ORIGINAL ABSTRACT

Cover crops (CC) usage induce soil characteristics changes as well as microbial community composition and functions that may impact glyphosate and aminomethylphosphonic acid (AMPA) soil contents, following glyphosate-based herbicide (GBH) application. To test this hypothesis, two corn-soybean-wheat rotations, one with and one without CC, were both established three years prior to soil sampling at two open field sites. During the wheat year, crops residues were removed from the surface of the soil and soil cylinders were collected from the fields and brought into a greenhouse. GBH or distilled water was applied on soil cylinders which were sampled at five time points following GBH application. Glyphosate dissipation followed a double first-order in parallel equation and did not differ significantly between sites and CC history. Overall, AMPA contents were not higher in GBH-treated soils compared to water-treated soils. Microbial community analyses revealed significant effects of site and CC usage with CC associated with prokaryotic abundance and eukaryotic diversity enhancement along with an increase in mycorrhizal fungal abundance and reduction in the abundance of pathogenic fungi. GBH application was associated to an increase in total fungal abundance along with a change in the abundance of few prokaryotic, fungal, and eukaryotic ASVs. Functional potential analysis showed that GBH application did not significantly affect bacterial pathways related to glyphosate degradation, pointing toward glyphosate leaching instead of degradation as main dissipation pathway. Overall, removing CC residues from soil surface seemed to negate slowdown of glyphosate dissipation associated to CC without removing CC impacts on soil microbial community.