Sorption, desorption, and leaching of S-metolachlor in two soils amended with maize stover

S-metolachlor is widely used across various crops for its effectiveness as a grass herbicide and is considered pivotal, particularly amidst multiple cases of resistance in weed biotypes of the Poaceae family. In no-till systems, the presence of surface residues can modify herbicide retention, release, and mobility. This study evaluated the influence of maize stover on the behavior of S-metolachlor in two contrasting soils from Paraná, Brazil: a clayey Oxisol and a sandy Ultisol. Maize stover was applied at rates ranging from 0 to 1% (0 to 20 t w/w). Sorption and desorption were determined using the batch equilibrium method, and leaching was assessed in PVC columns under simulated rainfall (60 mm). Herbicide quantification was performed by high-performance liquid chromatography. Maize stover significantly altered soil chemical attributes (pH, C, P, and K), resulting in increased S-metolachlor sorption. Freundlich sorption coefficients (K fs ) ranged from 3.24 to 5.98 mg (1−1/n) L 1/n kg − 1 in the Oxisol and from 1.48 to 2.61 mg (1−1/n) L 1/n kg − 1 in the Ultisol. Maximum sorption was observed at the 1% residue rate, reaching approximately 64% in the Oxisol and 37% in the Ultisol. In the absence of maize stover, more than 70% of the herbicide remained within the 0–5 cm soil layer. In the Oxisol, 9–30% of the applied herbicide was retained in the stover layer across application rates ranging from 5 to 20 t ha⁻¹, with greater accumulation in the surface soil. S-metolachlor was detected in the leachate at lower residue rates. These findings demonstrate that maize stover enhances S-metolachlor retention, reduces leaching potential, and may improve herbicide persistence and efficacy while mitigating environmental risks.