4.1. Differences among layers of soil microbial communities
In our study, the bacterial community had lower diversity in the middle layer, while the diversity was higher in the top and bottom layers. In the fungal community, there was no significant trend in the various indicators, and the results of measurements at different soil depths were similar. One possible reason is that soil depth primarily shaped bacterial communities, while plant species structure influenced fungi [44]. Former studies showed that the predicted bacterial diversity in the topsoil was higher than in the corresponding subsoil [30]. Fungal diversity was generally higher in the 0–5 cm layer [45]. Compared with surface soils, the amount of bacterial biomass was much lower in deeper soils and microbial turnover was significantly slower [46–49].
The composition of fungal species changed significantly with soil depth, while the composition of bacterial communities remained relatively stable in our study. Fungi interact with roots at various depths, which may promote different life strategies among fungal taxa. Fungi are more prevalent than bacteria and actinomycetes in surface soils due to the lower soil pH. Earlier studies have reported that fungi were dominant in acidic soil conditions and topsoil (0–10 cm) [50, 51]. The soil in the study area is mostly acidic. This is due to the self-toxicity of organic acids and phenolic substances secreted by Chinese fir, as well as the increasingly serious acid rain pollution. As a result, the acidity of the soil is been further aggravated [52]. Soil fungi in deeper layers of the soil profile contribute to carbon and nutrient cycling, soil formation, and xenobiotic degradation. They also have close relationships with plant rhizosphere, which will improve water and nutrient absorption [53, 54]. Based on the above viewpoints, it is necessary to analyze the changes in fungal communities. And the distribution of Aspergillus in the soil at depths of 0-20cm exhibited specificity. Microbes play a crucial role in the natural process of making phosphorus accessible to plants, specifically through solubilization. Low phosphorus content in acidic red soil in the study site reduces the productivity of Chinese fir forests. The available phosphorus content that can be absorbed and utilized by trees is low [55]. However, phosphorus-solubilizing fungi are a mere 0.1–0.5% of the entire microbial population in the soil. This group includes many species, especially Aspergillus [56, 57].
4.2. Effects of different ages on soil microbial communities
Stand age is a key factor in evaluating soil biomass dynamics, carbon storage, contributing diversity, and other ecological processes [58]. The structure of bacterial communities is influenced by stand age, soil vegetation, and soil physical and chemical properties, and can quickly responds to changes in the soil environment [59]. There is a strong correlation between the growth of Chinese fir and soil properties, microbial communities, and environmental conditions [60, 61]. RDA analysis shows that soil fungal communities in Chinese fir plantations strongly affected water content, organic matter, available phosphorus, and available potassium [55]. The highest stability of soil was observed in the MIF, and it has the highest microbial biomass and diversity but lacks uniformity in the YOF [55, 62].
Our study found that older forests had a higher species richness, diversity, and OTUs in soil fungal communities. Soil properties changed with forest development, and with soil porosity and field capacity increasing in a high-low-high pattern. The low-high-low pattern was found in the PLFAs of bacteria, fungi, and the ratio of fungi number to bacteria in non-rhizosphere soil [63]. Cao et al. (2021) showed that both the microbial diversity index and OTU increased with the increase in the forest age of Chinese fir plantations. Soil porosity, bulk density, and moisture content follow a general pattern of high-low-high changes, the bacterial community gradually changed to a K-strategy, while the fungal community prioritize quality and quantity in their reproduction [60]. In this study, with the change of stand age, the fungal community also showed a trend of high and low patterns. Therefore, an appropriate extension of the plantation cultivation time is conducive to the restoration of soil physical and chemical properties and fungal communities to improve soil quality.
4.3. Effects of different seasons and climates on soil microbial communities
Seasonal changes have led to significant alterations in the structure of microbial communities [64]. Few studies have investigated the impact of environmental changes on the structure of microbial communities, and the key factors that influence these structures across seasons [65, 66]. There are differences in seasonal microbial activity, but overall the microbial biomass and metabolic diversity remain relatively stable [67].Our analysis of differences in microbial community diversity across different seasons revealed that species richness of the bacterial community and species diversity of the fungal community varied greatly, with peaks occurring in summer and winter.
Fungal communities in the same season had less similarity. Compared with fungi, due to the interaction between soil sampling time and land management, seasonal change had a more complex effect on bacterial diversity [39, 68, 69]. Previous research has suggested that environmental factors had a more significant effect on microbial diversity than plant diversity [70]. In our study, the average and maximum temperature had a more significant effect on microbial communities. Similarly, community composition was mostly driven by temperature rather than other environmental factors, and the community diversity and distribution were regulated by the interaction and comprehensive regulation of various environments [71]. Soil temperature was the main factor influencing differences in microbial community structure [72]. The temperature rises weakened species interactions, in particular, the combination of increased precipitation and warming significantly increases the bacterial richness and decreases fungal richness [73]. Climatic factors, edaphic factors, and spatial patterning are the best predictors of soil fungal richness and community composition at the global scale [74]. The soil microbial community shows a sensitive and phylogenetical response to the changes [75]. Therefore, the research on the effects of different seasons and climates on soil microbial communities helps to understand the mechanism of soil fertility changes and then provides theoretical support and prediction basis for future management measures.
