The role of two organic amendments to modify the environmental fate of S-metolachlor in agricultural soils.

S-metolachlor is a widely used herbicide that may contaminate groundwater when applied to irrigated crops, especially when the soil has a low organic carbon (OC) content. The objective here was to assess the capacity of two organic wastes, namely, green compost (GC) and pelletised organo-mineral manure fertilizer (PM), applied to two soils (S) with different textures at a rate of 10% dry weight to modify the fate of S-metolachlor. The herbicide's Freundlich adsorption coefficient (Kf) increased within a range of 3.2-8.2 times in S + GC and 3.8-6.8 times in S + PM. A positive correlation between adsorption and OC and the coefficient of variation of the OC normalised adsorption coefficients (Kfoc) higher than 20% indicated the evident influence on this process of soil OC content and its nature. The increase in adsorption did not prevent the dissipation of S-metolachlor in the amended soils, although the degradation rate decreased up to ~2 times or was not significantly modified across the different soil types. The S-metolachlor metabolites, metolachlor ethane sulfonic acid and metolachlor oxanilic acid, were detected in the herbicide's dissipation in the unamended soils, but they were not detected in the amended soils. The mobility experiments indicated leached amounts of S-metolachlor higher than 50% in unamended soil. The amounts decreased 1.1-1.7 times and 1.7-1.8 times in the S + GC and S + PM when a saturated flow was applied. Moreover, breakthrough curves indicated a slow leaching kinetics of herbicide in amended soils, with low concentrations continuously detected in the leachates together with a decrease in the maximum peak concentration. The results show the effect of the application of organic wastes especially in sandy soils to promote the immobilisation and/or degradation of S-metolachlor, avoiding its transfer to other environmental compartments.