## 4 Discussion
purpurea_ has a short growth cycle and the most active life activities is around 120 days [38]. This is also one of the reasons why the level of PLFAs represented by bacteria and fungi changed significantly in 120 days. Arbuscular mycorrhizal fungi (AMF) can form mutually beneficial symbioses with more than 90% of vascular plants on the planet. A large number of reports indicate that AMF can significantly improve the efficiency of plant rhizosphere remediation of PAH-contaminated soil and can affect plant uptake of PAHs [29,39]. Studies by [PERSON] et al. (2011) proposed that AMF can be used to remediate organically contaminated soil [40]. AMF can promote the degradation and transformation of toxic organic pollutants, reduce the amount of residual organic pollutants in the soil, and have a good effect on the remediation of toxic organic pollution. Therefore, it is important to study the relative biomass changes of PLFAs characteristic of AMF in the soil. It is not difficult to find that the relative biomass of PLFAs characteristic of AMF in the soil changes significantly before and after planting (Figure 2A). The activity of AMF in the rhizosphere of group S was higher than that of group C. The activity was strong and reached a peak at 120 days and a decrease at 150 days, which was consistent with the change in the total amount of fungal-associated phospholipid fatty acids and related to the degradation efficiency of _E. purpurea_ against PAHs. In organically contaminated soil, plant mycorrhiza promote the absorption of mineral nutrients and water in the soil by plants. This is conducive to plant growth and phytoremediation [41]. Many studies have pointed out that the presence of mycorrhizae can often promote plant growth and PAH degradation. [PERSON] et al. (2003) studied the effect of mycorrhizal fungi on the removal of PAHs in contaminated soil. After planting clover and ryegrass for a period of time, it was found that the presence of mycorrhizae promoted plant growth and PAH degradation, and the degradation rate of rhizosphere PAHs was higher than that of non-sterile rhizospheres [42]. Research by [PERSON] (2014) showed that in an experiment on remediating PAH-contaminated soil with _E. purpurea_, the removal rate of the four PAHs showed an increasing trend with increasing time after planting, and the growth rate gradually decreased [43]. At 120 days, the removal rate increased the most. The removal rates of tetracyclic pyrene (Pyr) and chrysene (CHR) reached 46.11% and 71.91%, respectively, and those of pentacyclic benzo(b)fluoranthene (BbF) and benzo(k)fluoranthene (BkF) were 30.00% and 46.72%, respectively. At this time, the soil microbial community was the most active, and the PLGA level reached its peak. At 150 days, the removal rate increased, that is, the microbial community activity decreased, the PLGA content decreased, and the removal rate appeared to reach the maximum. The removal rates of Pyr and CHR were 66.10% and 70.30%, respectively. The removal rates of BbF and BkF were 40.50% and 60.40%, respectively. The main reason for this is related to the growth period of the plant. At 120 days, the plant grew vigorously, and the rhizosphere secretions (including polyphenol oxidase, dehydrogenase, peroxidase, etc.) contributed to the degradation of PAHs [18,43]. These secretions are conducive to the life activities of AMF, that promotes the growth of plant roots [44] and the degradation of PAHs. In addition, we speculate that rhizosphere exudates of _E. purpurea_ may contain factors that activate genes of AMF to efficiently degrade PAHs, but the specific mechanism needs further study. After planting _E. purpurea_, the growth trend of Gram-positive bacteria was not significant compared with the non-planted case. It may be that Gram-positive bacteria have poor tolerance under this condition, and the activated carbon source required by Gram-positive bacteria cannot be adequately supplied.