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‐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 7–10) and subsequent slower transformation rate, pointing to sorption‐limited degradation. Glyphosate transformation was well described by considering only equilibrium sorption. Model simulations suggested that only 0.02–0.13% 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] = 3.9 min), and, despite strong equilibrium sorption, total glyphosate in soil dissipated quickly (DT50 = 2.4 d). 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 (0.27–3.32%), but biodegradation of dissolved AMPA was slower (DT50 = 1.9 h). The model‐based data interpretation suggests that kinetic sorption strongly reduces AMPA bioavailability, leading to increased AMPA persistence in soil (DT50 = 12 d). 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.

Glyphosate contamination in European rivers not from herbicide application?

“…Site U6, Bogue Phalia and U10, Yazoo River are described to have an intense use of glyphosate in their catchments, 23 which may lead to the accumulation of the more persistent AMPA. 24 This argument is supported when looking at the broad and similar concentration maxima of dechlorometolachlor, which is also more persistent than its parent metolachlor (Table S1). 25 The logarithm of the AMPA to glyphosate concentration ratio, log(A:G), at the Sope Creek (Site U12) and at the South Fork Iowa River near New Providence (U7) fluctuate around zero (median = 0.3) with either AMPA or glyphosate dominating at a time as can be expected for a small catchment.…”

Impacts of Cropping Systems on Glyphosate and Aminomethylphosphonic Acid Contents and Microbial Community in Field Crop Soils in Quebec (Canada)

“…Maccario et al [23] also found that soil texture did not have an impact on AMPA content. This could be explained by the soil's higher affinity to AMPA compared to glyphosate [14,83].…”

“…Glyphosate half-life is reported to range between 1 and 197 days, while AMPA half-life is between 23 and 958 days in temperate agricultural soils [12]. Once in the soil, AMPA is more resistant to biodegradation and has a higher affinity to soil particles compared to glyphosate, which increases its half-life in the soil [13,14]. Nonetheless, both molecules have similar physico-chemical properties with high water solubility, low lipophilicity, and very low volatilization potential [15,16].…”

Assessing the Effect of Organic Amendments on the Degradation of Profoxydim in Paddy Soils: Kinetic Modeling and Identification of Degradation Products

“…Indeed, pesticide residues, which include DPs and metabolites, can usually be found in environmental compartments (Le Coadou et al, 2017). Diverse studies have confirmed that many DPs of pesticides can be more persistent, toxic and/or mobile than their parent compounds (Rani et al, 2020;Wimmer et al, 2023). Transformation reactions often lead to smaller, more polar and thus less hydrophobic molecules, increasing their water solubility.…”