Glyphosate and aminomethylphosphonic acid (AMPA) behavior in loess soils and off‐site transport risk assessment

Glyphosate is the most used herbicide in agricultural lands worldwide, with more than 825 000 tons sold globally in 2014. Such great use is mostly a result of the introduction of glyphosate-resistant (GR) crops in 1996 by Monsanto company. Loess soils, on the other hand, are amongst the most productive and fertile soils and, consequently, are intensively used for agriculture and to grow GR crops. Consequently, they are heavily subject to the application of glyphosate-based herbicides every year. Despite being the most used pesticide worldwide, the environmental fate of glyphosate and its main metabolite aminomethylphosphonic acid (AMPA) is still not well understood. Therefore, this PhD thesis aims at better understanding the environmental fate of glyphosate and AMPA in the loess soil environment. Special focus is given to: 1) the decay kinetics of glyphosate and the formation and decay kinetics of AMPA; and 2) the off-site transport of glyphosate and AMPA associated to the particle-bound phase, as a result of wind and water erosion. These processes were studied under laboratory and field conditions. The field study was performed in agribusiness fields of the loess Pampas of Argentina. The results of this PhD thesis have shown that: 1) the decay of glyphosate and AMPA in loess soils is mostly a microbiological process and is fastest under warm and moist soil conditions and slowest under cold and dry soil conditions; 2) AMPA persists longer in loess soil than glyphosate, and tends to accumulate; 3) the type of decay kinetics followed by glyphosate in loess soils is mostly temperature dependent, but abrupt soil moisture changes from dry to moist also play a role; 4) glyphosate degradation into AMPA was extremely variable (5-100%) amongst different temperature conditions and between laboratory and field conditions; 5) glyphosate and AMPA contents are highest in eroded soil particles <10 µm (PM10) and, consequently, their long-range off-site transport risk with wind erosion (dust) is very high: 6) during water erosion events, the particle-bound transport of glyphosate and AMPA is as or even more important than the water-dissolved transport; and 7) the risk of deposition of glyphosate and AMPA into off-target downslope fields during water erosion events can be considerable. It is concluded that repeated glyphosate applications, particularly under dry soil conditions, increase the risk of accumulation of glyphosate and AMPA in loess soils and, consequently, of on-site soil pollution and off-site transport with wind and water erosion.