The Effects of Glyphosate On Soil Microbial Community Structure and Potassium Dynamics.

Glyphosate, a non-selective, broad-spectrum herbicide, is widely used worldwide.  Since the introduction of genetically modified glyphosate tolerant (GT) crops, the use of glyphosate in American agriculture has increased tenfold.   Although early research on short-term effects on soils was inconclusive, long-term applications of glyphosate for GT crops maybe shifting microbial communities and causing plant nutrition deficiencies, including anecdotal reports of potassium deficiency.  Our hypothesis is that glyphosate causes an interaction of soil biology and mineralogy that may be inducing K deficiency in plants.  Glyphosate can stimulate a specific fungal genus or cause a general shift in the microbial community towards fungal dominance which could be immobilizing plant available K.  The objective of this study was to determine the effect of glyphosate on three soil microbial properties:  (1) respiration, (2) community structure, using ester linked fatty acid methyl ester (EL-FAME) profiling, and (3) microbial biomass potassium (K_mic), measured by fumigation-extraction.  A twenty-eight day in vitro incubation study comparing soils with and without a history of glyphosate application was conducted.  Both soils received a single dose of glyphosate (47 µg g^-1 a.i.).  The soil with a history of glyphosate application revealed no differences between the glyphosate treatment and the untreated control.  Yet, in the soil which had never had a previous glyphosate application, EL-FAME analysis showed a response in levels of two fatty acids associated with gram positive bacteria after glyphosate treatment.  This suggested that adding glyphosate does cause a shift in a sub population of the soil.   Another study was conducted with repeated glyphosate applications over 6 months to simulate long term glyphosate use.  Results will be discussed.