Submitted to: Pest Management Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/14/2006
Publication Date: 3/1/2007
Citation: Weaver, M.A., Krutz, L.J., Zablotowicz, R.M., Reddy, K.N. 2007. Effects of glyphosate on soil microbial communities and its mineralization in a mississippi soil. Pest Management Science 63:388-393. Interpretive Summary: The herbicide, glyphosate, is an important tool for weed management in soybeans. A series of field and laboratory experiments assessed the effect this herbicide has on soil microorganisms. In two years of field evaluation no significant effects were detected on soil or root microbial communities after the second in-season glyphosate application. Exposure of soils to glyphosate in a laboratory experiment resulted in small, short-term changes in the microbial community and a brief inhibition of microbial activity. Glyphosate did not persist in the soil used in this study, likely due to microbial degradation.
Technical Abstract: Transgenic glyphosate-resistant (GR) soybean [Glycine max (L.) Merr.] has enabled highly effective and economical glyphosate-based weed control. The concomitant increased application of glyphosate to such fields could lead to shifts in the soil microbial community. The objective of these experiments was to evaluate the effect of application of glyphosate on the soil microbial community structure, function and activity. The field assessments of glyphosate effects on soil microbial communities were conducted in 2003 and 2004 on a Dundee silt loam, at Stoneville, MS. Surface soil was collected at time of planting, before initial glyphosate application and 14 d after two post-emergence glyphosate applications. In 2004, rhizosphere soil was isolated from soybean roots. Microbial community structure was assessed by analysis of fatty acid methyl esters (FAMEs) from soil samples. In 2003, analysis of the total FAME profile by Principal Component Analysis (PCA) revealed no differentiation between the treatments, although relative abundance of several individual fatty acids differed significantly. In 2004, there was also no significant herbicide effect in the bulk soil or rhizosphere soil. Collectively, these findings indicate that application of relatively high rates of glyphosate caused no meaningful whole microbial community shifts in this time period, even when applied at greater than label rates. An in vitro study, including up to three-fold label rates of glyphosate resulted in up to 19% reduction in soil fluorescein diacetate hydrolytic activity and small, and brief changes in the soil microbial community. Thirty-two to thirty-seven percent of the applied glyphosate was mineralized when applied at field rates with about nine percent forming bound residues. These results support the conclusion that field rates of glyphosate have only small and transient effects on the soil microbial community.