Drought and Nitrogen Deposition Drive Community Restructuring of Endophytic Fungi and Functional Regulation by Dark Septate Endophytes in Quercus dentata

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.