**4. Discussion**

#### *4.1. Changes in Soil and Litter Chemical Characteristics*

The transformation of vegetation-stand types has a certain impact on the physical and chemical properties of soil as well as litter and nutrient cycling [31]. Compared with Japanese cedar forests, the soil NH4 +-N contents of mixed 2 and moso bamboo increased significantly, soil and litter organic matter carbon decreased significantly, and total nitrogen showed a trend of first increasing and then decreasing. The carbon–nitrogen ratio of Japanese cedar forest was significantly higher than that of moso bamboo, indicating that Japanese cedar forest has strong carbon-sequestration ability and strong carbon-assimilation ability. So, the expansion of moso bamboo in Japanese cedar forests has a certain impact on the soil carbon and nitrogen cycle. However, due to the characteristics of moso bamboo, the expansion of moso bamboo will lead to the death of other surrounding plants due to the failure of nutrient competition. Therefore, the underground biomass of plants will decrease, resulting in a decrease in soil organic carbon and total nitrogen content [10]. Conversely, the decreased soil organic carbon and total nitrogen may be related to the decomposition rate of the litter. The rate of decay of Japanese cedar litter is lower than that of moso bamboo litter. This may, therefore, accelerate the degradation of soil organic carbon and the total nitrogen absorption of moso bamboo forests [32,33]. The total nitrogen content of the soil under moso bamboo expansion decreased, indicating that the overall absorption of nitrogen during moso bamboo expansion was higher than that of Japanese cedar, which may be related to the increased available nitrogen in the plant. Moso bamboo expansion resulted in increased soil pH, which may have altered some ammonia-oxidizing bacteria and ammonia-oxidizing archaea or may also be due to changes in chemical properties such as cation exchange [34].

#### *4.2. Changes in Fungal Community Structure*

In this study, there were grea<sup>t</sup> differences in the soil fungal-community structure among the four stand structures. Changes in vegetation types will also alter soil fungal communities. The number of soil fungi OTUs of moso bamboo showed an increasing trend. The soil fungi diversity, as measured by the Chao1 and observed species indexes, indicated that *Basidiomycota* and *Mucoromycota* were significantly higher in mixed 2 and moso bamboo than in Japanese cedar. Therefore, the expansion of moso bamboo in Japanese cedar forests significantly increased soil fungal diversity, resulting in increased number of soil pathogens and symbiotic groups, altered soil fungal community structure and function, and enzyme activities [35]. The expansion of moso bamboo is also associated with differences in plant biomass inputs and outputs, thus affecting the composition of soil microbial communities [36,37].

#### *4.3. Changes in Bacterial Community Structure*

Soil bacteria are sensitive indicators of soil change. Increased plant diversity leads to increased soil microbial diversity. In this study, in terms of the simple index of bacterial diversity, *Acidobacteria*, *Chloroflexi*, and *Gemmatimonadetes* were significantly higher in Japanese cedar and mixed 1 than in moso bamboo. This is consistent with the findings of Lin et al. [24], whereby the expansion of moso bamboo reduced the plant diversity and the soil microbial bacterial-community structure and diversity. The allelopathy produced by the leaves of moso bamboo leads to a decrease in the diversity of the bacterial-community structure [13]. Additionally, plant diversity decreases with moso bamboo expansion, resulting in weakened competition between vegetation types, which, in turn, gradually increases the mutual benefit and encourages the frequent exchange of substances between the rhi-

zosphere, and the resulting changes in rhizosphere exudates and decline in community productivity may lead to changes in the composition of the bacterial community [34].

#### *4.4. Relationship between Soil Physical and Chemical Properties and Microbes of Moso Bamboo*

The transformation of vegetation types plays a vital role in the structure of the soil microbial community [28]. This is mainly related to changes in the chemical composition of litter and root exudates and related soil carbon and nitrogen substrate concentrations, which alter the soil microbial activity and community structure [22,38]. Farmers picking bamboo shoots can cause disturbances to the soil. This process is similar to tillage and may reduce soil organic-matter content, which in turn alters bacterial communities [24]. Japanese cedar has high soil carbon and nitrogen content, has complex bacterial microbial diversity and community structure, and is associated with leaf density [26,39]. Understory plants and rhizosphere resources, such as root exudates and nutrient content, all affect the structure of the soil microbial community. Soil fungal communities are decomposers of soil organic matter [40]. Xiao et al. [41] found that soil organic carbon mineralization is directly affected by fungal diversity, and fungal diversity is also affected by pH. In this study, the expansion of soil organic carbon and total nitrogen in moso bamboo was significantly negatively correlated with fungal community diversity (Chao1 and observed species) and species abundance (*Basidiomycota* and *Mucoromycota*).

Compared with soil fungi, the soil bacterial-community structure is more sensitive to soil physical disturbances and changes in chemical properties [42]. In this study, moso bamboo increased the soil organic matter carbon and total nitrogen, and the simple bacterialdiversity index indicated that *Acidobacteria*, *Chloroflexi*, and *Gemmatimonadetes* all exhibited a downward trend. The number of soil bacterial OTUs was significantly higher than the number of fungal OTUs. There were certain differences in the soil microbial-community structure among the four forest-stand structures, demonstrating that the differences among different vegetation types have shaped different soil bacterial communities. The differing chemical compositions of the root exudates produced by different forest-stand structures will have a certain impact on the soil bacterial community structure.
