Impacts of Repeated Glyphosate Use on Wheat-Associated Bacteria Are Small and Depend on Glyphosate Use History

Abstract

Glyphosate is the most widely used herbicide worldwide and a critical tool for weed control in no-till cropping systems. However, there are concerns about the nontarget impacts of long-term glyphosate use on soil microbial communities. We investigated the impacts of repeated glyphosate treatments on bacterial communities in the soil and rhizosphere of wheat in soils with and without long-term history of glyphosate use. We cycled wheat in the greenhouse using soils from 4 paired fields under no-till (20+-year history of glyphosate) or no history of use. At each cycle, we terminated plants with glyphosate (2× the field rate) or by removing the crowns, and soil and rhizosphere bacterial communities were characterized. Location, cropping history, year, and proximity to the roots had much stronger effects on bacterial communities than did glyphosate, which only explained 2 to 5% of the variation. Less than 1% of all taxa were impacted by glyphosate, more in soils with a long history of use, and more increased than decreased in relative abundance. Glyphosate had minimal impacts on soil and rhizosphere bacteria of wheat, although dying roots after glyphosate application may provide a "greenbridge" favoring some copiotrophic taxa. Glyphosate (Roundup) is the most widely used herbicide in the world and the foundation of Roundup Ready soybeans, corn, and the no-till cropping system. However, there have been recent concerns about nontarget impacts of glyphosate on soil microbes. Using next-generation sequencing methods and glyphosate treatments of wheat plants, we described the bacterial communities in the soil and rhizosphere of wheat grown in Pacific Northwest soils across multiple years, different locations, and soils with different histories of glyphosate use. The effects of glyphosate were subtle and much less than those of drivers such as location and cropping systems. Only a small percentage of the bacterial groups were influenced by glyphosate, and most of those were stimulated, probably because of the dying roots. This study provides important information for the future of this important tool for no-till systems and the environmental benefits of reducing soil erosion and fossil fuel inputs.

Soil microbial communities in diverse agroecosystems exposed to glyphosate

“…And while both barcoding and culture surveys detected other pathogens, none responded to glyphosate (Supplementary Data). Our results are consistent with previous metabarcoding studies [20, 21].…”

“…Greenhouse studies with GR wheat conducted in the Pacific Northwest found only minor effects of glyphosate on microbial communities, and determined site was a major driver of soil microbial community structure [20, 21]. While these studies did detect effects of glyphosate on the prevalence of a few microbial taxa, they applied glyphosate at twice the recommended rate, increasing the likelihood that the microbial community experienced a significant effect.…”

“…Both studies applied culture-based methodology to quantify these microbial groups, with molecular analysis of the ribosomal internal transcribed spacer region (ITS) for fungi; however, the identification techniques employed were not sufficiently discriminatory to distinguish pathogenic and beneficial genotypes for either group. Studies using culture-free methodology to characterize microbial communities have failed to detect substantial glyphosate effects on pathogen abundance [20, 21]. The key to conclusive determination of glyphosate effect on microbial communities of GR crops is to carefully compare glyphosate sprayed and non-sprayed treatments within an agronomic context.…”

Identification of the first glyphosate transporter by genomic adaptation

“…A recent study revealed that the exposure of honeybees to glyphosate alters the bee gut microbiota and increases the susceptibility to infection by opportunistic pathogens like Serratia marcescens (Motta et al, 2018). In contrast, a role of glyphosate in affecting the soil microbial activity is controversially discussed because studies are available suggesting a positive and a negative effect of the herbicide on the microbial activity of the soil (Haney et al, 2002;Barriuso et al, 2011;Schlatter et al, 2017;Dennis et al, 2018). Thus, the effect of glyphosate on the bee microbiota seems to be unambiguous.…”

“…Thus, the effect of glyphosate on the bee microbiota seems to be unambiguous. In contrast, a role of glyphosate in affecting the soil microbial activity is controversially discussed because studies are available suggesting a positive and a negative effect of the herbicide on the microbial activity of the soil (Haney et al, 2002;Barriuso et al, 2011;Schlatter et al, 2017;Dennis et al, 2018). Moreover, the intensive use of the herbicide clearly stimulates the evolution of glyphosate-resistant plants and the use of genetically modified glyphosate-resistant crops decreases the diversity and abundance of wild plants (Heap, 2014;Schütte et al, 2017;Bagavathiannan and Davis, 2018).…”