Assembly and enrichment of rhizosphere and bulk soil microbiomes in Robinia pseudoacacia plantations during long-term vegetation restoration

Despite myriad studies on root–soil–microbe interactions, the assembly and enrichment dynamics of rhizosphere and bulk soil microbiomes remain poorly understood. Here, we characterized soil bacterial and fungal communities in rhizosphere and bulk soil along a 15–45-year chronosequence of forest vegetation restoration. The neutral model, modified stochasticity ratio, niche breadth index, co-occurrence network, and source tracker were used to assess microbial community assembly, species enrichment, and filtering processes in the rhizosphere and bulk soil of Robinia pseudoacacia plantations. The relative importance of deterministic processes in microbial community assembly differed markedly between the rhizosphere and bulk soil. Microbial network complexity was higher in the rhizosphere soil than in the bulk soil. Both rhizosphere and bulk soil microbial networks were dynamically associated with forest age. Overall, the rhizosphere microbial community was mainly derived from the bulk soil, and filtered by the soil environment and plant selection. The soil environment was less selective for bacteria than for fungi, thus increasing bacterial community migration ratio. Furthermore, soil microbes formed distinct clusters in different niche compartments, with greater interaction and niche sharing potential in the rhizosphere soil than in the bulk soil.