Biochar derived from maricultural solid waste: an innovative strategy to sustainably mitigate sulfamethoxazole and antibiotic resistance genes pollution in saline-alkali soil

The accumulation of antibiotics and antibiotic resistance genes (ARGs) in saline-alkali soils not only poses a potential threat to human health but also further impedes the remediation of such soils. Given that the harsh conditions of saline-alkali soils may limit conventional ARG mitigation methods, it is imperative to develop effective strategies (e.g., biochar) for mitigating ARG contamination. In this study, maricultural solid waste was valorized through pyrolysis at 700 °C to produce biochar (BC700). Subsequently, the biochar was further modified by H₃PO₄ (Biochar-P), KOH (Biochar-K), and FeCl₃ + MgCl₂ (Biochar-Fe), respectively. In saline-alkali soil, BC-Fe showed a 41.58–133.04% increase in SMX removal efficiency compared with CK during the 90-day incubation, and the strongest reduction in ARG relative abundance was observed at day 60, reaching 80.7% relative to CK. This study further showed that biochar reduced the abundance of MGEs, suggesting reduced levels of HGT-related indicators, while the observed reduction in SMX concentration may have alleviated antibiotic selection pressure. In addition, BC700 and its modified biochars reduced the abundances of microbial taxa potentially associated with ARG abundance, including Bacteroidota at the phylum level and Turicibacter at the genus level. Furthermore, the biochar–Suaeda salsa–saline-alkali soil system showed that Biochar-Fe reduced the abundance of ARGs in Suaeda salsa leaves and promoted plant growth. These results indicate the potential of BC700 and its modified biochars, particularly Biochar-Fe, for SMX removal and ARG mitigation in saline-alkali soils, while providing possible mechanistic insights into the attenuation process. This study also provides a potential strategy for valorizing maricultural solid waste and supporting plant establishment in antibiotic-contaminated saline-alkali soils.