Mechanisms of the removal and remediation of phenanthrene and pyrene in soil with mixed cropping of alfalfa and rape.

【Objective】 To evaluate the potential of multispecies rhizoremediation in decontaminating polycyclic aromatic hydrocarbons (PAHs) contaminated soil and its impacts on plant accumulations of PAHs. 【Method】 Pot experiments were conducted to investigate the mechanisms of the removal and remediation efficiencies of PAHs under different planting models using two plant species, alfalfa (Medicago sativa L.) and rape (Brassica campestris L.) in a greenhouse. 【Result】 Results showed that multispecies rhizoremediation rather than monoculture have the evident potential to enhance pollutants removal in soils with initial phenanthrene concentrations of 20.05-322.06 mg·kg-1 and pyrene of 20.24-321.42 mg·kg-1. At the end of the 70 d experiment, the extractable PAHs in soils with mixed cropping of alfalfa and rape were lower than that in monoculture. About 65.17%-83.52% of phenanthrene and 60.09%-75.34% of pyrene were removed from the soils in mixed cropping, respectively. As compared to monoculture with rape, mixed cropping removed in 43.26% of phenanthrene and 40.38% of pyrene from soils in average and relative to monoculture with alfalfa, 11.03% of phenanthrene and 16.29% of pyrene was removed. Alfalfa or rape did take up PAHs from the soils obviously; the concentrations of PAHs in root or shoot monotonically increased while ones in the soils were higher, and under the same treatment conditions, the concentrations of PAHs in root or shoot were lower in mixed cropping than that in monoculture, and in the shoot lower than in the root. Despite the presence of vegetation evidently enhanced the remediation of PAHs in soil environment, contributions of abiotic loss, plant accumulation and degradation was much lower than ones of microbial degradation and plant-microbial interactions in the process of phytoremediation. Thus plant-microbial interactions are the main mechanisms for the remediation enhancement of soil PAHs pollution under mixed cropping models. 【Conclusion】 Results from this study suggested a feasibility of the establishment of multispecies phytoremediation to improve the efficiency of bioaugmentation in decontaminatingPAHs contaminated soils, decreasing crop accumulations to PAHs and reducing risks associated with PAHs.