then increasing. The latent heat flux decreased in the senescent leaf, while the sensible heat flux increased rapidly. There was no significant effect of elevated O₃ on the net radiation, soil heat flux, sensible and latent heat flux, as well as their energy distribution from the wheat jointing to the milk maturity stage. The elevated O₃ also accelerated the senescence of wheat leaf in the period from milk ripening to ripening, leading to a reduction in the chlorophyll content. There was a great decrease in the mean and peak value of latent heat flux and evapotranspiration. However, the elevated O₃ had no significant effect on evapotranspiration, grain yield and water use efficiency. As such, excellent O₃ resistance was achieved to cultivate this variety under the elevated O₃ , especially for the high grain yield and water use efficiency in the wheat field. This finding can provide new evidence to evaluate the water and heat flux of winter wheat farmland, in order to simulate the farmland water use and grain yield under elevated O₃ and variety replacement.