S-metolachlor is a chlorocetanilide herbicide widely used as a pre-emergence and post-emergence weed control in a variety of crops such as maize (Zea mays L.), cotton (Gossypium hirsutum L.) and soybean (Glycine max L.). S-metolachlor and its metabolites are commonly found in surface and groundwater close to fields where this molecule is applied. This study aimed to evaluate the effect of crop residues on the interception, retention, degradation and mineralisation of 14C-S-metolachlor. One bare soil modality, four modalities with cover crop residues from one single species such as winter turnip rape (Brassica rapa L. subsp. oleifera), oat (Avena Sativa), red clover (Trifolium rubens L.) and phacelia (Phacelia tanacetifoli L.), one modality with four months aged maize residues (Zea mays L.), and one modality with a mix of residues of four species were used in this study. The crop residues were efficient in intercepting 14C-S-metolachlor and enhancing its degradation. The fresh cover crop residues were also fixing a large amount of 14C-S-metolachlor residues under the form of bond residues. This suggests that the presence of cover crops residues could potentially have a beneficial effect and reduce the environmental impact of the S-metolachlor used in agriculture. However, further investigation in the field would be necessary to validate the founding of this study.

EI Purpan, INRA Grignon