Application-dependent effects of tea waste biochar on PFAS mobility and N₂O and CH₄ emissions in agricultural soil

Per- and polyfluoroalkyl substances (PFAS) in agricultural soils can migrate into crops and may alter soil greenhouse gas emissions, yet remediation strategies rarely address these risks simultaneously. This study evaluated tea waste biochar (TWB) produced at 400, 500, and 600 °C and applied at 5-20 g per pot by whole-soil mixing or surface-layer placement in a simulated PFAS-contaminated soil-leachate-plant system. Perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) distributions, nitrous oxide (N₂O) and methane (CH₄) fluxes, and microbial responses were examined. TWB produced at 500 °C showed the most favorable combination of pore accessibility, surface hydrophobicity, and interfacial charge, with material-associated PFOS and PFOA enrichments of 0.12 and 0.57 μg/g, respectively. Whole-soil mixing with TWB-500 lowered soil and leachate PFAS levels and reduced PFAS concentrations in plant shoots by approximately 36% relative to the contaminated control. TWB treatments also reduced cumulative N₂O emissions and enhanced net CH₄ uptake. Metagenomic analysis showed lower relative abundances of genes associated with nitrogen fixation, ammonia oxidation, and several N₂O-producing pathways, whereas CH₄-cycling genes responded differently to the two application methods. Organic fluorine transformation genes were not enriched, indicating that PFAS control mainly resulted from physicochemical retention rather than enhanced microbial defluorination. Overall, TWB-500 can integrate PFAS stabilization with greenhouse gas management, but the optimal placement depends on the remediation objective: whole-soil mixing favors PFAS immobilization, whereas surface-layer application provides greater greenhouse gas mitigation.