Ensuring safe utilization of hydrochars in sustainable agriculture: Insights into the phytotoxicity and plant-promoting effects of low molecular weight organic compounds (LMWOCs) based on the untargeted metabolomics analysis.

Hydrothermal carbonization (HTC) converts agricultural residues into hydrochar for soil amendment, yet its phytotoxicity limits agricultural application. This study investigates hydrochar-derived dissolved organic matter (DOM) to unravel phytotoxicity mechanisms and proposes co-HTC as a mitigation strategy. Untargeted metabolomics identified low molecular weight organic compounds (LMWOCs) in hydrochar extracts from lignocellulosic biomass (ZB), swine manure (SM), and their 1:1 mixture. Seed germination assays revealed phytotoxicity (germination index [GI] reduction by 18.6-96.0 %) linked to phenolic derivatives, PAHs and furans, which increased with HTC temperature and lignocellulosic content. Co-HTC of ZB with SM reduced phytotoxic LMWOCs by 63.9-90.7 % through feedstock interactions, while enriching growth-promoting indoles and amino acids. Random forest modeling identified 4-methylcatechol, tyrosol and 5-HMF (r = -0.81 to -0.88) as dominant phytotoxins, validated via dose-response assays. Response surface optimization demonstrated non-toxic hydrochar (GI >70 %) production at 200 °C with 40-60 % ZB, balancing waste valorization and crop safety. This work elucidates the chemical basis of hydrochar phytotoxicity and establishes co-HTC as a sustainable strategy for safe agricultural reuse.