4.1 Dynamic changes in rhizosphere microbial diversity at different forest ages
In the course of R. pseudoacacia vegetation restoration, we observed that rhizosphere bacterial β-diversity decreased over time, whereas bacterial richness did not change significantly under different restoration stages. Moreover, rhizosphere fungal β-diversity was much greater at the 45-year-old forest site than at the 15-year-old site, and fungal richness changed significantly with an increase in restoration stage. At the phylum level, the relative abundances of dominant bacteria, such as Acidobacteria and Proteobacteria , did not change significantly under different restoration stages. Nonetheless, among the dominant fungi, the relative abundance ofBasidiomycota was much higher at the 15-year-old forest site than at other sites, while the relative abundance of Ascomycota was the highest at the 35-year-old forest site. The trends we observed in rhizosphere microbial community structure could be attributed to improvements in rhizosphere soil nutrient conditions induced by R. pseudoacacia growth from a forest age of 15 years to a forest age of 35 years.
Large-scale planting of R. pseudoacacia could result in the deterioration of soil structure and an increase in soil nutrients; soil phosphorus and fine root phosphorus contents were the key factors regulating soil microbial community composition under a R. pseudoacacia plantation no more than 35 years old (34, 38). The finding is partly inconsistent with our results in the present study. Here, the key soil factors regulating rhizosphere microbial β-diversity associated with R. pseudoacacia included soil NH4+-N, TN, and TP contents. Soil NH4+-N content was a primary factor driving β-diversity of rhizosphere microbial communities, whereas soil TP and pH were secondary factors driving bacterial and fungal β-diversity, respectively. The discrepancy between the findings of the previous studies and the present study could be attributed to the different forest ages selected (15–35 years vs. 15–45 years), as well as the potential influencing factors analyzed (soil and plant variables vs. soil nutrient and pH levels).
The soil–plant–microbe system is a complex multitrophic system, and its stability is key to maximizing ecosystem functionality (53). Strategies of enhancing the robustness of the soil–plant–microbe system through artificial intervention or regulation is an emerging research topic. A previous study conducted in the Less Plateau region reported that soil microbial communities shifted their survival strategy (from r- to k-strategists) both at the phylum and genus levels, which in turn influenced the expression of genes related to C and N metabolism during secondary succession in Quercus liaotungensis forests (55). Another study reported that active plant-associated microbial communities shifted from bacteria-dominated to fungi-dominated communities, and the fungal community shifted from fast-growing and pathogenic species to a community consisting of beneficial and relatively slow-growing fungi during secondary succession following agriculture abandonment (16). In the present study, the relative abundances of the phyla Basidiomycota and Proteobacteria(r-stategists) (16, 55) decreased gradually, while the relative abundances of Ascomycota and Acidobacteria (k-stategists) (14, 51) increased with an increase in R. pseudoacacia forest age. Accordingly, we hypothesized that rhizosphere microbial communities could undergo dynamic changes in terms of life strategies or trophic levels in the course of R. pseudoacacia vegetation restoration, thereby mediating ecosystem functionality. Our findings provide support for the rational regulation and control of ecosystem function stability in R. pseudoacacia plantations.