Desertification Drives Functional Reassembly of Rhizosphere Fungal Communities from Arbuscular Mycorrhizal Fungi to Dark Septate Endophytes in Temperate Grassland

Desertification strongly alters soil microbial communities in dryland ecosystems, yet the reassembly of fungal functional groups and their interactions under increasing aridity remain unclear. This study aimed to determine how desertification reshapes arbuscular mycorrhizal fungi (AMF) and endophytic fungal groups in the rhizosphere of Stipa breviflora and Artemisia frigida, as well as how these shifts are associated with fungal network fragmentation. Rhizosphere soil internal transcribed spacer (ITS) sequencing and AMF-specific amplicon sequencing, combined with root colonization assessment, functional annotation, co-occurrence network analysis, and partial least squares path modeling (PLS-PM), were used to assess shifts in fungal communities along the desertification gradient. Desertification significantly reduced soil multifunctionality and fungal diversity, accompanied by a shift in community composition from environmentally sensitive taxa to stress-tolerant groups. Along the desertification gradient, AMF diversity and colonization decreased, whereas FUNGuild-inferred endophytic fungal abundance and microscopically observed dark septate endophytes (DSEs) colonization increased. FUNGuild-inferred endophytic fungal abundance was negatively correlated with AMF diversity. Co-occurrence networks showed reduced connectivity and increased fragmentation under desertification, especially at the desert steppe and steppe desert stages. PLS-PM analysis revealed that desertification directly increased fungal network fragmentation and indirectly promoted fragmentation through increased FUNGuild-inferred endophytic fungi and reduced AMF diversity, whereas soil multifunctionality mainly reflected environmental deterioration along the gradient. These findings demonstrate the functional reassembly of rhizosphere fungi under desertification and suggest that compensatory shifts among fungal guilds may contribute to ecosystem stability in dryland grasslands.