Litter C/N ratio is associated with POC-to-MAOC transformation potential across forest types in subtropical restoration.

Forest type is a critical determinant of soil organic carbon (SOC) dynamics during ecological restoration, yet how forest type shapes microbial community assembly and functional gene abundance to govern the partitioning of soil carbon into particulate (POC) and mineral-associated (MAOC) fractions remains poorly resolved. In May 2025, we collected soil samples from 12 plots representing three typical forest types (coniferous, mixed, and broad-leaved forests) in the Lingnan Nature Reserve and applied metagenomic sequencing to characterize soil microbial communities and functional processes. Following over three decades of restoration, SOC in mixed (25±1.5 g/kg) and broad-leaved forest (26±2.1 g/kg) soils increased by ∼18% and 23%, respectively, compared to coniferous forests (21±1.6 g/kg). Litter C/N was lower in mixed and broad-leaved forests, corresponding with their higher SOC. Structural equation modeling further linked litter C/N ratio to POC and MAOC accumulation via microbial biomass carbon (MBC) as a key node, with POC, MAOC, and MBC increasing by 108-134%, 20-22%, and 26-31%, respectively, in mixed and broad-leaved versus coniferous soils. At the community level, variations in forest types selectively enriched Acidobacteriota or Actinomycetota, while co-occurrence network analysis revealed a shift from predominantly negative toward predominantly positive associations among bacterial taxa in broad-leaved and mixed forests, along with enhanced cross-module metabolic flow. Functionally, compared to coniferous forests, mixed and broad-leaved forests exhibited ∼15%/38% and 21%/47% increases in RPKM values of carbon fixation/degradation gene, respectively. GO enrichment analysis further indicated that litter inputs may be converted into stable humus via glycolysis and amino acid synthesis pathways. By integrating community-level microbial ecology, co-occurrence network analysis, and metagenomic functional profiling, this study provides novel mechanistic insight into how forest type shapes soil carbon fraction dynamics during restoration. These findings indicate the gene abundance variation in POC-to-MAOC transformation might be a plausible mechanistic link in the plant-microbe-soil carbon nexus and suggest that promoting broad-leaved or mixed forest restoration may represent a potentially effective strategy for enhancing soil carbon accumulation in subtropical regions.