Drought and nitrogen deposition are major drivers of global change that can influence forest ecosystems and plant–microbe interactions, yet their combined effects on endophytic fungal communities and the roles of dark septate endophytes (DSE) remain unclear. In this study, we examined the diversity of culturable endophytic fungi in leaves and roots of Quercus dentata under different drought and nitrogen deposition treatments and evaluated the functional effects of representative DSE strains on host growth and physiology. A total of 1488 fungal isolates were obtained, revealing distinct organ-specific community patterns. Root-associated communities showed greater compositional stability across treatments, whereas leaf communities were more responsive to environmental variation. Severe drought reduced the dominance of several genera and promoted community restructuring, while nitrogen deposition had contrasting effects on α-diversity in leaves and roots. Beta diversity analyses indicated significant interaction effects between drought and nitrogen addition. Inoculation with four DSE strains produced strain-dependent effects on plant biomass, photosynthesis, water-use efficiency, physiological traits, and nutrient contents. These results indicate that drought and nitrogen deposition jointly influence endophytic fungal communities and that functional differences among DSE strains may affect host responses to combined stress.

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.