Analyzing the impact of soil properties on glyphosate degradation

In year 1974, Monsanto introduced the glyphosate [N-(phosphonomethyl) glycine] product RoundUp and in the 1990s also glyphosate resistant crops. Since then, and increasingly after the expiry of the patent, glyphosate has become the most commonly used herbicide worldwide. The estimation of the worldwide use of glyphosate as an active ingredient amounts to more than 800,000 tons/year (estimation from 2014). A herbicide of such wide spread use, commercial in agriculture and for personal use in gardens, has been found in many different environmental compartments (e.g. surface waters and food) and with negative impact on non-target organisms (e.g. glyphosate resistance in weeds, or bactericidal effects). Glyphosate persistence and degradation of the compound differ between strongly between soils. In this study, we focus on elucidating the factors contributing to the persistence and degradation of glyphosate in two contrasting soils. Different chemical additives (N, P, DOM), as well as pH change and microbial transfer alongside glyphosate application were investigated in a ¹⁴C-glyphosate multi-labeling approach, upon their effect on the glyphosate degradation. The study shows that pH initially has a strong positive impact the mineralization in both soils and the DOM addition only increased the mineralization slightly. On the other hand, phosphate addition shows contrasting results in both soils, and nitrate addition lowered the mineralization significantly. Microbial transfer did not have any significant effect on the mineralization. Furthermore, we identify the impact of adsorption of glyphosate in soil as one of the major factors reducing glyphosate degradation.