Topographic slope effects on soil bacterial assembly and network stability after 20-year karst ecological restoration

Karst ecosystems are highly vulnerable to degradation, with restoration outcomes largely dependent on belowground microbial processes. Yet, how long-term ecological restoration shapes the topography-driven patterns of soil bacterial community assembly and network stability remains inadequately understood. Here, we examined the diversity, assembly processes, and co-occurrence networks of soil bacterial communities along contrasting slope directions (sunny vs. shady) and slope positions (lower, middle, upper) in a 20-year restored karst canyon. Results showed that bacterial diversity declined significantly with increasing elevation on sunny slopes but remained stable on shady slopes. Neutral community models and null model-based βNTI analyses jointly indicated that stochastic processes—particularly homogenizing dispersal—dominated at lower slope positions and shady slopes. Network analyses revealed decreasing complexity (nodes, links, avgK, avgCC) but increasing stability (robustness) with elevation, especially on sunny slopes. Environmental filtering by soil water content, nutrients, and microbial biomass was the major driver of spatial variation. These findings highlight that slope topography governs bacterial community assembly through hydrological redistribution and niche filtering, offering microbial indicators for karst soil erosion risk and restoration monitoring.