Fig. 3 (a)-(b): Compositions and structures of soil microbial
communities between different stand ages in the L. gmeliniiplantations. The NMDS, the dynamics of microbial community during the
increase of L. gmelinii plantations age and then assessed through
a PERMANOVA.
The plot of NMDS (Figs. 3c and 3d) suggests variations in microbial beta
diversity among the soil samples, with the response of soil bacterial
beta diversity to afforestation time being larger than that of fungal
beta diversity (bacteria, PERMANOVA, r = 0.48,p < 0.001; fungal, PERMANOVA, r = 0.31,p < 0.001), despite the significant variation in both
species with afforestation time, and the differences in diversity
between the restorative stages were much larger than the differences in
diversity among soil depths.
Six bacterial phyla (Acidobacteria, Actinobacteria, Proteobacteria,
Rokubacteria, Gemmatimonadetes, and Chloroflex) dominated forest soils,
accounting for 95% of bacterial abundance. The relative abundances of
these taxa varied by forest ages stages and soil depth. Notably,
Rokubacteria significantly increased with increasing forest ages.
Proteobacteria showed the opposite trend. Other phyla showed different
responses with increasing stand age but these responses were
insignificant. In the case of the fungal community, fungi could be
classified into two different phyla, predominantly Ascomycota and
Basidiomycota. The relative abundance of Basidiomycota increased but
Ascomycota decreased with increasing forest age. The relative abundance
of Basidiomycota gradually decreased with increasing soil depth in the
younger stand (10 years old).
Using random-forest machine learning to discriminate samples with
different samples based upon the microbiota composition (Zhang et al.,
2018). The importance of indicator prokaryotic classes was assessed by
10-fold cross-validation (Xue et al., 2021). When the cross-validation
error curve was at its lowest, a total of 24 OTUs were detected as
biomarker fungi taxa and 42 OTUs as biomarker bacteria taxa. Among these
classes, taxa exhibited different relative abundances in soils of
different stand ages, and these results suggest that these taxa may be
useful as biomarkers of soil microorganisms in planted forests (Fig.
S2).
Using FUNGuild, 11 of all OTUs were assigned to putative fungal
functional groups, which accounted for 54.5% of total biomarker fungi
taxa. Pathotroph-saprotroph-symbiotroph (45.8%) were the abundant
functional groups across all samples (Fig. S2). For trophic modes, the
relative abundances of some dominant taxonomic groups changed apparently
with forest ages. The relative abundances of saprotroph and symbiotroph
in 30 and 50 years forests were greater than those of 10 years forest
group, while the relative abundance of pathotroph aerobic
chemoheterotrophy (AC) and aromatic compound degradation (ACD) was
significantly lower than that of 10 years forest. Relative abundance of
aerobic nitrite oxidation (ANO) significantly increased with stand age.
These changes are closely related to soil nutrient limitation.