Property-Dependent Regulation of Phenanthrene Biodegradation by Carbon Nanomaterials in Agricultural Soil: Bioavailability and Indigenous Microbial Responses

Abstract

Environmentally released carbon nanomaterials can alter polycyclic aromatic hydrocarbon (PAH) attenuation in agricultural soils, yet how their properties influence biodegradation in association with contaminant bioavailability and indigenous microbial responses remains unclear. This study compared nano-biochar (NBC), pristine and hydroxylated multi-walled carbon nanotubes (MWCNTs; 4–6 nm and >50 nm) in phenanthrene-contaminated soil, using raw biochar (RBC) as parent reference material. NBC showed the strongest promotion, reaching 68.17% biodegradation at 60 d and a maximum rate of 3.00 mg/kg/d, compared with 58.31% and 2.00 mg/kg/d in the unamended phenanthrene-spiked control. RBC exerted a weaker positive effect, suggesting biochar nanosizing favored biodegradation, accompanied by moderate bioavailability reduction and stimulation of microbial abundance, enzyme activity and diversity. In contrast, MWCNTs inhibited biodegradation, with reduced β-HPCD-extractable phenanthrene, suppressed polyphenol oxidase activity and more pronounced community shifts, while higher abundance of PAH-degradation gene nidA did not correspond to enhanced biodegradation. Pristine MWCNTs showed the lowest initial β-HPCD-extractable phenanthrene and biodegradation rates, whereas small-diameter MWCNTs most strongly limited final biodegradation extent, consistent with greater surface reactivity and intensified cell-contact stress. Hydroxylated MWCNTs exhibited weaker inhibition than pristine counterparts. These findings highlight the importance of material properties in shaping PAH fate and microbial ecology in agricultural soils.