Ecotoxicity of two contrasting soils with biochar derived from cattle manure and rice husk

• Biochar type has a dominant effect on biotoxicity. • CMB and RHB were classified as highly toxic and non-toxic. • Biotoxicity in biochar-amended soils showed variable responses and sensitivities. • Biochar could be "good" or "bad" from an ecotoxicological point. This study investigates the ecotoxicological effects of two contrasting biochars, produced from cattle manure (CMB) and rice husk (RHB), applied to two agricultural soils with divergent properties: a calcareous silt loam (Norwood) and an acidic clay (Alligator). The biochars differed markedly in their physicochemical profiles: CMB exhibited high pH (11), electrical conductivity (12 dS m⁻¹), and K content (89 g kg⁻¹), while RHB showed much greater surface area (174 vs. 1.7 m² g⁻¹). Ecotoxicity was assessed through four bioassays ( Vibrio fischeri, Pseudokirchneriella subcapitata, Sorghum saccharatum, and Lepidium sativum ). CMB caused strong toxicity in all aqueous bioassays, while RHB showed no adverse effects. When applied to soils, CMB reduced the toxicity of acidic Alligator soil to P. subcapitata (from 54 % to 16 %) and reclassified its hazard level from class III to I, likely due to aluminum immobilization (Al³⁺). Conversely, CMB slightly increased toxicity in Norwood soil while RHB improved ecotoxicological profiles in both soils. These results highlight the critical role of soil-biochar interactions in modulating environmental risk. This study links biochar composition and soil properties to bioassay outcomes, offering a practical approach to assess bioavailability-driven toxicity across realistic combinations of biochar and agricultural soils under controlled conditions. These findings underscore the need to align biochar–soil combinations with specific management goals, whether agronomic, environmental, or both.