Contrasting responses of soil bacterial and fungal communities to the spatial structure of mycorrhizal–associated trees in a warm–temperate forest

Mycorrhizal associations regulate soil microbial diversity through nutrient economy strategies. In warm-temperate montane forests, Arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) tree species often show distinct spatial structure along topographic—edaphic gradients. Few studies have explicitly tested whether AM/ECM dominance gradients, together with the horizontal spatial structure of the mycorrhizal tree abundance, density, and diversity, differentially regulate the assembly of non-mycorrhizal soil bacteria versus fungi. We quantified AM/ECM dominance gradients in a 25-ha forest dynamics plot in the Qinling Mountains and integrated soil physicochemical properties, topographic variables, and 16S rRNA gene and ITS amplicon data. We found that AM dominance was associated with higher soil nutrient availability and lower elevation, whereas ECM dominance aligned with nutrient-poor soils and higher elevation. Bacterial β-diversity was primarily driven by soil pH and was dominated by heterogeneous selection; moreover, AM dominance significantly increased microbial network complexity. In contrast, fungal communities showed strong dispersal limitation, and mycorrhizal tree density–particularly AM density—was the strongest predictor of fungal community composition, outweighing the effects of mycorrhizal tree abundance or diversity. Overall, AM dominance reshaped microbial β-diversity through two coupled pathways: strengthening deterministic selection in bacteria while weakening dispersal limitations in fungi. By positioning the spatial structure of tree mycorrhizal types as a key aboveground–belowground linkage, our study advances a mechanistic framework that jointly explains divergent assembly controls on bacteria and fungi across mountainous landscapes.