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Title: Subtle impacts of repeated glyphosate use on wheat-associated bacteria are small and depend on glyphosate use history

Author
item Schlatter, Daniel
item YIN, CHUNTAO - Washington State University
item HULBERT, SCOT - Washington State University
item BURKE, IAN - Washington State University
item Paulitz, Timothy

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/26/2017
Publication Date: 11/1/2017
Citation: Schlatter, D.C., Yin, C., Hulbert, S., Burke, I., Paulitz, T.C. 2017. Subtle impacts of repeated glyphosate use on wheat-associated bacteria are small and depend on glyphosate use history. Applied and Environmental Microbiology. https://doi.10.1128/AEM.01354-17.
DOI: https://doi.org/10.1128/AEM.01354-17

Interpretive Summary: Glyphosate is the most widely used herbicide for weed control in agriculture, but concerns of non-target impacts on soil and plant-associated microbiomes are persistent. We examined the response of soil and plant-associated bacterial communities to repeated glyphosate exposure and the potential for adaptation within microbial communities with a history of glyphosate exposure. We found that the effects of glyphosate on microbiomes were small and inconsistent across soils, plant proximities, and cropping histories. However, a greater number of bacteria responded to glyphosate in soils with a history of exposure, indicating that some taxa have adapted to glyphosate use. We suggest that the small shifts in microbiomes due to glyphosate are primarily a result of nutrients released by dying roots.

Technical Abstract: Glyphosate (Roundup) is the most widely used herbicide in the world and a critical tool for weed control in no-till wheat cropping systems. However, there are persistent concerns about non-target impacts of long-term glyphosate use on soil communities. We investigated the impacts of repeated glyphosate treatments on bacterial communities in the soil and rhizosphere of wheat across a geographic range of soils with and without long-term history of glyphosate exposure. We cycled wheat in the greenhouse using soils from 4 paired fields under no-till (20+ years history of glyphosate) or no history of glyphosate. At each cycle, we terminated plants with glyphosate (2x field rate) or by removing the crown. After 4 cycles we sampled bulk and rhizosphere soil, extracted DNA, and characterized bacterial communities with Illumina MiSeq of 16S rRNA genes. Location, cropping history, year, and proximity to the roots had much stronger effects on bacterial communities than did glyphosate, which only explained 2-5% of variation in communities. Significantly more taxa were affected by glyphosate in soil with a long history of use than in soils with no history of use. However, less than 1% of all taxa were impacted by glyphosate, and more of these significantly increased than decreased in relative abundance. Thus, glyphosate has minimal impacts on soil and rhizosphere bacteria of wheat, though dying wheat roots after glyphosate application may favor some copiotrophic taxa.