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.