4.2 Effects of soil depth and plantation restoration period on
microbial community structure
The microbial community structure changed at different stand ages, and
the microbial diversity was higher in the surface soils, with different
community structures and species composition at different soil depths.
Vertical gradients in microbial community structure have been reported
in other soil environments that are associated with differences in soil
properties with depth (Du et al., 2021; Sun et al., 2020).
Microorganisms promote the decomposition of soil humus and accelerate
the turnover rate of soil organic matter, therefor afforestation leads
to changes in soil microbial diversity and also affects the soil carbon
content in afforested ecosystems (Chen et al., 2021; Kang et al., 2018).
Significant differences in the microbial communities were found in the
different sampling sites in NMDS plots.
Changes in bacterial diversity were only expressed in the deeper soils,
probably due to the richer nutrient content and composition of the
topsoil, which can support a more diverse microbial community. The
relative abundance of Acidobateria in the soil was significantly higher
in the later stages of afforestation restoration (50 years) compared to
the early stages of restoration (10 years). Acidobateria was thought to
be able to use soil polysaccharides (especially microbially synthesized
polysaccharides) to supply its production, and organic matter in the
soil is more present in the form of microbially synthesized carbon as
the stand age increases (Shao et al., 2019). Proteobacteria showed a
gradual decreased in relative abundance with increasing recovery time,
and this was closely related to the organic matter availability content
(DOC/SOC). Changes in the composition and availability of soil organic
matter determine the role of these taxa.
In general, fungi exhibited slower biomass turnover rates but higher
decomposition efficiency than bacteria in early silviculture, and as the
age of the forest increases, the structure and amount of organic matter
in the deep soil changes and humification decreases the diversity of
fungi (Bastida et al., 2021; Heijboer et al., 2018). It was reported
that in the acidic soil fungi represented by Ascomycota and
Basidiomycota are the main decomposers (Shi et al., 2021; Zhou et al.,
2017). Basidiomycota played an important role in degrading lignin (Zhang
et al., 2020), and the relative abundance of Basidiomycota is higher in
plantation forests with younger stand ages (in 10 years old soil
environments) due to the high cellulose content and aromatic compounds
in the topsoil. While in the deep soil layer due to the new carbon
generated by the disturbance of early anthropogenic activities was
transferred and buried deeper in the soil (Lorenz and Lal, 2005), the
lignin content in the soil was low relative to the surface soil, and
Basidiomycota phylum would be gradually replaced by Ascomycota
(S-strategist) (Ho et al., 2017; Li et al., 2021).
The transfer and decomposition of soil organic matter should be carried
out from top to bottom, and the organic matter of the substratum of the
short-term vegetation restoration was still dominated by the organic
matter imported under human disturbance.
With increasing afforestation age, litter input from old vegetation
would be terminated and replaced by litter from new vegetation, while
the soil C derived from the former litter would be decomposed and
mineralized by soil microbes (Ramírez et al., 2020b), the content of
easily available ammoniacal nitrogen in the soil increased and the
content of soil nutrients increased (Hoffland et al., 2020). Organic
matter breakdown results from a combination of physical fragmentation
and the activities of microorganisms (bacteria and fungi).
Bacterial community structure was linked to the function of the
bacterial community, the function of degradation of aromatic compounds
was increased with increasing afforestation age, and the relative
abundance of functional flora associated with nitrogen cycling was also
increased. In contrast, the bacterial community of chemical heterotrophs
needs to consume a large number of organic molecules in the soil as a
source of energy (Ma et al., 2020), and the composition of organic
matter in the soil environment of the 10 years afforestation forest was
significantly more complex than that of the 30 and 50 years ones, so the
relative abundance of chemical heterotrophic bacteria was higher in the
10 years forest. For fungi, the relative abundance of Saprotroph and
Symbiotroph increased with the age of the stand. Fungi decomposing
organic matter in the soil to obtain nutrients, and nitrogen in the soil
which is mainly produced by the decomposition of organic matter by
symbiotic fungi (Wang et al., 2021). Increasing forest age makes fungi
simple and efficient, and humification of organic matter may be
particularly important in the functions performed by fungi in these
ecosystems.
In a coniferous forest soil environment, the early apoplastic material
was not easy to be decomposed, result in the high content of aromatic
and cellulose in the soil to be high (Jílková et al., 2019). As the age
of the forest increased, the substances in the soil that were difficult
to be decomposed gradually decreased, and the role played by
microorganisms changes from poor to eutrophic, and in the soil
difficulty in the decomposition of organic matter determines the
distribution and composition of microorganisms (Amarasinghe et al.,
2021).
In this study, the association of bacteria and fungi increased with
increasing forest age, the bacterial-fungal diversity network became
simple and specialized, the products to be metabolized and decomposed in
the soil decreased with increasing forest age, difficulty to decompose
substances such as lignin increased, and the multiplicity of network
relationships gradually increased with the singularity of nutrient
substrates (Jiang et al., 2021; Zheng et al., 2021). Increasing the age
of the trees did not increase the complexity of the microbial network,
and those different ages possessed different microbiota, indicating that
microbes were highly selected by apoplastic properties and reflecting
that increasing stand age makes microbial decomposition of apoplastic
material more specialized and efficient. Although only speculative
(Freilich et al., 2018), increasing forest age makes bacteria and fungi
more specific and bacterial-fungal associations greater.
Afforestation/reforestation was recognized as an important approach for
agricultural land restoration (Austin et al., 2020). While previous
studies had mainly provided analyses of soil microbial community
abundance and diversity in forests of different ages, the use of network
analysis to assess soil microbial differences due to stand age can
provide new insights into the potential mechanisms between recovery time
and microbes during vegetation restoration.