Abstract   The residual soil material resulting from biomass thermochemical transformation during carbon separation, known as biochar, has been introduced as a soil amendment because of its numerous environmental benefits, including uses for contaminated land management. Adsorption and leaching of fomesafen in soils amended with 3 different rates of rice hull biochar (0.5%, 1%, and 2% w/w) under laboratory conditions were investigated, and studies were performed following a batch equilibration adsorption–desorption procedure and a column experiment for leaching. Adsorption–desorption data fit with the Freundlich equation well. The adsorption coefficient of fomesafen sharply increased from 0.59 to 0.99 to 8.02 to 22.23 when the amount of biochar amendment in the soil increased from 0% to 2% (w/w). In addition, a strong correlation was found between the amount of adsorbed fomesafen and the rate of amended biochar (r > 0.992, p < 0.01). Furthermore, biochar amendments reduced the desorption percentage of fomesafen in the soils. The outcomes of the leaching experiment also illustrated that the lowest fomesafen concentration in the leachate (21.4%) occurred in the soil amended with 2% (w/w) biochar. Moreover, the adsorption coefficients (Kfads) of the soil were positively correlated with the total amount of adsorbed fomesafen in the corresponding soil columns (r = 0.990, p < 0.01) and negatively correlated with the leachate percentage (r = 0.987, p < 0.05). The results of the present study suggest that biochar amendments in agricultural soils likely alter the fate of herbicides by decreasing their transport through enhanced adsorption. Environ Toxicol Chem 2015;34:1258–1266. © 2015 SETAC