Investigation of Residual Glyphosate Accumulation in Glyphosate Tolerant Cropping Systems.

Application of glyphosate is a very common form of weed control, and yet there still is not a clear understanding of the long term ecological effects. Although it is thought to have low environmental impacts, after 10 or more years of glyphosate tolerant cropping (GTC) field observations by farmers and emerging reserach suggest that long-term glyphosate usage is having cumulative and non-target effects on soil microbial properties. The breakdown product of glyphosate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) is more toxic biologically than glyphosate but there is very little information on whether it is accumulating with long-term GTC. Therefore, the objective was to investigate the changes in microbial properties, and the accumulation of glyphosate and AMPA in soil that had never received glyphosate in a simulated 8 yr GTC system in corn (Zea mays) and soybean (Glycine max) rotation. The greenhouse experiment was a 2 X 2 factorial with a completely randomized design that had 4 replications and the following glyphosate treatments: Monsanto Powermax®, Agrisolutions Cornerstone®, Cornerstone® with Agro-Plus Grozyme® and AgSpectrum Glycure®, and no treatment . All glypohosate treatments were applied to the crop foilage at their recomended rates twice per growth period. Crops were grown in soil with and without a history of glyphosate application and both speices were present for every growth period. Rhizosphere and intra-root soil from growth periods 5 and 8 was collected from rhizo-boxes that allowed non-destructive soil sampling 1 and 7 days after each glyphosate application which were analyzed for extractable glyphosate and AMPA, and profiled for microbial functional groups using the PLFA biomarkers. Results show an accumulation of the glyphosate breakdown product AMPA, and changes in the microbial functional groups between growth periods.