Effects of simulated nitrogen deposition on growth and photosynthetic characteristics of one-year-old Toona sinensis seedlings

With global industrial and agricultural modernization, the characteristics and processes of various ecosystems have been profoundly affected due to increasing nitrogen deposition. Toona sinensis is a unique, dual-use plant widely distributed in China. It is used for medicinal purposes and is also a fast-growing timber tree species. To determine the threshold value of nitrogen deposition that could be sustained at the seedling stage of T. sinensis and to provide some theoretical basis for the cultivation of T. sinensis, a preliminary study was performed to test the response of annual T. sinensis seedlings to nitrogen deposition in summer. Annual T. sinensis seedlings were grown under the simulated nitrogen deposition experiment in summer. Urea was used as the nitrogen source, with six levels of nitrogen addition:0 kg(N)·hm^-2·a^-1, 20 kg(N)·hm^-2·a^-1, 40 kg(N)·hm^-2·a^-1, 80 kg(N)·hm^-2·a^-1, 120 kg(N)·hm^-2·a^-1, and 180 kg(N)·hm^-2·a^-1. The ground diameter, seedling height, biomass, and photosynthesis of T. sinensis seedlings were then measured. The results showed that nitrogen addition contributed to an improvement in the diameter, height, and biomass of T. sinensis seedlings. Ground diameter, height, and biomass were the highest after the addition of 180 kg(N)×hm^-2·a^-1 nitrogen and were 42.5%, 64.4%, and 304.9% higher, respectively, than the control values. The biomass of T. sinensis seedlings was distributed more to the root and leaf. The SPAD of seedling leaves increased with increasing levels of nitrogen addition. At the highest nitrogen addition of 180 kg(N)·hm^-2·a^-1, SPAD values were 73.9% higher than the control. The apparent quantum yield (AQY), maximum net photosynthetic rate (P_nmax), light saturation point (LSP), light compensation point (LCP), and the dark respiration rate (R_d) increased at first and then decreased with increasing levels of nitrogen addition. Except for LCP, which was highest at 80 kg(N)·hm^-2·a^-1, AQY, P_nmax, LSP, and R_d were the highest at 120 kg(N)·hm^-2·a^-1. This study showed that a certain level of nitrogen addition could promote the growth and enhance the photosynthetic ability of T. sinensis seedlings. However, there is a limit to the nitrogen deposition level tolerance of T. sinensis seedlings and when the nitrogen level increases to a certain point, photosynthetic ability begins to decline.