Control of the Field Herbicide Dissipation by Cover Crop Mulch in Conservation Agriculture

The effects of mulch on the dissipation of S-metolachlor-SMOC, foramsulfuron-FORAM, and thiencarbazone-methyl-TCM and the formation of their main degradation metabolites were studied here. The herbicides were jointly applied in preemergence of maize on two separate occasions to two agricultural soils under conventional tillage (CT) and non-tillage (NT) over two wheat-maize cycles. Herbicide concentrations were determined in topsoil samples at different times after both applications, and they were fitted to kinetic models. The half-life (DT50) values for SMOC were higher under CT management than under NT (mean values: 25.6 and 7.38 days, respectively) in both soils over the two years. The faster herbicide dissipation with mulch could be because it is partially intercepted and strongly adsorbed/retained through different potential pathways, especially biodegradation, which was supported by the detection of SMOC-ESA and SMOC-OA metabolites. The mean DT50 values for FORAM (6.15 and 6.07 days, respectively) were very close for both soils under NT and CT management over the two-year experiment. The mulch had a lesser impact than for SMOC due to the former's higher water solubility and lower adsorption, with dissipation being controlled mainly by biodegradation and likely also by leaching. TCM recorded intermediate DT50 values (mean value 20.8 days) in both soils+CT in the two-year experiment compared to SMOC and FORAM. The mulch effect on TCM dissipation was observed only after the second application because the DT50 values were higher in soils+NT after the first application (mean value: 26.9 days) than after the second one (mean value: 5.9 days). The amount of soil surface covered by the mulch controlled the herbicide dissipation, and soil and herbicide properties determine their adsorption behaviour by both mulch and soils.