Effects Of Free-Air CO₂ Enrichment On Rice And Wheat Concopy Microclimate And Energy Balance

In this study, the FACE system (setup at Xiaoji, Jiangdu, Jiangsu province) was used to investigate the effects of FACE on rice and wheat canopy microclimate and energy balance. The rice canopy microclimate observations were carried out from July 18 to October 15, 2006, when the rice crops were at the elongation to maturing development stage. The wheat canopy microclimate observations were carried out from March 19 to May 24, 2007, when the wheat crops were at the elongation to maturing development stage. The results showed that:(1) FACE reduced the rice and wheat leaf stomatal conductance. FACE reduced rice stomatal conductance by 25%, 24%, and 18% significantly (p<0.01) in upper leaves, middle leaves, and in lower leaves respectively. FACE reduced wheat stomatal conductance by 28% ,32%,and 26% significantly (p<0.01) in upper leaves, middle leaves, and in lower leaves respectively, but the reduction rates subsequently decreased after filling stage.(2) FACE increased daytime canopy temperature. The daily maximum rice canopy temperature difference between FACE and ambient reached 0.4～1.9℃. The average daytime rice canopy temperature difference was about 0.45℃.The maximum daytime mean temperature difference of the wheat canopy between FACE and ambient reached 1.58℃in response to stomatal closure under E-CO₂ at the flowering stage. And the average daytime wheat canopy temperature difference was about 0.77℃.(3) Daytime air temperature inside the canopy was higher in FACE plot than in ambient plot. The value of daytime air temperature difference between FACE and ambient increased with the increase of solar radiation and varied with height. The maximum daytime air temperature difference between FACE and ambient were 0.45℃and 0.29℃at middle of rice canopy and rice canopy height, respectively. Air humidity inside rice canopy was not significantly affected by FACE. The maximum daytime air temperature difference varied between 0.12～0.98℃inside the canopy, and 0.03～0.7℃at the canopy height. whereas daytime air humidity inside the canopy was lower in FACE plot than in ambient plot. The maximum daytime air humidity difference between FACE and ambient varied between -0.3%～-7% at about 14:00.(4) Net radiation above rice canopy was not significantly affected by FACE, about larger 1.3% than ambient. The net radition in wheat FACE was higher higher by 0.9% before anthesis, and lower by 2.5% thereafter. (5) We estimated energy balances using the residual energy balance method. The daytime total canopy sensible and latent heat fluxes of FACE were higher and lower than those of ambient respectively, throughout the observation period. The maximum difference of rice canopy sensible and latent heat flux between FACE and ambient varied between 13～63 W/ m² and -12～-66W/ m² respectively. The maximum difference of wheat canopy sensible and latent heat flux between FACE and ambient varied between 12～78 W/ m² and -12～-63W/ m² respectively. Daily average latent heat flux was lower in the FACE plots, by about 3.7% and 3.5% respectively.(6) Canopy conductance from P-M equation agreed well with the measured leaf conductance of rice and wheat, which showed that the microclimate data and the latent and sensible heat fluxes calculated with the residual energy balance method are relatively right. The correlation determination (R²) and the root mean squared error (RMSE) between the simulated and measured conductance based on the 1:1 line are 0.91, 0.59cm/s for ambient, 0.89, 0.57cm/s for FACE of the rice paddy; 0.84, 1.07cm/s for ambient, 0.73, 1.10cm/s for FACE of wheat conopy,respectively.(7) Regression analysis of the relationship between stomatal conductance and environmental factors showed that PAR and VPD are the vital factors affecting plant stomatal conductance. The Jarvis-type stomatal conductance models are proposed in the paper adapted for FACE. The combination of the stomatal conductance models with P-M equation gave satisfactory estimation of evapotranspiraton. The simulated rice evapotranspiraton and daily evapotranspiraton measured by lycimeter agreed well, which R²=0.88, RMSE=0.29mm for FACE, and R²=0.90, RMSE=0.39mm for ambient.(8) Total water use by rice and wheat throughout the observation period was saved 10mm and 26mm in E-CO₂ respectively. Reduced transpiration combined with increased total biomass resulted in a 12% and 19% increase in water use efficiency in E-CO₂ in the rice and wheat canopy respectively.