Compound-specific effects of biochar on herbicide dissipation and ecotoxicity in contrasting soils

The objective of this study was to assess the effects of a sugarcane bagasse-derived biochar (309 m² g⁻¹; pH 9.3) on herbicide dissipation and ecotoxicity in two contrasting soils (Norwood and Alligator) to identify potential environmental implications. Biochar effects on herbicide dissipation were strongly compound-specific and soil-dependent. Biochar enhanced the dissipation of metolachlor (MET), reducing its half-life from 141 to 78 days in Alligator soil, whereas it inhibited the dissipation of trifluralin (TRI), increasing its persistence, with residual concentrations up to 6-fold higher in amended soils. These responses were linked to herbicide physicochemical properties and their interactions within biochar-soil systems. Ecotoxicity responses were also organism-specific. Biochar-amended soils increased inhibition of Vibrio fischeri (up to ~42%), while consistently reducing phytotoxicity to Sorghum saccharatum , with inhibition values falling below the 20% toxicity threshold earlier than in non-amended soils. Soil type influenced the magnitude of ecotoxicological responses, with generally higher inhibition observed in Norwood soil. Overall, biochar altered both herbicide fate and biological effects, revealing a trade-off between reduced phytotoxicity and increased persistence or toxicity in the bacterial bioassay depending on the compound. These findings emphasize the need to consider herbicide properties and soil characteristics when evaluating biochar applications and support the inclusion of multiple bioassays in environmental risk assessment.