Rhizosphere microbial diversity and community assembly in tree species–slope aspect coupled mountain forests of the Qilian Mountains, northeastern Qinghai–Tibet Plateau

Abstract

Soil microorganisms underpin nutrient cycling and organic matter decomposition, and elucidating community assembly is pivotal for ecosystem functioning. On the northeastern Qinghai–Tibet Plateau, Juniperus przewalskii typically dominates sunny slopes whereas Picea crassifolia occurs on shady slopes, forming a characteristic tree species–slope aspect coupling that provides a natural contrast for rhizosphere microbiomes in mountain forests. However, it remains unclear whether rhizosphere microbial diversity and inferred assembly processes differ between these coupled systems, and whether bacteria and fungi respond similarly. Here, we compared rhizosphere bacterial and fungal communities between J. przewalskii–sunny and P. crassifolia–shady slope forests to characterize diversity patterns, inferred assembly processes, and environmental associations. Across 19 plots (n = 100 rhizosphere samples), the J. przewalskii system exhibited a “high-diversity–low-nutrient” pattern, whereas the P. crassifolia system showed “low-diversity–high-nutrient.” Bacterial communities showed stronger stochastic signals (NST > 0.5), with stronger neutrality in J. przewalskii (R² = 0.687) than P. crassifolia (R² = 0.623); fungal communities displayed deterministic signals (NST < 0.5) and weak neutrality (R² = 0.113–0.125). Network topology indicated cross-kingdom complementarity, with more connected bacterial networks under J. przewalskii and larger, more modular fungal networks under P. crassifolia. Plant diversity and soil nutrients were the strongest correlates of microbial differentiation. This study suggests that slope-aspect differences are associated with distinct plant–soil–microbe coupling patterns, with bacteria and fungi showing differentiated assembly signals and network organization, providing an observational basis for discussing potential stability–efficiency trade-off hypotheses in mountain forest ecosystems under extreme environments and global environmental change.