## 4 Discussion  
The current study showed that Cd, Zn, Cu and Pb were the dominant heavy metals at the investigated Futian mangrove soils according to their levels against the thresholds set for Class I marine sediment equality in the National Standard of China [15]. This is in good agreement with other published work showing that Cd, Cu and Zn were the major heavy metals in the Pearl River Estuary [19; 20]. The more heavily contaminated status in Locations 1-4 than in Locations 5-8 was previously noted by [PERSON] et al. [21] and [PERSON] et al. [16]. This was due to the closer proximity of Locations 1-4 to urban roads, as compared to Locations 5-8. The higher total carbon concentration of soils collected in January 2019 than that in July 2018 reflected the seasonal influence on soil carbon dynamics. It is well established that decomposition of organic matter tended to be slow during the winter season, resulting in the accumulation of soil organic carbon [22]. It is expected that spatial and temporal variations in the key soil parameters have impacts on microbial activities [23]. [PERSON]'s correlation analysis (Figure 5) between various environmental factors and the dominant bacteria at the phylum level indicated Proteobacteria and Chlamydia had a positive correlation with TC and TN (\(p\leq 0.01\)), while the Chloroflexi showed a negative correlation with TC (\(p\leq 0.01\)). The pH was also a key factor influencing the community structure, as shown by Planctomycetes that had a strong negative relationship with pH (\(p\leq 0.01\)). Furthermore, the canonical correlation analysis (CCA) between OTU and the basic soil properties allowed 65.35% of the variation in the species data being explained (Figure 6a). [PERSON] showed that the contribution from the environmental factors towards the bacterial community structure was in the following decreasing order: TC \(>\) TN \(>\) Cd \(>\) EC \(>\) pH (Figure 6b). The dominant influence of the total carbon may reflect its role in providing carbon and energy sources for microbial growth [8]. Figure 4: Heatmap of the distribution of the 20 most abundant bacterial genera present in the soil samples. The relative abundance of bacterial genera was indicated by color intensity. Figure 5: Effects of environmental factors on the community structures of dominant bacteria at the phylum level by [PERSON]’s correlation analysis. Note: the figure shows the correlation between environmental factors and microorganisms with a warm tone indicating positive correlation and cool tone indicating a negative correlation. (* indicates \(0.01<p\leq 0.05\), ** indicates \(0.001<p\leq 0.01\)). Figure 6: Canonical correlation analysis (CCA) ordination plots for the two dimensions to show the relationship between the bacterial community structure and environmental parameters using a 16S rRNA gene sequence. (**a**) Correlations between environmental variables and CCA axes are represented by the length and angle of arrows (environmental factor vectors). PERMANOVA shows the explanation of single factors in the community structure of bacteria; (**b**) A single asterisk sign “**” next to a bar denotes significance at \(p<0.05\); A two-asterisk sign “**” next to a bar indicates significance at \(p<0.01\). Heavy metal pollution directly impacts the microbial ecosystem, including alterations in the physiology, diversity and abundance of microorganisms [10; 24]. [PERSON]'s correlation analysis (Figure 5) showed that Cd had a strong positive correlation with the relative abundance of Nitrospirae and Acidobacteria phylum (\(p\leq 0.01\)). The investigated area can be divided into two subareas based on the status of heavy metals in the soils (Figure 2): (a) higher-polluted area (Location 1-4) and lower-polluted area (Location 5-8). The relative abundance of Nitrospirae and Acidobacteria in the higher-polluted area was greater than that in the lower-polluted area (Figure 7).