Leaching and degradation of ¹³C₂-¹⁵N-glyphosate in field lysimeters

Glyphosate (GLYP), the globally most important herbicide, may have effects in various compartments of the environment such as soil and water. Although laboratory studies showed fast microbial degradation and a low leaching potential, it is often detected in various environmental compartments, but pathways are unknown. Therefore, the objective was to study GLYP leaching and transformations in a lysimeter field experiment over a study period of one hydrological year using non-radioactive ¹³C₂-¹⁵N-GLYP labelling and maize cultivation. ¹⁵N and ¹³C were selectively measured using isotopic ratio mass spectrometry (IR-MS) in leachates, soil, and plant material. Additionally, HPLC coupled to tandem mass spectrometry (HPLC-MS/MS) was used for quantitation of GLYP and its main degradation product aminomethylphosphonic acid (AMPA) in different environmental compartments (leachates and soil). Results show low recoveries for GLYP (< 3%) and AMPA (< level of detection) in soil after the study period, whereas recoveries of ¹⁵N (11-19%) and ¹³C (23-54%) were higher. Time independent enrichment of ¹⁵N and ¹³C and the absence of GLYP and AMPA in leachates indicated further degradation. ¹⁵N was enriched in all compartments of maize plants (roots, shoots, and cobs). ¹³C was only enriched in roots. Results confirmed rapid degradation to further degradation products, e.g., ¹⁵NH₄⁺, which plausibly was taken up as nutrient by plants. Due to the discrepancy of low GLYP and AMPA concentrations in soil, but higher values for ¹⁵N and ¹³C after the study period, it cannot be excluded that non-extractable residues of GLYP remained and accumulated in soil.