Abstract: Based on natural nitrogen deposition in the field (23 kg·ha⁻¹·year⁻¹), this research employed low (LN, 23 kg·ha⁻¹·year⁻¹), moderate (MN, 46 kg·ha⁻¹·year⁻¹), and high (HN, 69 kg·ha⁻¹·year⁻¹) nitrogen levels to simulate natural nitrogen deposition, using no nitrogen addition used as a control (CK). The goal was to explore the physiological and ecological effects of excessive nitrogen deposition on two broad-leaved forest species, i.e., Manchurian ash (Fraxinus mandshurica Rupr.) and Mongolian oak (Quercus mongolica Fish. ex Ledeb). Results showed that 1) CO₂ diffusional limitations (i.e., stomatal limitation, l_sc, mesophyll limitation, l_m) of both species decreased by more than 10% after nitrogen addition, then increased with increasing nitrogen supply, while biochemical limitation (l_b) increased by more than 10% after nitrogen addition, then decreased with increasing nitrogen supply. 2) Both l_sc and l_m reached minimum values of 18.4% and 18.0% (Manchurian ash-August), 21.6% and 19.7% (Mongolian oak-July), and 21.6% and 20.1% (Mongolian oak-August), while l_b reached a maximum value of 63.6% (Manchurian ash-August) and 59.7% and 58.3% (Mongolian oak-July and August) under MN treatment, indicating that soil nitrogen addition of 46 kg·ha^-1·year^-1 had the greatest photosynthesis-promoting effect. 3) The enhancement of plant photosynthetic capacity during soil nitrogen supply predominantly resulted from the weakening of CO₂ diffusional limitations, in which stomatal conductance to CO₂ (g_sc, i.e., l_sc) was the primary limiting factor affecting plant photosynthesis. 4) The three photosynthetic limitations (l_sc, l_m, and l_b) did not show any significant differences between July and August, indicating that the primary photosynthetic role of l_sc may lack seasonal variation. 5) Soil nitrogen addition within a certain content range did not significantly affect the water use potential of plants.