Contrasting roles of plant, bacterial, and fungal diversity in soil organic carbon accrual during ecosystem restoration: A meta-analysis

Plant and microbial diversity plays vital roles in soil organic carbon (SOC) accumulation during ecosystem restoration. However, how soil microbial diversity mediates the positive effects of plant diversity on carbon accumulation during vegetation restoration remains unclear. We conducted a large-scale meta-analysis with 353 paired observations from 65 studies to examine how plant and microbial diversity changed over 0-160 years of natural restoration and its connection to SOC accrual in the topsoil (0-10 cm). Results showed that natural restoration significantly increased plant aboveground biomass (122.09 %), belowground biomass (153.05 %), and richness (21.99 %) and SOC accumulation (32.34 %) but had no significant impact on microbial diversity. Over time, bacterial and fungal richness increased and then decreased. The responses of major microbial phyla, in terms of relative abundance, varied across restoration and ecosystem types. Specifically, Ascomycota and Zygomycota decreased more under farmland abandonment than under grazing exclusion. In forest, Bacteroidetes, Ascomycota, and Zygomycota significantly decreased after natural restoration. The increase in SOC and Basidiomycota was higher in forest than in grassland. Based on standardized estimates, structural equation modeling showed that plant diversity had the highest positive effect (0.55) on SOC accrual, and while fungal diversity (0.15) also had a positive effective, bacterial diversity (-0.20) had a negative effect. Plant diversity promoted SOC accumulation by directly impacting biomass and soil moisture and total nitrogen and indirectly influencing soil microbial richness. This meta-analysis highlights the significant roles of plant diversity and microbial diversity in carbon accumulation during natural restoration and elucidates their relative contributions to carbon accumulation, thereby aiding in more precise predictions of soil carbon sequestration.