Change Patterns of Understory Vegetation Diversity and Rhizosphere Soil Microbial Community Structure in a Chronosequence of Phellodendron chinense Plantations

The effects of Phellodendron chinense plantations on soil properties, microbial characteristics, and the plant diversity across forest age remain poorly understood. In this study, four forest ages (2-, 5-, 8-, and 12-year-old) were examined to compare soil nutrient status, rhizosphere microbial community composition, and plant diversity. Our results showed that understory vegetation comprised 56 plant species from 29 families, with species richness significantly increasing with forest age. Rhizosphere soils showed a marked decline in pH and a significant increase in organic carbon, while nutrient dynamics followed distinct trends: P and Mg exhibited continuous accumulation; N displayed unimodal patterns; and K and Ca initially decreased before rising. Microbial community structure shifted significantly with forest age—the dominant bacterial phylum transitioned from Proteobacteria in young stands to Acidobacteriota in mature forests, whereas fungal communities underwent a successional sequence from Basidiomycota (2a) to Ascomycota (5–8a) and finally to Rozellomycota (12a). Correlation analyses demonstrated that plant diversity (S index) was positively correlated with P, K, Ca, and Mg, whereas fungal Shannon diversity was primarily driven by soil N and pH. These findings indicate that forest age mediates plant–soil-microbe interactions through rhizosphere environmental changes. For sustainable plantation management, we recommend (1) dynamically optimizing understory vegetation composition, (2) regulating soil pH and moisture during key growth stages, and (3) selecting compatible companion plants to enhance rhizosphere conditions.