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.