Meng Zhang

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.

Biochar is increasingly applied to cropland, yet co-occurring dissolved organic matter (DOM) jointly regulates polycyclic aromatic hydrocarbons (PAHs) fate and food-safety risks. We amended phenanthrene-spiked agricultural soil with wheat-straw biochar and three DOM types (citric acid, oxalic acid and humic acid) and linked biodegradation to bioavailable pools and microbial responses. Citric and oxalic acids increased bioavailable fractions and raised microbial removal from 43.98 ± 0.72% (control) to 55.84 ± 0.87% and 54.74 ± 2.29%, whereas humic acid lowered effective bioavailability and reduced removal to 39.65 ± 0.73%. Biochar alone increased removal (48.20 ± 0.98%) despite decreasing initial bioavailability. Biochar-driven microhabitat improvement coupled with citric/oxalic-acid-enhanced bioavailability enriched key degraders and nidA copies (initiating phenanthrene oxidation), yielding the highest removal (64.20 ± 0.49% and 60.52 ± 0.09%). In contrast, biochar plus humic acid promoted sequestration and showed the lowest removal (36.55 ± 0.97%). DOM chemistry modulates whether biochar favors bioremediation or stabilization.

The present study investigated the combined effects of wheat straw biochar (BC) and biosurfactant rhamnolipid (RL) on the biodegradation kinetics of phenanthrene by indigenous microorganisms in agricultural soil, focusing on dynamic responses of both bioavailability and community structure. The combined treatment (BC + RL, 60.63%) significantly enhanced phenanthrene biodegradation compared to RL alone (54.74%) and the control (45.98%), while BC amendment alone (42.55%) notably inhibited biodegradation by reducing phenanthrene bioavailability despite increasing bacterial abundance, enzyme activity, and community diversity. Both RL and BC + RL treatments promoted bioavailability by transforming phenanthrene from tightly bound (very slowly desorbing fraction, F_vslow) to readily bioavailable fractions (rapidly and slowly desorbing fractions, F_rapid and F_slow), as revealed by sequential Tenax extraction. The RL-mediated increase in phenanthrene bioavailability to microbes by 11.93–17.90% via solubilization greatly enriched PAH-degrading bacterial genera and the nidA gene, contributing to enhanced biodegradation. The BC + RL combination outperformed the single application of RL in improving phenanthrene biodegradation due to their synergy in stimulating microbial population and activity (e.g., Bacillus, Massilia, Sphingomonas, and polyphenol oxidase) as a growth stimulus. These findings demonstrate that BC and RL co-application enhances PAH removal through improved bioavailability and optimized microbial communities, offering a promising strategy for soil bioremediation to ensure agricultural product safety.

The potential of biochar to mediate shifts in soil microbial communities caused by polycyclic aromatic hydrocarbon (PAH) stress in farmland, thus assisting in the bioremediation of contaminated soil, remains uncertain. This study introduced wheat straw biochars generated at 300 °C (W300) and 500 °C (W500) at varying levels (1% and 2% w/w) into agricultural soil contaminated with phenanthrene at 2.5 and 25 mg/kg. The aim was to investigate their effects on microbial community structure and phenanthrene degradation by indigenous microbes. Biochar application in both slightly (PLS) and heavily (PHS) contaminated soils increased overall microbial/bacterial biomass, preserved bacterial diversity, and selectively enriched certain bacterial genera, which were suppressed by phenanthrene stress, through sorption enhancement and biotoxicity alleviation. The abundances of PAH-degrading genera and nidA degradation gene were promoted by biochar, especially W300, in PHS due to soil nutrient improvement, enhancing phenanthrene biodegradation. However, in PLS, biochar, particularly W500, inhibited their abundance due to a reduction in phenanthrene bioavailability to specific degraders, thus hindering phenanthrene biodegradation. These findings suggest that applying wheat straw biochar produced at appropriate temperatures can benefit soil microbial ecology and facilitate PAH elimination, offering a sustainable strategy for utilizing straw resources and safeguarding soil health and agricultural product quality.

