Glyphosate and AMPA dynamics during the transition towards conservation agriculture: Drivers under shallow groundwater conditions

The environmental fate of glyphosate and its metabolite AMPA were investigated in buried lysimeters in a factorial combination of two cropping systems (conservation and conventional agriculture) and two water table depths (120 and 60 cm). Glyphosate was applied at a rate of 1.44 kg ha-1 a.i., and pore-water was sampled for 48 non-consecutive days along the soil profile (15, 30 and 60 cm) as well as the groundwater. Glyphosate and AMPA concentration in soil was detected to compare the effect of different soil-crop managements on transformation/ dissipation dynamics and a full characterization of the soil profile was performed. Freundlich adsorption co-efficients were calculated down to 110 cm depth and correlated to laboratory-estimated soil properties. Clay minerals, soil organic carbon, phosphate content, Al and Fe amount were the driving factors influencing most of the glyphosate sorption, regardless of the crop and water management system that did not differentiate between adsorption dynamics. In contrast, conservation practices differently affected glyphosate concentration in the soil profile, although its adoption was limited to two years. Moreover, the occurrence of a shallow water table influenced glyphosate transport in the vadose zone, and its detection in groundwater supported the hypothesis of fast transport of the molecule as mediated by preferential pathways, compromising groundwater quality, espe-cially in agricultural lands. On the contrary, AMPA has never been detected in the groundwater, giving evidence of a diverse adsorption and transport dynamic compared to glyphosate.