Long-term effects of olive mill waste amendment on the leaching of herbicides through undisturbed soil columns and mobility under field conditions

•Long-term applications of oiled and de-oiled olive mill wastes increased soil porosity.•Both amendments increased MCPA and S-metolachlor adsorption.•Leaching losses of both herbicides were reduced for amended soils in the undisturbed columns.•The amendments reduced vertical movement of MCPA and especially of S-metolachlor in the soil profiles.•Both amendments may be effective for reducing the risk of groundwater contamination by these herbicides.

Olive mill waste has recently been proposed for use as soil amendment in Mediterranean environments because of its potential for soil carbon sequestration, improving the soil’s quality and productivity. MCPA and S-metolachlor are herbicides applied worldwide in agricultural soils. They readily leach down to lower soil depths. The aim of this study was to evaluate the impact of oiled and de-oiled olive mill waste amendments (OW and DW, respectively) on the sorption, leaching through undisturbed soil columns, and movement and persistence of MCPA and S-metolachlor under real field conditions, in a sandy clay loam soil with low organic matter content which had received addition of these wastes for nine years. The soil had been amended with 30 and 60 Mg ha−1 yr−1 of OW (OW30 and OW60) and 27 and 54 Mg ha−1 yr−1 of DW (DW27 and DW54). The OW or DW addition resulted in significantly enhanced sorption and sorption irreversibility for both herbicides. Compared with unamended soils, leaching losses of the herbicides were reduced in the undisturbed soil columns by 84 and 97% of the applied MCPA and by 85 and 92% of the applied S-metolachlor for the OW60 and DW54 amended soils, respectively, despite the greater volume of soil macropores in OW and DW-amended soils. Under field conditions, slower vertical movement was observed for MCPA and especially for S-metolachlor in the OW- and DW-amended than in the unamended soil profiles. While the OW and DW addition extended the field persistence of S-metolachlor, increasing its half-life from 53 to 69 and 63 days for unamended, OW60, and DW54 soils, respectively, MCPA dissipation was not significantly affected by these amendments. The results suggest that previous laboratory persistence studies performed in unamended, OW, and DW-amended soils might have underestimated the real half-lives of herbicides in the field. Both OW and DW addition could be beneficial in reducing the mobility of MCPA and S-metolachlor in agricultural soils with low organic matter content, thus, decreasing the risk of groundwater contamination by these herbicides.