Mechanistic modeling indicates rapid glyphosate dissipation and sorption‐driven persistence of its metabolite AMPA in soil

Abstract

Residual concentrations of glyphosate and its main transformation product aminomethylphosphonic acid (AMPA) are often observed in soils. The factors controlling their biodegradation are currently not well understood. We analyzed sorption￢ﾀﾐlimited biodegradation of glyphosate and AMPA in soil with a set of microcosm experiments. A mechanistic model that accounts for equilibrium and kinetic sorption facilitated interpretation of the experimental results. Both compounds showed a biphasic dissipation with an initial fast (up to Days 7￢ﾀﾓ10) and subsequent slower transformation rate, pointing to sorption￢ﾀﾐlimited degradation. Glyphosate transformation was well described by considering only equilibrium sorption. Model simulations suggested that only 0.02￢ﾀﾓ0.13% of total glyphosate was present in the soil solution and thus bioavailable. Glyphosate transformation was rapid in solution (time required for 50 % dissipation of the total initially added chemical [DT50]ￂﾠ=ￂﾠ3.9ￂﾠmin), and, despite strong equilibrium sorption, total glyphosate in soil dissipated quickly (DT50ￂﾠ=ￂﾠ2.4ￂﾠd). Aminomethylphosphonic acid dissipation kinetics could only be described when considering both equilibrium and kinetic sorption. In comparison to glyphosate, the model simulations showed that a higher proportion of total AMPA was dissolved and directly bioavailable (0.27￢ﾀﾓ3.32%), but biodegradation of dissolved AMPA was slower (DT50ￂﾠ=ￂﾠ1.9ￂﾠh). The model￢ﾀﾐbased data interpretation suggests that kinetic sorption strongly reduces AMPA bioavailability, leading to increased AMPA persistence in soil (DT50ￂﾠ=ￂﾠ12ￂﾠd). Thus, strong sorption combined with rapid degradation points to low risks of glyphosate leaching by vertical transport through soil in the absence of preferential flow. Ecotoxicological effects on soil microorganisms might be reduced. In contrast, AMPA persists, rendering these risks more likely.