4.2 Divergent drivers and co-occurrence patterns for microbial
communities along the chronosequence
In the past, researchers mainly focused on how restoration stages or
forest ages influence soil microbial diversity, but rarely reported
microbial co-occurrence patterns (especially in the rhizosphere) and the
divergent drivers along a chronosequence, for example, in R.
pseudoacacia plantations. In the present study, we observed that from
25 years to 45 years of R. pseudoacacia forest age, different
clusters of rhizosphere microbial communities formed distinct modules in
the microbial network. Modulization indicates habitat heterogeneity,
aggregation of phylogenetically related species, niche overlap, and
species coevolution (41). Therefore, a similar module means a similar
habitat, and species in the same module due to niche overlap have more
interactions with each other. Such an interaction may in the form of
cooperation or competition for resources. Conversely, there are
relatively less interactions between modules, probably due to isolation,
which means that the co-occurrence of species is inhibited. Our results
suggest that clusters located on the same branch of a phylogenetic tree
form unified modules in the microbial network, which indicates that
similarity in rhizosphere microbial community structure plays a role in
determining the modularized structure of the microbial network.
We observed tight interactions and close aggregations among microbes in
Module 1 and Module 2 for both bacteria and fungi in the rhizosphere
soils in the R. pseudoacacia plantation. However, Module 1 and
Module 2 were not fully separated from Module 3, indicating that forest
age was not the only factor driving microbial community module
differentiation. In addition, the relationship between rhizosphere soil
multinutrient index and bacterial and fungal taxa relative abundance
varied in different modules. Consequently, rhizosphere soil functions
potentially influenced microbial co-occurrence patterns in different
modules; the microbial responses to rhizosphere soil functions varied
over time and space. Furthermore, we observed that different forest ages
targeted specific fungal lineages, but not bacterial lineages. Not only
the stability but also the connectivity of the bacterial network was
higher than those of the fungal network.
Notably, the microbial co-occurrence networks illustrated the
correlations among different taxa, including real ecological
interactions and non-random processes; therefore, the network may not
really reflect the direct interactions between different groups (13,
40). Because habitats have complex regulatory mechanisms, microbial
networks are useful tools for exploring complex microbial community
structures, particularly in rhizosphere microenvironments with complex
interactions. Microbial networks may also facilitate the exploration of
the interactions among multi-level biological systems in the future.
Long-term observations are still required to select microbes with
specific functions that could facilitate plant selection of mutually
beneficial microbiota, which would enhance plant adaptation without
adversely affecting plant growth.