Tracking Glyphosate and AMPA Degradation Using Phosphate Oxygen Isotope Ratios.

Glyphosate [N–(phosphonomethyl) glycine], an active ingredient of most common herbicide Roundup, and its main metabolite, aminomethylphosphonic acid (AMPA) have been reported to be present in soils and other environments several months after application. Their persistence in the environment and adverse effects on soil biota and human health have raised public concern on their fate in the environment. In this study, we analyzed degradation of glyphosate and AMPA by manganese oxide in ¹⁸O labeled air and waters, and under variety of ambient conditions to unravel the degradation kinetics and external oxygen incorporation into the generated orthophosphate. The rate of glyphosate degradation was higher than AMPA in all experimental conditions studied, and both the rate and extent of degradation was lowest under anoxic conditions for both compounds. Oxygen isotope ratios (δ¹⁸O_P) of orthophosphate generated from glyphosate and AMPA degradation suggested that one external oxygen atom was incorporated into the released orthophosphate solely from ambient water, with other three inherited from parent compound. We further identified that no oxygen from ambient air and mineral was incorporated into the generated orthophosphate. Interestingly, δ¹⁸O_P values of all commercial glyphosate studied were found to be the lightest among all orthophosphate known so far. This finding indicates that δ¹⁸O_P values could be used to distinguish glyphosate sources and from other organophosphorus compounds as well as to track glyphosate degradation and to identify degradation products. Overall, our results highlight the rapid degradation of glyphosate and AMPA by manganese oxides and potential applicability of δ¹⁸O_P values as a tracer to distinguish different glyphosate sources as well as degradation products in the environment.