Fate and availability of glyphosate and AMPA in agricultural soil

Abstract

The fate of glyphosate and its degradation product aminomethylphosphonic acid (AMPA) was studied in soil. Labeled glyphosate was used to be able to distinguish the measured quantities of glyphosate and AMPA from the background values since the soil was sampled in a field where glyphosate had been used formerly. After addition of labeled glyphosate, the disappearance of glyphosate and the formation and disappearance of AMPA were monitored. The resulting curves were fitted according to a new EU guideline. The best fit of the glyphosate degradation data was obtained using a first-order multi compartment (FOMC) model. DT₅₀ values of 9 days (glyphosate) and 32 days (AMPA) indicated relatively rapid degradation. After an aging period of 6 months, the leaching risk of each residue was determined by treating the soil with pure water or a phosphate solution (pH 6), to simulate rain over a non-fertilized or fertilized field, respectively. Significantly larger (p < 0.05) amounts of aged glyphosate and AMPA were extracted from the soil when phosphate solution was used as an extraction agent, compared with pure water. This indicates that the risk of leaching of aged glyphosate and AMPA residues from soil is greater in fertilized soil. The blank soil, to which 252 g glyphosate/ha was applied 21 months before this study, contained 0.81 ng glyphosate/g dry soil and 10.46 ng AMPA/g dry soil at the start of the study. Blank soil samples were used as controls without glyphosate addition. After incubation of the blank soil samples for 6 months, a significantly larger amount of AMPA was extracted from the soil treated with phosphate solution than from that treated with pure water. To determine the degree of uptake of aged glyphosate residues by crops growing in the soil, ¹⁴C-labeled glyphosate was applied to soil 6.5 months prior to sowing rape and barley seeds. After 41 days, 0.006 ± 0.002% and 0.005 ± 0.001% of the applied radioactivity was measured in rape and barley, respectively.

Keywords:

• Glyphosate
• AMPA
• aged residues
• degradation
• leaching
• plant uptake
• phosphate

Acknowledgments

The authors are grateful to senior scientist Solvejg K. Mathiassen for providing the soil and for her valuable advice regarding the plant uptake study. We also thank Kirsten Heinrichson, Anne G. Mortensen, Bente Laursen and Kirsten Jensen for excellent technical support.

Notes

^a % of oven-dry soil.

^b P, K, Mg, Ca (mg/100 g dry soil) were determined by extraction with ammonium lactate (Al) or hydrochloric acid (HCL).

^a Based on applied glyphosate (applied in this study and at the field in 2003 for the samples and blank, respectively).

^b Total quantities found in samples were calculated based on measured amounts of labeled compounds, hence amounts originating from glyphosate present in the soil prior to this study, were not included.