Glyphosate and phosphorus leaching and residues in boreal sandy soil

Glyphosate [(N-(phosphonomethyl)glycine)] is a widely used herbicide and it is known to compete for the same sorption sites in soil as phosphorus. Persistence and losses of glyphosate were monitored in a field with low phosphorus status and possible correlation between glyphosate and phosphorus leaching losses was studied. Glyphosate and its metabolite AMPA (aminomethyl phosphonic acid) residues in soil samples were analysed after a single application in autumn. Twenty months after the application the residues of glyphosate and AMPA in the topsoil (0-25 cm) corresponded to 19% and 48%, respectively, of the applied amount of glyphosate, and traces of glyphosate and AMPA residues were detected in deeper soil layers (below 35 cm). These results indicate rather long persistence for glyphosate in boreal soils. Surface runoff and subsurface drainflow were collected continuously all year round for 20 months and analysed for glyphosate, AMPA, dissolved phosphate, total phosphorus and total suspended solids. The glyphosate concentrations in the surface runoff water were highest, with 99% of the total leaching losses obtained, during the periods of snow melting and soil thawing in the first winter following the autumn application. The total leaching of glyphosate was 5.12 g ha(-1) and that of AMPA 0.48 g ha(-1), corresponding to about 0.51% and 0.07%, respectively, of the applied amount of glyphosate. No residues of glyphosate and AMPA were detected in the subsurface drainflow. The correlations between concentrations of glyphosate and dissolved orthophosphate as well as glyphosate and total phosphorus in surface runoff were significant (p < 0.01).