Microbial Community Stable Isotopic Probing and Fate of 13C-Glyphosate in Crop Rhizospheres.

Glyphosate tolerant cropping (GTC) is extensively used in the US and worldwide.  Effects of glyphosate on microbial communities and crops over short periods has yielded mixed results but relatively little is known about the longterm impact; which may be linked to increased crop diseases and possibly to yield reductions now noted with GTC. Therefore, the objective was to deterimine which microbial groups are involved in taking up glyphosate C in the crop rhizosphere in soils that had been under zero or longterm (>10 years) GTC. This was done in a greenhouse study that simulated 8 years of a GT corn-soybean rotation where ¹³C labeled glyphosate was applied to the foliage (2 times/crop).  Rhizoboxes allowed for sampling of rhizosphere soil (1 and 7 days after glyphosate application) which was analyzed for ¹³C-phospholipid fatty acids (PLFA) and subjected to microbial community biodiversity and structural analysis. The information obtained from our research will increase our understanding of the effect of glyphosate applications on soil microbial communities in glyphosate resistant cropping systems.