Complexity of bacterial communities within the rhizospheres of legumes drives phenanthrene degradation

Determining the mechanisms underlying complex plant-microbe interactions and feedbacks is crucial for effective rhizoremediation of contaminated soils. Here, rhizosphere bacterial communities of 10 common legumes planted in arable and aged oil-contaminated soils for 90 days were used to degrade phenanthrene (PHE). Bacterial community structures were analyzed to reveal the mechanisms underlying the varying PHE degradation capacities. We found that bacterial beta-diversity within the rhizospheres played a major role in predicting PHE degradation rates, and more complex bacterial communities exhibited higher PHE degradation capacities. The highest PHE degradation rates were obtained with bacterial communities of Coronilla varia and Vigna unguiculata planted in oil-contaminated soils (almost 80%). There were significant differences in bacterial community composition between arable and contaminated soils, and among different legumes rhizospheres and the unplanted soils, although minimal differences were observed in bacterial alpha-diversity. The potential degradation-related taxa were more abundant in bacterial communities from contaminated soils, indicating more effective PHE degradation than in arable soils. Network analysis revealed the vital ecological roles of the potential degradation-related taxa in the maintenance of complex associations among bacterial taxa. Oil contamination reduced the differences in bacterial community composition between rhizosphere soils with high and low PHE degradation rates. These results suggest that the formulation of effective rhizoremediation strategies ought to focus on more diverse and complex interactions between rhizosphere-inhabiting organisms from aged oil-contaminated soils through the selection of optimal plant-microbiota association.