Bacterial and Fungal Community Shifts Across an Alpine Forest–Grassland Ecotone in the Yili River Valley, Xinjiang

Alpine forest–grassland ecotones represent sensitive transition zones where drastic shifts occur in vegetation, soil properties and biotic communities. Nevertheless, how soil microbial community composition, inter-taxon interactions and community assembly processes respond to grassland–ecotone–forest transitions remains poorly understood. In this study, we investigated the shifts in soil bacterial and fungal communities along an alpine forest–grassland ecotone, and explored key edaphic drivers as well as underlying assembly mechanisms. Our results showed that bacterial α-diversity in forest soils was significantly higher than that in grassland soils, with 15.44% higher Chao1 index and 4.73% higher Shannon index. Adonis analysis revealed clear community compositional differentiation across grassland, ecotone and forest habitats, with the greatest divergence between grassland and forest, and the highest similarity between ecotone and forest. Species turnover dominated the β-diversity variation in both bacterial and fungal communities along the gradient. Bacterial network complexity, connectivity and cohesion reached maximum values in the ecotone, suggesting intensified potential interspecific interactions and elevated niche overlap for bacterial taxa within transitional habitats. Neutral community model and NST analyses indicated that bacterial community assembly was primarily governed by stochastic dispersal processes. By contrast, fungal communities were controlled by deterministic environmental filtering. Along the grassland-to-forest transition, shifts in host plant traits, mycorrhizal symbiosis and litter inputs intensified habitat filtering, resulting in the continuous simplification of fungal network structure, decreased cohesion and rising modularity. Soil pH, electrical conductivity and total phosphorus acted as core edaphic determinants for bacterial communities, whereas fungal communities were additionally sensitive to ammonium nitrogen. The ecotone functioned as an important ecological buffer zone: stochastic dispersal promoted complex and stable bacterial co-occurrence networks, while deterministic filtering reshaped fungal community structure and inter-taxon associations. This work highlights the divergent assembly strategies of bacteria and fungi across alpine ecotone gradients, and provides insights for understanding microbial community shifts and biotic stability in mountain ecotone ecosystems.