Is the Dissipation of Herbicides favored by Conservation Agricultural Practices?

The conservation agriculture (NT) is a current agricultural practice characterized by no tillage, the use of cover crops and accumulation of their residues on the soil surface (mulching). This type of agriculture can induce changes in the soil physical, chemical and biological properties, which can strongly modify herbicides dissipation compared to that in a conventional tillage system (CT). The objective of this work was to study at field scale: i) the dissipation of the herbicides S-metolachlor (SMOC), foramsulfuron (FORAM) and thiencarbazone-methyl (THIEN), ii) the formation of metabolites, and iii) the soil dehydrogenase activity (DHA) in both conventional (CT) and conservation (NT) tillage practices to analyse the effect of herbicides and type of farming practice on soil microbial activity in an agricultural soil with two different surface organic carbon contents (S1 and S2). Herbicide dissipation curves were fitted to a single first-order (SFO) kinetic model or a first-order multi-compartment (FOMC) model. Herbicide half-life (DT50) values were calculated for each of the treatments tested. Soil DHA in each of the tested treatments was determined on fresh soil samples. Herbicide dissipation was faster in NT than in CT soils, taking place mainly on the mulch surface. The DT50 values under CT varied in the range: 20.9-30.2 (SMOC), 5.0-8.8 (FORAM) and 18.7-25.2 (THIEN) days, with a dissipation order of : FORAM > THIEN > SMOC. Under NT, the DT50 values varied in the range: 4.0-1.7 (SMOC) , 3.7-5.1 (FORAM) and 6.6-5.2 (THIEN) days, following the dissipation order of : FORAM ≈ SMOC >THIEN (S1+NT), and SMOC> THIEN ≈ FORAM (S2+NT). The lower amount of metabolites observed in the soils under NT agreed with the lower amount of herbicides that reached the soil surface as a result of their partial interception by the mulch. Soil DHA was higher in soils untreated with herbicide and under NT condition