Combined effects of maize straw biochar and oxalic acid on the dissipation of polycyclic aromatic hydrocarbons and microbial community structures in soil: A mechanistic study

Highlights: • Biochar and root exudates synergistically promoted high-ring PAH dissipation. • Biochar and root exudates enhanced microbial biomass and activity. • Biochar and root exudates synergistically affected microbial community structures. • Biochar and root exudates enhanced the genera associated with PAH degradation. • Biochar and root exudates enhanced the genes associated with PAH degradation. -- Abstract: Whether the rhizodegradation of organic contaminants occurs in biochar- amended soil and its potential mechanisms have rarely been reported. Therefore, a study was conducted to investigate the combined effects of root exudates and biochar on the dissipation of polycyclic aromatic hydrocarbons (PAHs) and on the microbial community structures in soil. As a major component of the root exudates of ryegrass, oxalic acid (OA) significantly enhanced the dissipation of high- and low-ring PAHs in the studied soil with or without maize straw biochar amendment (p < 0.05). However, biochar alone enhanced only the dissipation of high-ring PAHs. The activities of three enzymes (urease, polyphenol oxidase and dehydrogenase) were the highest in soil amended with both maize straw biochar and 0.5 mg kg⁻¹ of OA. Moreover, soil microbial biomass and the abundances of genera and genes associated with PAH degradation were significantly enhanced with the tandem application of biochar and OA (p < 0.05). These changes led to a synergetic effect of biochar and OA on the shifts in microbial community structures and on the dissipation of PAHs, especially for high-ring PAHs. The results in this study suggested that a combined biochar-rhizosphere approach should be a feasible remediation strategy for PAH-contaminated soil.