Glyphosate is a broad-spectrum herbicide used extensively worldwide to control broadleaf weeds in agriculture. Research suggests that repeated application causes a change in soil microbial properties which could be affecting soil quality and productivity. Although glyphosate is generally regarded as having relatively low environmental impact, after 10 or more years of widespread use, field observations by farmers and emerging research suggest that long-term glyphosate tolerant (GT) cropping is having cumulative and non-target effects on soils and crop productivity. There is very little information on the effects of GT residue when added to soils. Therefore, the objective was to determine shifts in the soil microbial community during GT residue decomposition in soils with and without a history of glyphosate exposure. Soybean residues from a simulated long-term GT cropping system were used in laboratory incubation. The experiment was a 2x3x4 factorial design with 2 soils (with or without glyphosate), three residue types (leaf, stem,or root), and 4 soybean residue treatments (GT residue exposed to glyphosate with potassium salt carrier, GT residue exposed to glyphosate with isopropylamine salt carrier, untreated GT genotype, and untreated non-GT genotype). These soils were profiled using phospholipid fatty acid analysis to determine shifts in soil microbial community structure due to the addition of GT residue to soil. The results showed that microbial shifts during decomposition of GT soybean residue varied between soils with or without long-term exposure to glyphosate. There was also a trend that GT material that had been exposed to glyphosate cause a differential shift in the communities over GT residue that had not been exposed to glyphosate. Commercially available glyphosate formulations have two major types of salt carriers; potassium salt and isopropylamine salt which could be a factor besides glyphosate in affecting the chemistry of GT residues and subsequently microbial response during decomposition. However, the results showed that carrier did not significantly affect PLFA profiling in soils regardless of the soil's history of glyphosate exposure. Ratios of saturated to monounsaturated PLFAs are used as indicators of microbial stress. Our results showed that soil history of glyphosate exposure significantly affected microbial stress. There were also significant differences in stress between glyphosate residue treatments in soil with a history of glyphosate exposure.

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