Abstract: Aims To study the effects of different nitrogen (N) addition levels and NH4+-N to NO3–-N ratios on the photosynthetic and chlorophyll fluorescence parameters of Schima superba of the main subtropical tree species, so as to clarifying the short-term response mechanism of photosynthesis physiology and growth of S. superba to different N nutrient environments. Methods One-year-old seedlings of S. superba were cultured in sands with three N addition levels of 0.5 (low N), 1.0 (medium N), and 2.0 (high N) mmol L-1, and seven ratios of NH4+-N to NO3–-N being as 10:0, 8:2, 6:4, 5:5, 4:6, 2:8, 0:10. After 180 days of treatment with different nutrient solution, the photosynthetic characteristics, chlorophyll fluorescence parameters, chlorophyll content, biomass, and root-shoot ratio of S. superba seedlings were determined. Findings (1) Under the 21 experimental treatments with different N addition levels and NH4+-N to NO3–-N ratios, the net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), chlorophyll (a+b) content, aboveground biomass, and total biomass of S. superba seedlings were the highest at the ratio of 4:6 under the high and medium N addition levels. However, these traits at the ratio of 10:0 were highest under the low N addition level. The underground biomass was the highest at the ratio of 4:6 under high N addition level, and the highest at the ratio of 6:4 under medium and low N addition levels. The root-shoot ratio was the highest under both the high and medium N addition levels at the ratio of 10:0, and under the low N addition level at the ratio of 6:4. (2) The Pn, Gs, Tr, chlorophyll (a+b) content, aboveground biomass, and total biomass of S. superba seedlings were the highest under the high N addition level at the ratios of 0:10, 8:2, 2:8, 6:4, 4:6, 5:5, except the ratio of 10:0. The water use efficiency, underground biomass, and root-shoot ratio were the highest under the low N addition level. (3) The fluorescence intensity of S. superba was more substantial under the high N addition level, and it significantly decreased at the ratio of 4:6 compared to other ratios under the low N addition level. The deviation of the OJIP curve at the ratio of 0:10 was the largest under the medium N addition level. The fluorescence values in the J-I segment at the ratio of 10:0, 0:10, 8:2, 2:8 under the low N addition level, the O-J segment and J-I segment at the ratio of 0:10, 8:2, 2:8, 4:6 under the medium N addition level, the J-I segment at the ratio of 0:10, 2:8, 6:4, and the I-P segment at the ratio of 0:10, 4:6 were increased under the high N addition level. There was no significant difference in the degree of deviation of the OJIP curve under three N addition levels that showed that the low N addition level (N stress) and the highly unbalanced ammonium to nitrate N ratio (10:0 and 0:10) cultivation environment did not cause stress to the growth of S. superba seedlings. Overall, the energy utilization of the PSII reaction center could be optimized, the photosynthetic capacity could be effectively improved, the aboveground growth and biomass accumulation could be promoted of S. superba seedlings in the cultivation environment with the proper N addition level and the appropriate ratio of NH4+-N to NO3–-N. Under the low N addition level, the S. superba can obtain more nutrients for its growth and development by enhancing the growth of underground roots. However, but the photosynthetic capacity was weak and the biomass accumulation was less.

Key words: nitrogen supply level, nitrogen form ratio, photosynthetic characteristics, chlorophyll fluorescence, Schima superba