3.3 Soil microbial diversity
The soil bacterial and fungal α-diversity index were differed among the three groups (Fig. 4a, b, f, g ). Compared with Control, petroleum pollution led to a significant reduction in soil bacterial diversity (p <0.05). For fungi, soil fungal diversity had been significantly reduced by petroleum pollution regardless of the contamination duration. Moreover, correlations between microbial diversity index and soil physicochemical properties had been calculated (Fig. S3 ). The results showed that the Chao1 index was negatively correlated with soil TPH and EC (p <0.05) for bacteria. For fungi, the Chao1 index was negatively correlated with soil EC (p <0.001). The PCoA analysis of microbial community profiles proved that groups could separate soil bacterial and fungal communities (Fig. 4c, d, h, i ). In addition, the results of ANOSIM uncovered that the total discrepancy among groups was more significant than within groups.
Through VPA analysis, the relative contributions of soil sampling points and soil groups, soil physical and chemical properties and metals to the composition of soil microbial community were revealed. The effects of soil sampling sites and groups, soil physical and chemical properties, metallomes and their interactions on the composition of soil microbial community were analyzed. For bacterial community (Fig. 4e ), these variables collectively explained 53% of observed discrepancies, leaving 43% unexplained. Among those separate variables, soil physical and chemical properties explained the most significant part of the difference in bacterial community profiles (12%, p <0.05), followed by soil metallome (8%, p <0.05), sampling location and sampling group (7%, p <0.05). Their interactions interpreted 36 percent of the total discrepancy in bacterial community composition (p <0.05).
For fungal communities (Fig. 4j ), the same variable explained 60% of the observed discrepancies, and 40% of it was unexplained. Among these independent variables, sampling sites and sampling groups contributed the most to the change of fungal community composition (11%, p <0.05), while soil physicochemical properties only accounted for 6% of total variation (p <0.05). Interactions between soil physicochemical properties and soil metallomes explained 13% of the discrepancies in fungal community profiles, while the interactions between all variables explained 54% of the variation (p <0.05).