Abstract The amendment of anthropogenic soils with natural biochar (without pyrolysis process) presents several environmental and agronomic benefits. However, there is little information available on the interaction of natural biochar with the herbicides applied directly into the soil. To address this knowledge gap, the influence of natural biochar on the sorption–desorption and mobility of diclosulam and pendimethalin herbicides in an agricultural soil was evaluated. The biochar was extracted from the profile of an Oxisol and post-dates the Holocene. The Ultisol-Typic Hapludalf used in this study was amended with natural biochar at rates of 0 (control), 0.1, 0.5 and 1.0% (m m−1). The 14C-diclosulam and 14C-pendimethalin showed specific activities of 0.002587 and 5.55 MBq mg−1, respectively. The sorption–desorption process was assessed by batch–equilibrium experiments. Liquid scintillation counting and soil thin-layer chromatography were used to determine herbicides adsorption and mobility. Application of natural biochar slightly increased the linear sorption coefficient (Kd) of each herbicide (from 0.22 to 0.50 L kg−1 for diclosulam, and from 25.55 and 37.81 L kg−1 for pendimethalin) in the soil, and the corresponding sorption percentages were 18.53–34.14% and 96.22–97.42%, respectively. However, the soil amendment did not alter the mobility of either herbicide, with retention factor (Rf) values of ~0.99 and 0.20 for diclosulam and pendimethalin, respectively, regardless of the natural biochar amount. Therefore, the presence of biochar in soil may not interfere with the bioavailability of diclosulam and pendimethalin in weed control under field conditions.