Root exudates enhance the PAH degradation and degrading gene abundance in soils

Highlights: • A soil microcosm study was used to explore the root exudates-influenced PAH degradation. • 10–100 mg DOC/kg root exudates promoted the PAH degradation in soils. • Absolute abundance of PAH degraders and degrading genes strongly correlated to PAH degradation. • Root exudates enhanced PAH degradation due to the increased PAH-degraders and degrading genes. Root exudates could influence the bioavailability of polycyclic aromatic hydrocarbons (PAHs), provide nutrients for soil microorganisms, and affect PAH biodegradation. However, it remains unclear how a bacterial community and its PAH-degrading genes play crucial roles in PAH biodegradation and respond to root exudates. In this study, a 32-day soil microcosm study was conducted to explore the impacts of artificial and actual root exudates on PAH degradation, degrading genes, and bacterial community structure. The results showed that 10–100 mg DOC/kg artificial and actual root exudates promoted the degradation of naphthalene, phenanthrene, and pyrene in soils, and their percent removal increased initially and then decreased with the increasing root exudates. Quantitative polymerase chain reaction analysis and 16S rRNA gene high-throughput sequencing suggested that the artificial root exudates significantly promoted the Nocardioides and Arthrobacter genera, which may harbor the nidA gene (the representative PAH-degrading gene from Gram-positive bacteria). In contrast, actual root exudates significantly stimulated the Pseudomonas genus that may harbor the nahAc gene (the representative PAH-degrading gene from Gram-negative bacteria). The correlation analysis further indicated that the absolute abundance of PAH degraders and degrading genes had strong correlations with PAH degradation efficiency. Therefore, these findings suggest that root exudates enhanced PAH biodegradation probably due to increases in abundance of both PAH-degraders and their degrading genes.