Abstract: We investigated the effects of simulated nitrogen deposition on the physiological and ecological characteristics of Sinocalycanthus chinensis seedlings by adding various amounts of NH₄NO₃ in a pot experiment. Three levels of NH₄NO₃ included: low-N (2 g N m^-2·a^-1), medium-N (8 g N m^-2·a^-1), and high-N (32 g N m^-2·a^-1), with 0 g N m^-2·a^-1 as the control. After one year, we measured the photosynthetic characteristics, relative chlorophyll content, chlorophyll fluorescence parameters, membrane lipid peroxidation, and antioxidant enzyme activities in leaves of S. chinensis seedlings among the different treatments. The diurnal mean values of net photosynthetic rate at the low, medium, and high levels of N addition were 14.7%, 54.5%, and 26.4% higher than that of the control, respectively; the maximum net photosynthetic rates were 8.3%, 66.1%, and 30.9% higher than that of the control, respectively. The highest values for net photosynthetic rate and maximum net photosynthetic rate were recorded for the medium-N level. The daily mean stomatal conductance was higher in all N treatments than in the control, and the highest value was recorded for the medium-N level. However, the intercellular CO₂ concentrations were lower in all N treatments than in the control; the lowest value was recorded in the medium-N treatment. The light compensation point and dark respiration rate among the four treatments were in the order of medium-N > high-N > low-N > control, with highly significant differences among the treatments. In terms of the light saturation point, the treatments were ranked as follows: medium-N > low-N > control > high-N, with highly significant differences among the treatments. The apparent quantum yield values were ranked as follows: medium-N > low-N > control; however, there was no significant difference between the high-N treatment and the control in the apparent quantum yield value. The relative chlorophyll contents in seedlings in the three N treatments were significantly higher than that in the control, with the highest values recorded in the medium-N treatment. The primary chemical efficiency and potential activity of photosystem II in S. chinensis seedlings were ranked as follows: medium-N > low-N > control > high-N, with highly significant differences among the treatments. The superoxide dismutase activity, peroxidase activity, and malondialdehyde contents in S. chinensis seedlings in the medium and high-N treatments were significantly higher than those in the control, but there was no significant difference in these parameters between the low-N treatment and the control. The membrane permeability of S. chinensis seedlings was ranked from most permeable to least permeable as follows: high-N > medium-N > low-N > control, with highly significant differences among the treatments. These results suggested that appropriate nitrogen levels, especially the medium-N level, can enhance the photosynthetic ability of S. chinensis seedlings, and also affect other physiological indexes to some extent.

Key words: interactive effects warming, changing precipitation, elevated CO₂ concentration, crops.