Dynamics of Glyphosate and Aminomethylphosphonic Acid in Soil Under Conventional and Conservation Tillage

This study investigates the adsorption and dissipation of glyphosate and the formation/dissipation of AMPA in non-tilled (NT) and conventionally tilled (CT) soil at 0-5 and 5-20 cm depth. Glyphosate adsorption was mainly related to the different NT and CT soil properties (clay and amorphous Al oxides), whereas an effect of the soil management could not be identified. Glyphosate dissipation was initially fast, and it slowed down later. The initial glyphosate concentration in NT soil at 0-5 cm was significantly lower than the dose applied due to the interception by the weeds and crop residues. AMPA began to form early after treatment and persisted longer than glyphosate. The DT50 range was 8-18 days for glyphosate and 99-250 days for AMPA. Longer glyphosate and AMPA DT50 were observed in NT soil compared to CT soil but, for glyphosate, the difference was significant only at 5-20 cm. Higher glyphosate and AMPA concentrations were detected in NT than in CT soil at the end of the study at 0-5 cm. The differences in glyphosate and AMPA DT50 and persistence were mainly attributable to the influence of different NT and CT soil characteristics. However, other factors could have contributed to the different glyphosate and AMPA dynamics between the soils, like glyphosate wash-off from crop residues on NT soil with the rainfall, the delayed glyphosate return to the soil by weed root exudation or weeds decomposition, and the NT soil compaction which may have reduced the microbial degradation of glyphosate at low concentrations.