Glyphosate and AMPA dissipation at different depths in conventional and conservation agriculture

Glyphosate is the most used herbicide worldwide, especially in conservation agriculture, where the lack of mechanical weed control often necessitates chemical inputs. In conservation agriculture, the elimination of tillage operations leads to changes in soil physical, chemical, and biological properties. Consequently, herbicide environmental fate may be potentially altered relatively to conventional tillage systems. The aim of this study was, therefore, to investigate the effect of conservation agriculture and conventional tillage on the adsorption of glyphosate and on the dissipation of glyphosate and its primary metabolite aminomethylphosphonic acid (AMPA) at two depths, 0−5 and 5−20 cm.

The field trial was conducted from October 2018 to April 2019 at the Padua University Experimental Farm, North-East Italy. Glyphosate was applied as a formulated product (Roundup Power 2.0) at a dose of 1.44 kg/ha of the active ingredient. The dissipation of glyphosate and the formation/dissipation of AMPA were followed for 182 days after their application. The concentrations of glyphosate and AMPA in the soil were analysed by Ultra-High Performance Liquid Chromatography coupled with mass spectrometry. The dissipation of glyphosate was described by the first-order multicompartment model (FOMC), whereas the model for AMPA was composed of an FOMC degradation model for glyphosate and the single first-order degradation model for AMPA. The estimated trend of concentrations over time for both glyphosate and AMPA were used to derive their DT50 (time required for 50% dissipation of the initial concentration).

The results indicate an increase in glyphosate adsorption in non-tilled soil compared to the tilled soil, at both depths. Glyphosate initial dissipation was fast, followed by a slower decline. At 0–5 cm no significant difference was observed in glyphosate persistence between the two soil managements, whereas at 5–20 cm glyphosate was more persistent in non-tilled soil (DT50 18 days) than in tilled soil (DT50 8 days). The fast initial dissipation of glyphosate was reflected in an increase in the concentration of AMPA. AMPA persisted longer than glyphosate but, for this metabolite, no apparent effect was observed in response to the different soil management. The higher persistence of glyphosate under conservation tillage might increase the risk of on-site soil pollution due to the accumulation of this chemical, especially in the case of repeated applications of glyphosate. Nevertheless, high glyphosate adsorption observed in non-tilled soil may reduce the leaching potential to lower soil depths.

This abstract falls in the group “Soil contamination” and the subgroup “Experimental assessment”.