Dominant Influence of Soil Properties over Plant Diversity in Shaping Key Functional Microbial Communities during Natural Restoration of Degraded Karst Forests

The restoration of degraded karst forests represents an effective ecological restoration method, particularly in Southwest China where fragile limestone landscapes dominate. While natural restoration facilitates aboveground recovery, its impact on belowground ecosystems, particularly the dynamics of functionally specialized soil microbial communities, remains inadequately characterized. To address this, we conducted plant surveys, soil physicochemical analyses, and metagenomic sequencing across a restoration chronosequence of a degraded karst forest (TG, shrubland; SG, secondary-growth forest; and OG, old-growth forest). Our findings reveal that restoration drives a significant reorganization of the soil microbiome, increasing overall microbial diversity and altering the composition of functional groups. Specifically, microbial communities involved in carbon (C), nitrogen (N), and phosphorus (P) cycling displayed distinct and decoupled successional trajectories, indicating group-specific adaptations to shifting environmental conditions. Structural equation modeling identified soil properties, rather than aboveground plant diversity, as the primary drivers of these microbial shifts. Among the edaphic factors, exchangeable calcium (ACa) exhibited the strongest statistical association with community restructuring, highlighting its pivotal role in karst soil ecology. The communities were consistently dominated by the phyla Proteobacteria, Actinobacteria, and Acidobacteria, though their relative abundances varied throughout succession. Collectively, this study demonstrates that the restoration of soil physicochemical conditions, more than vegetation dynamics, governs the reassembly of soil functional microbial communities during karst forest recovery. These insights provide a microbiological foundation for evidence-based ecosystem management in karst regions.