The dynamic effect of biochar amendment in contaminated soil on the bioavailability of polycyclic aromatic hydrocarbons (PAHs) and microbial communities and how it comprehensively affects PAH biodegradation remain unclear. This study investigated the effects of wheat straw-derived biochars obtained at 300 and 500 °C at different amendment levels (0.03% and 0.3%) on the mineralization kinetics of phenanthrene with different initial concentrations (2 and 20 mg kg⁻¹) in soil by indigenous microorganisms. The results revealed that the addition of biochar inhibited both the rates and extents of mineralization in low-concentration phenanthrene-contaminated soil (PLS) by 38.9–78.3% and 23.9–53.6%, respectively. This was because biochar amendment in the PLS greatly reduced the bioavailable fraction of phenanthrene for degradation owing to its strong sorption and also decreased that to specific degrading bacterial genera, which hindered their growth and reduced their abundances by 1.37–36.6%. However, biochar addition into the soil contaminated with high concentrations of phenanthrene (PHS) resulted in its effective mineralization and enhanced mineralization rates and extents at high amendment levels by 32.4–86.7% and 32.0–44.7%, respectively. This was because biochar amendment in the PHS significantly promoted the abundances of the total bacterial communities (29.9–80.4%) and potential degrading genera (1.89–25.9%) by providing nutrients and stimulated the specific PAH-degradative nidA gene abundance by 1–2 times. These findings will guide the use of biochar to remediate soils with different PAH pollution levels based on the two roles that they play (i.e., immobilizing PAHs or facilitating PAH degradation). Graphical Abstract

The impact of multi-walled carbon nanotubes (outer diameter 50 nm: MW50) and fullerene (C₆₀) at 300 or 3000 mg kg⁻¹ on phenanthrene bioaccumulation and elimination kinetics (L, 1.37 mg kg⁻¹; H, 16.14 mg kg⁻¹) in a geophagous earthworm (Metaphire guillelmi) was investigated. Prior to worm exposure, the residual phenanthrene concentrations in soil decreased by 56.4% (L) and 59.9% (H) after 12 h of equilibrium. Phenanthrene accumulation in earthworms exhibited a bell-shaped pattern for all treatments. However, both the rate and extent of bioaccumulation and elimination were significantly affected by the CNMs, dependent on both the type and amendment level. C₆₀ and MW8 at 300 mg kg⁻¹ in the L system significantly decreased the uptake rate of phenanthrene, resulting in delayed maximum accumulation at 2 d. All other treatments had little impact on the uptake rate, with the exception of a slight decrease induced by MW8 at 3000 mg kg⁻¹. The maximum phenanthrene accumulation followed an order of C₆₀ > MW8 > MW50 at each amendment level of these materials. All CNMs increased the uptake rate of phenanthrene by earthworms in the H system, with the exception of MW8 at 3000 mg kg⁻¹ where the maximum accumulation occurred earlier than the control. Furthermore, the uptake rate and maximum accumulation of phenanthrene in earthworms followed an order of C₆₀ > MW50 > MW8 at each level, which was the opposite of its strength of sorption to these materials. Interestingly, phenanthrene bioaccumulation in the H system amended with 3000 mg kg⁻¹ MW8 was 2.2–3.9 times that of all the other treatments at 35 d. For elimination, MW8 at this level increased phenanthrene amounts remaining in earthworms in both L (1.4–3.2 times) and H (1.1–15.2 times) systems. A higher burden of phenanthrene during the later bioaccumulation and elimination periods could result from ingestion and absorption of MW8; the accumulated phenanthrene may strongly bind to the internalized MW8, thereby reducing its elimination. Soil amendment with C₆₀ and MW50 at both levels and MW8 at 300 mg kg⁻¹ increased worm phenanthrene elimination in the L system, while that in the H system was not significantly affected. The results highlight the impact of CNMs on the bioaccumulation and elimination kinetics of phenanthrene in a typical geophagous earthworm, and provide important information for understanding of the potential risks of CNMs released into terrestrial systems.