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Biochar (BC) produced from oak sawdust by slow pyrolysis was investigated to check the potential inhibition to early growth of tomato for phytotoxicity assessment. An inverted-U-shaped dose-response relationship between BC dosage and seed germination/seedling growth can be observed. Half maximal effective concentration (EC), based on the inhibition rate of root and stem length, was 65.7 and 74.0 g kg, respectively. At the highest BC dosage of 80.0 g kg, germination rate, root, and shoot length were notably inhibited by 34.9, 62.3, and 62.2% compared with their corresponding controls (without BC). Fluorescence intensity, indicating reactive oxygen species (ROS) generation in leaf and root, was 177.7 and 344.5% higher than the control. Similar results on HO content in leaf and root were observed as well. Besides, membrane leakage from the leaf and root cells was 2.1- and 1.3-fold higher than the corresponding controls. These results proved that BC exhibited significant phytotoxicity in the early growth stage of tomato. Unlike BC, the inhibitions on seed germination and seedling growth, the ROS accumulation, and the plasma membrane damage were not obvious with increasing La-BC dosage. These results indicated that BC phytotoxicity can be greatly mitigated by La involvement in pyrolysis, which was potentially associated with the reduced organic compounds and free radicals in La-BC. Besides, bio-available La in La-BC was partially involved in mitigating the phytotoxicity.

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