4 | DISCUSSION
The highest TOC, SOM, TN, TC, NH4–N, and AP content levels were identified in the soil samples at the 0–20 cm depth, indicating that the chemical characteristics of the soil were in balance (Wang et al., 2017). The low-yielding G. jasminoides + C. oleifera forest had significantly increased TOC, HC, SOM, TN, TC NH4–N, AN, and AP content levels, and the availability of soil nutrients subsequently increased. This could be because agroforestry trees provide nutrients that fulfill agricultural demands. Another reason may be the increased abundance of related bacteria, including rhizobia, phosphate-solubilizing bacteria, and potassium-solubilizing bacteria, in soil nutrient cycling. However,G. jasminoides intercropping in a new afforestation area had marginal effects on these indicators. This supports the hypothesis that anthropogenic activities could considerably alter the original distribution of nutrients in low-yield forests by changing the planting patterns and altering the growth conditions to modify the soil nutrient status. This investigation found that slope position had few effects on the soil physicochemical properties.
Understanding the impacts of forestry practices on the soil microbiota will aid in the development of sustainable forestry practices. Our results support the hypothesis that intercropping alters the below-ground microbial community and composition. G. jasminoides supplementation in a new area ofC. oleifera afforestation increased the soil bacterial abundance and improved the bacterial alpha diversity. The soil depth and the slope position had no significant effect on the alpha diversity of C. oleifera. These findings were in contrast with previous studies, suggesting that bacterial diversity was significantly greater at the lower slope position compared with the upper slope position (Sun et al., 2018). Different bacterial taxa showed variable abundance with varying soil depths (Eilers et al., 2012). This suggests that forest types had more pronounced effects on the bacterial communities than the depth and the slope position in the same plot.
Based on high-throughput sequencing, Proteobacteria, Chloroflexi, and Acidobacteria were identified as the main bacterial phyla in the soil communities, and this was consistent with the findings of previous studies (Li et al., 2018, 2019). Proteobacteria and Acidobacteria are the primary soil bacteria taxa associated with SOM decomposition. Changes in the SOM content are highly correlated with the abundance of Proteobacteria (Fierer et al., 2007; Banerjee et al., 2016). The Proteobacteria abundance is highest in disturbed forest soils (Noble et al., 2020), implying a prominent role in carbon turnover. Proteobacteria were also found to be more abundant in the organic layer of deciduous forests (Eilers et al., 2012;, Mundra et al., 2021). This could be because higher levels of soil compaction in the deeper layers reduces O2 availability, limiting the growth and activity of many microorganisms (Hartmann et al., 2014). However, the reason behind the increase in the abundance of Proteobacteria in the 40–60 cm soil layer requires further investigation. Acidobacteria is the common bacterial phyla in soil and can degrade the cellulose and lignin of plant residues and reduce nitrate, nitrite, and potentially nitric oxide, which play a major role in the carbon cycle and nitrogen circuits (Ward et al., 2009; Kalam et al., 2020). Previous studies have demonstrated that Acidobacteria diversity is inversely related to soil depth. The abundance of Acidobacteria is highest in soils in unmanaged forests (Kuske et al., 2002; Sheng et al., 2019), which was largely confirmed in this study. This could be attributed to the Acidobacteria oligotrophic nature or the ecological K-strategy (Ward et al., 2009; Kielak et al., 2016. Chloroflexi have been identified in many environments through marine and freshwater sediments, and they exhibit biological activity in extreme soil environments (Neilson et al., 2012). Chloroflexi efficiently degrades chlorides (Zhang et al., 2020a) and can metabolize additional complex carbon sources (McGonigle et al., 2020). It is likely that Chloroflexi species play a role in the material circulation of the soil bio-chemical layers.
In our research, the dominant soil microbes significantly differed in their relative abundance among the different forest types. Our results confirmed that the G. jasminoides plantation altered the relative abundance of the dominant phyla and that there were no significant differences in the bacterial communities in relation to the depth and slope positions. These observations suggested that different forest substructures affected the soil ecological environment. A study on the effects of leguminous supplementation on the resilience of soil bacterial communities and nutrient content in Chinese fir plantations showed that functional plant supplementation significantly increases the diversity and richness of soil microbes, accelerates the transformation and absorption of soil nutrients, and promotes the growth of Chinese fir (Zhang et al., 2020c). Complex bacterial community structures increase soil resistance to adverse environmental factors and are important for protection of the soil ecosystem. As key components of the soil ecosystem, soil physicochemical properties and microorganisms have an irreplaceable role. These interspecific interactions can help maintain the stability and ecological functions of microbial communities and re-construct stable core microbial communities. Based on the LEfSe analysis, the variable distribution of these biomarkers across different forest types indicate that the influence of the soil bacterial community structure onC. oleifera may be highly important.
SEM analysis showed that the G. jasminoides plantation significantly influenced soil bacterial communities by changing NO3–N and secondarily affecting pH and EC. However, these implications in the new C. oleifera afforestation were less pronounced than in the low-yielding C. oleifera forest treatment and, may not have had as strong a stimulatory effect on the bacterial activity as expected. This indicates that the effects of the G. jasminoides plantation in C. oleifera forest may be more important than the depth and slope position when considering bacterial function in the ecosystem.