Nitrogen deposition and drought significantly influence plant growth and soil physicochemical properties. This study investigates the effects of nitrogen deposition and water stress on the growth and physiological responses of Quercus dentata, and how these factors interact to influence the overall productivity. Two-year-old potted seedlings were selected to simulate nitrogen deposition and water stress. Nitrogen was applied at rates of 0 kg·ha^-1·year^-1 (N0) and 150 kg·ha^-1·year^-1 (N150). The levels of water stress corresponded to 80% (W80), 50% (W50), and 20% (W20) of soil saturation moisture content. High nitrogen (N150) significantly increased stem elongation and stem diameter by enhancing photosynthetic parameters, including P_n (W80) and G_s (W50), and maintained higher water use efficiency. Under drought conditions, nitrogen enhanced leaf water content, stabilized electrical conductivity, regulated antioxidant enzyme activity, and increased the accumulation of proline. However, under severe drought, nitrogen did not significantly improve biomass, highlighting the critical role of water availability. Additionally, increased nitrogen levels enhanced soil enzyme activity, facilitated the uptake of crucial nutrients like K and Zn. Mantel tests indicated significant correlations between soil enzyme activity, water use efficiency, and leaf Fe content, suggesting that nitrogen deposition altered nutrient uptake strategies in Q. dentata to sustain normal photosynthetic capacity under water stress. This study demonstrates that nitrogen deposition substantially enhances the growth and physiological resilience of Q. dentata under W50 by optimizing photosynthetic efficiency, water use efficiency, and nutrient uptake. However, the efficacy of nitrogen is highly dependent on water availability, highlighting the necessity of integrated nutrient and water management for plant growth.

Tropospheric ozone (O₃) is a pervasive phytotoxic air pollutant, and its co-occurrence with increasing nitrogen (N) deposition poses an emerging hazard to forest health and carbon (C)-nitrogen balance. Understanding how trees respond to these interacting stressors is critical for predicting ecosystem resilience under global change. Here, we investigated the physiological, stoichiometric, and molecular mechanisms underlying Quercus aliena responses to elevated O₃ (160 ppb) and N addition (150 kg·ha^-1·yr^-1), alone and in combination, using an integrated approach combining growth traits, photosynthesis, nutrient analysis, and multi-omics profiling. Elevated O₃ severely inhibited chlorophyll synthesis, photosynthetic rate, and root biomass, and depleted key soluble sugars, indicating C assimilation failure. N addition alone enhanced photosynthetic performance, starch and sucrose accumulation, and biomass production. Strikingly, under combined O₃ and N exposure, seedlings exhibited partial recovery of growth and photosynthesis, reduced leaf C:N ratios, and increased N:P ratios, reflecting adaptive nutrient reallocation. Transcriptomic-metabolomic integration revealed that N supplementation under O₃ stress reprogrammed carbohydrate metabolism, up-regulating genes for sugar mobilization and osmoprotectant biosynthesis while down-regulating those for starch storage and glycolysis. Simultaneously, N assimilation shifted toward low-carbon-cost ammonium pathways, with increased glutamine synthetase and glutamate synthase activities and reduced nitrate reductase activity. These coordinated adjustments enhanced soluble C availability, optimized N utilization, and stabilized nutrient stoichiometry, collectively mitigating O₃-induced metabolic disruption. Our findings identify nutrient-driven metabolic plasticity as a key defense against atmospheric pollutant stress, providing mechanistic insight for targeted nutrient management to safeguard forest productivity in increasingly O₃-polluted environments.