Growth and photosynthetic responses of Fagus crenata seedlings to O3 under different nitrogen loads

To obtain the basic data for evaluating the critical level of ozone (O-3) to protect Japanese deciduous broad-leaved forest tree species, the growth and photosynthetic responses of Fagus crenata seedlings to O-3 under different nitrogen (N) loads were investigated. The seedlings were grown in potted andisol supplied with N as NH4NO3 solution at 0, 20 or 50 kg ha(-1) year(-1) and were exposed to charcoal-filtered air or O-3 at 1.0, 1.5 and 2.0 times the ambient concentration for two growing seasons. The interactive effect of O-3 and N load on the whole-plant dry mass of the seedlings at the end of the second growing season was significant. The O-3-induced reduction in the whole-plant dry mass of the seedlings was greater in the relatively high N treatment than that in the low N treatment. This interactive effect was mainly due to the difference in the degree of O-3-induced reduction in net photosynthesis among the N treatments. The degree of O-3-induced reduction in N availability to photosynthesis was greater in the relatively high N treatment than that in the low N treatment. In conclusion, the sensitivity of growth and photosynthetic parameters of F. crenata seedlings to O-3 become high with increasing amounts of N added to the soil. Therefore, N deposition from the atmosphere should be taken into account to evaluate the critical level of O-3 to protect Japanese deciduous broad-leaved forest tree species.