Effect of different nitrogen supply on light-response curves of Quercus variabilis seedlings

In order to provide a theoretical foundation for nitrogen fertilizer management of Quercus variabilis seedlings, using exponential nitrogen fertilization regimes, the effects of nitrogen treatments on light-response curves of Q. variabilis seedlings were studied under sand-cultured condition. The seedlings were planted in pots, supplied with four nitrogen application levels (0, 120.6, 216.4 and 329.2 mg/(seedling•year)). The results show that various nitrogen application levels had different effects on the light-response curves of Q. variabilis leaves, depending on the growth phases. At the initiation phase, all the simulation parameters of light-response curves showed no significant differences among treatments. At the fast-growth phase, the lack of nitrogen weakened the photosynthetic capability of seedlings in treatment CK (0 mg/(seedling•year)) sharply, which reduced the light saturation point (LSP), maximum net photosynthetic rate (Pmax) and the apparent quantum efficiency (α); meanwhile, the light compensation point (LCP) expressed a trend of increase. When the nitrogen level was raised from 120.6 to 216.4 mg/(seedling•year), no significant variation about Pmax was found between the two treatments; however, the LCP of the seedlings in 216.4 mg/(seedling•year) treatment was lower and the LSP was higher. Yet, excessive nitrogen supply (329.2 mg/(seedling•year)) played a negative role on the photosynthetic ability of Q. variabilis seedlings. At the later phase, relatively higher photosynthetic efficiency was maintained in the seedlings in 216.4 mg/(seedling•year) treatment, comparing with the common decline in other three treatments.