Aims This study aims to investigate the effects of varying nitrogen (N) addition levels and NH4+-N to NO3--N ratios on the photosynthetic and chlorophyll fluorescence parameters of Schima superba, a dominant subtropical tree species. The objective is to clarify the short-term response mechanisms of photosynthesis physiology and growth of S. superba to different N nutrient environments.
Methods One-year-old seedlings of S. superba were rooted in sand potted sand culture nutrient solution with three N addition levels: 0.5 (low N), 1.0 (medium N), and 2.0 (high N) mmol·L-1, and seven NH4+-N to NO3--N ratios: 10:0, 8:2, 6:4, 5:5, 4:6, 2:8, 0:10. After 180 d of treatment, the photosynthetic characteristics, chlorophyll fluorescence parameters, chlorophyll content, biomass, and root-shoot ratio of S. superba seedlings were determined.
Important findings (1) Across the 21 experimental treatments with varying 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 highest at the 4:6 ratio under high and medium N addition levels. However, these traits were highest at the 10:0 ratio under the low N addition level. The underground biomass peaked at the 4:6 ratio under high N addition, and at the 6:4 ratio under medium and low N addition levels. The root-shoot ratio was highest at the 10:0 ratio under high and medium N addition levels and at 6:4 ratio under low N addition. (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 across all ratios except 10:0. The water use efficiency, underground biomass, and root-shoot ratio were highest under the low N addition level. (3) The chlorophyll fluorescence intensity was more pronounced under the high N addition level and significantly decreased at 4:6 ratio compared to other ratios under low N addition. The deviation of the OJIP curve was greatest at 0:10 ratio under medium N addition level. The fluorescence values in the J-I segment at ratios of 10:0, 0:10, 8:2, 2:8 under low N addition level, the O-J and J-I segments at ratios of 0:10, 8:2, 2:8, 4:6 under medium N addition, the J-I segment at ratios of 0:10, 2:8, 6:4, and the I-P segment at ratios of 0:10 and 4:6 were increased under high N addition. There was no significant difference in the deviation of the OJIP curve across three N addition levels, indicating that low N addition (N stress) and highly unbalanced ammonium to nitrate N ratios (10:0 and 0:10) did not stress the growth of S. superba seedlings. Overall, the energy utilization of the PSII reaction center could be optimized, photosynthetic capacity effectively improved, and aboveground growth and biomass accumulation promoted in S. superba seedlings under appropriate N addition levels and NH4+-N to NO3--N ratios. Under low N addition, the S. superba enhanced underground root growth to obtain more nutrients for development, although the photosynthetic capacity was weaker and the biomass accumulation was lower.