Abstract: Aims To study the effects of different nitrogen addition levels and nitrogen form ratios on photosynthetic characteristics and chlorophyll fluorescence parameters of Cunninghamia lanceolata, with a view to providing a scientific basis for the management of nitrogen fertilization in C. lanceolata. Methods One-year-old seedlings of C. lanceolata were cultured in sands with three N addition levels of 0.5(N1),1.0( N2), and 2.0 mmol L-1(N3) and seven different N form ratios (NH4+-N to NO3－-N ratio being as 10:0(P1), 8:2(P2), 6:4(P3), 5:5(P4), 4:6(P5), 2:8(P6), and 0:10(P7). The chlorophyll (Chl) content, photosynthetic characteristics, chlorophyll fluorescence parameters and biomass in C. lanceolata were analyzed after 180 days treatment. Important findings (1) The Chl a and Chl b content of C. lanceolata were highest at P2 and P6 under N1 treatment, respectively. The Chl a, Chl b, and Chl (a+b) content were all highest at P4 under N2 treatment. Both Chl a and Chl b content were highest at P3 among the seven ratios under N3 treatment. The chl content showed an overall higher level of N3 and N2 than that of N1. The three of nitrogen addition level showed N3 and N2 than N1. (2) The net photosynthetic rate(Pn), stomatal conductance(Gs), transpiration rate (Tr), and water use efficiency (WUE) of C. lanceolata were all higher for higher ammonium N content than for higher nitrate N content. The Pn was highest at P1 under N3 treatment, but highest at P2 under N2 and N1 treatments. (3) The maximum photochemical efficiency of all treatments was in the normal range of 0.80~0.85, and the potential photochemical activity of photosystem Ⅱ was significantly higher under N3 treatment than that of N1 and N2 at P2. The fast chlorophyll fluorescence induction curve (OJIP) of leaves deviated greatly under N1 treatment at P1. The P1, P2, and P4 at N2 and P1, P2, and P5 at N3 gradually decreased at the I and P phases and gradually increased at the J phase. The apparent quantum flux per unit leaf cross-sectional area, heat dissipation capacity, and the number of active reaction centers per unit leaf cross-sectional area with the increase of nitrogen content at P2 and P3, while the absorption flux per reaction center, electron transport flux per RC, trapped energy flux per RC, and dissipated energy flux per RC at P6 decreased significantly with the increase of nitrogen content, the differences were not significant under the other ratios. (4) Total biomass and aboveground biomass of C. lanceolata were higher for higher ammonium N content than for higher nitrate N content, and significantly higher under the N2P2 treatment than under the other treatments. At the same N form ratio, it showed that N2 > N3 > N1. The root-shoot ratio of N1 and N3 was significantly higher than that of other treatments, and the ratio of P6 at N2 level was significantly higher than that of other treatments, and the ratio of P7 at N2 level was significantly higher than that of N2 and N3. The photochemical reaction efficiency of C. lanceolata seedlings could be improved and the energy utilization of the PSⅡ reaction center could be optimized by a higher nitrogen content and a higher ammonium nitrogen ratios, which would be more beneficial for photosynthesis and biomass accumulation.

Key words: nitrogen addition, nitrogen form ratios, photosynthetic characteristics, chlorophyll fluorescence, Cunninghamia lanceolata