Beyond the black box: Deconvoluting the inherent compounds in biochar phytotoxicity and soil impact

Biochar can support environmental remediation, but its inherent chemical constituents may also cause phytotoxicity and soil ecological disturbances. Here, biochars produced from kitchen waste (Kit), pig manure sludge (Pms), and rice straw (Ris) at 400 and 600 ℃ were evaluated to identify major chemical risk drivers. Water-extractable inorganic ions, DTPA-extractable metals and As, 16 priority polycyclic aromatic hydrocarbons (PAHs), and dissolved organic matter (DOM) were characterized by ICP-MS, GC-MS, and FT-ICR-MS. Short-term radish bioassays, a 180-day soil incubation, correlation analysis, and structural equation modelling were used to link chemical fingerprints with biological responses. KitBC showed the strongest phytotoxicity, particularly KitBC600 at the 1:20 exposure ratio, where germination rate and germination index decreased to 31.11% and 13.15, respectively. RisBC400 contained the highest total PAH concentration (63.46 mg kg^-1), which decreased to 6.57 mg kg^-1 at 600 ℃. Across feedstocks, increasing pyrolysis temperature reduced total PAHs by 77.6-89.6% and decreased the number of detected DOM molecular formulas, whereas soluble ions and extractable metals showed feedstock-dependent responses. Na/K salts, extractable Cu, and hydrophobic DOM were identified as major contributors to phytotoxicity and soil ecological responses. These findings demonstrate feedstock- and temperature-dependent chemical risk fingerprints and support targeted screening and management of biochar.