Feedstock and pyrolysis conditions of biochars: influence on soil phytotoxicity and water ecotoxicity

The use of biochar for soil restoration requires understanding ecological trade-offs, particularly how feedstock selection, dose, and production methods influence soil and aquatic ecotoxicity. The ecotoxicological effects of nine biochars derived from vineyard residues, Acacia wood, and olive pomace were evaluated after mixing them at rates of 1.5–5 % into two agricultural soils. Additionally, specific details of the biochar production method were assessed: blending ratios (vine pruning:stalks), pyrolysis temperature, (for Acacia wood) and hydrothermal activation method (for olive pomace). Physicochemical characterization—pH, electrical conductivity, organic matter, carbon and nitrogen content, polycyclic aromatic hydrocarbons (PAHs), FTIR spectroscopy and inertinite content—was combined with ecotoxicological assessment (Lactuca sativa L. phytotoxicity test and aquatic lethal and sub-lethal bioassays with Daphnia magna and Thamnocephalus platyurus). Vineyard pruning and shredded Acacia biochars, which had higher OM contents and lower EC and PAH concentrations, showed the lowest toxicity in soils and aqueous extracts. Soil mixed with biochar at 3–5 % blends optimally restored acidic soils through pH neutralization and moisture retention, which favoured seed growth. The aquatic assays showed stimulatory effects on D. magna feeding rates, increasing by 20–90 % at 5 % biochar concentration. Finally, production assessment revealed that both blending ratios and pyrolysis temperature caused minimal variability in organisms' responses. Hydrothermal activation reduced PAH content (<0.08 mg kg⁻¹) but failed to reduce salinity-driven ecotoxicity. These results suggest that 3–5 % wood-derived biochars are suitable to restore soils without risk to aquatic ecosystems. Olive pomace and vine stalk alternatives need a pre-application screening to detect PAHs and salinity conditions, essential factors affecting physicochemical properties of agricultural soils and environmental safety.