4.4. Differences among ages and seasons from the perspective of species composition
Results of species composition showed differences when considering the interaction between season and age, as compared to separate factors. Bacterial community species composition exhibited similar characteristics in the same season, while fungal communities tended to cluster by stand age (Fig. 8). Fungi have an important role in soil ecology by cycling nutrients and carbon, supporting plant nutrition and protection, and contributing to the diversity of pathogens [76]. Fungal guilds are key integrators of plant richness-stock relationships, with fungal growth dominating the forest soil [77–79].
In our study, Proteobacteria, Actinobacteria, and Acidobacteria were the most common phyla in bacterial communities during summer and winter, with significant differences between these two seasons. In particular, the species composition in the OMF was characterized by Actinobacteria and Bacteroidetes. The species composition in the YOF was characterized by Firmicutes. At the genus level, Streptomyces, Burkholderia-Caballeronia-Paraburkholderia were enriched in summer and autumn. Fungal communities had similar species composition and abundance across seasons at the phylum level, with Trichoderma as the most common genus (Fig. 8). In similar studies, Proteobacteria, Acidobacteria, and Actinobacteria were the most prevalent soil bacteria in South China [80–82].
Microbial communities at the phylum level of bacterial classification exhibit the most prominent features after the interaction, regarding specific functions. Proteobacteria fix nitrogen, alleviate soil phosphorus limitations, increase bacterial diversity, stimulate microbial groups, and prompt lipopolysaccharide biosynthesis and carbohydrate metabolism [83, 84]. Acidobacteria and Proteobacteria were most affected by land-use change and were the most abundant taxonomic groups of soil bacteria [85, 86]. They were more abundant in summer and in young and over-mature forests, which corresponded to high bacterial alpha diversity in our study. Acidobacteria had oligotrophic nature or ecological K-strategy [87]. They decompose organic matter, recycle nutrients, regulate biogeochemical cycles, decompose biopolymers, and promote plant growth. The biofertilizer increases nutrients by Acidobacterial inoculation [88]. Future research could explore initiatives aimed at manipulating crop rhizosphere with Acidobacterial populations to increase plant growth [89]. They were more abundant in autumn, especially in young forests, where bacterial species richness was high and the within-group difference was small, indicating good uniformity. Actinobacteria produce beneficial metabolites such as antibiotics, biopolymers, and biocatalysts. Actinobacteria have an important influence on the turnover of recalcitrant plant organic matter in rhizosphere microbial communities. The rhizosphere region is considered one of the best habitats for isolating these microbes [89, 90]. In the soil, Bacteroidetes is mattered with complex organic matter, especially the polysaccharides and proteins [91]. And Bacteroidetes in the soil secrete diverse arrays of CAZymes which target the highly varied glycans [92]. In soil, Firmicutes species possess iron and sulphate reduction abilities and have a critical role in soil disease control [84, 93]. Members of the Firmicutes group which have iron and sulfate reducing abilities can be developed as effective bioenhancers in future bioremediation applications [94].
It has been found in fungal community diversity studies that species richness began to recover at the mature forest stage, and it makes sense to focus on species that change significantly in relative abundance during this stage (Fig. 8). In autumn, Arcopilus was significantly more abundant in near-mature forest soils, whereas Tolypocladium was more abundant in over-mature forests. Few research has been done on soil Arcopilus, it is a genus recently proposed after the taxonomic restructuring of Chaetomium [95]. In the forests soil, a potential function of Arcopilus in the environment is bioremediation of soils contaminated with organic matter and abnormal pH [96]. One form of Tolypocladium in soil is saprophytic, It can produce the immunosuppressant cyclosporine under certain conditions, and the condition that significantly affects Tolypocladium growth and survival is temperature [97]. Tolypocladium inflatum isolated from soils with no history of lead contamination was as tolerant to lead as those isolated from lead-rich soils, it is more abundant in metal-rich soils and may be more tolerant to metals [98].
In the plantations, extending the planting period of plantations appropriately could help maintain the diversity of soil bacterial communities and improve soil quality [99]. According to our study, microbial community diversity in plantations increased when forests are mature or over-mature, which indicated that we should pay more attention to the management of cultivation time to achieve the goal of soil fertility maintenance.