At the time of spring pre-emergent herbicide application, the soil surface in conservation agriculture is most of the time covered by cover crops (CC) mulches. The state of these mulches depends on their destruction date and on the selected species. Sorption and degradation of 14C-S-metolachlor on and within 8 decaying CC-covered (2 species¿×¿4 initial decomposition state) soils corresponding to conservation agriculture were compared to its fate in bare soil (BS) corresponding to conventional agriculture. 14C-S-metolachlor and its metabolites distribution between mineralized, extractable and non-extractable (NER) fractions was determined at 5 dates during a 20¿°C/84-d period. Herbicide mineralization was weak (<2%) for both CC and BS. Extractability of 14C in BS was intermediate between CC that were decomposed 28 or 56¿days and 0 or 6¿days before application. Degradates consisted in up to 43% of total radioactivity, with specificities according to the CC or soil compartment. NER formation was equivalent in BS and in the much decomposed CC-amended microcosms, and was stronger in less decomposed CC. S-metolachlor DT50 was 23-d in BS, and 9, 15, 39 and 25-d for CC ordered by increased decomposition state at the time of application. These results were attributed to the proportion of 14C intercepted by CC, and to higher levels of organic matter and microbial activity in less decomposed CC as compared with more decomposed ones. Then the state of decomposition level of CC residues determines the behaviour of SMOC (S-metolachlor) sprayed on the mulch in the conditions of conservation agriculture.