Location, Root Proximity, and Glyphosate-Use History Modulate the Effects of Glyphosate on Fungal Community Networks of Wheat

Abstract

Glyphosate is the most-used herbicide worldwide and an essential tool for weed control in no-till cropping systems. However, concerns have been raised regarding the long-term effects of glyphosate on soil microbial communities. We examined the impact of repeated glyphosate application on bulk and rhizosphere soil fungal communities of wheat grown in four soils representative of the dryland wheat production region of Eastern Washington, USA. Further, using soils from paired fields, we contrasted the response of fungal communities that had a long history of glyphosate exposure and those that had no known exposure. Soil fungal communities were characterized after three cycles of wheat growth in the greenhouse followed by termination with glyphosate or manual clipping of plants. We found that cropping system, location, year, and root proximity were the primary drivers of fungal community compositions, and that glyphosate had only small impacts on fungal community composition or diversity. However, the taxa that responded to glyphosate applications differed between rhizosphere and bulk soil and between cropping systems. Further, a greater number of fungal OTUs responded to glyphosate in soils with a long history of glyphosate use. Finally, fungal co-occurrence patterns, but not co-occurrence network characteristics, differed substantially between glyphosate-treated and non-treated communities. Results suggest that most fungi influenced by glyphosate are saprophytes that likely feed on dying roots.

Influence of plant genotype and soil on the wheat rhizosphere microbiome: Evidences for a core microbiome across eight African and European soils

“…This fungus is described as a saprotrophsymbiotroph [26] capable of solubilizing phosphate in the rhizosphere [27]. Our results are consistent with other studies that identified Mortierella as a keystone taxa of the wheat rhizosphere unsensitive to cropping practices in Switzerland or the USA [9,28] with potential positive effects on wheat yield in Canada [29]. Altogether, these findings encourage future research to develop targeted cultivation approaches of the wheat-associated Mortierella fungi that seem to play a central role in the wheat microbiome across the world.…”

“…Chaetomium (Ascomycota, 60% of samples) described as pathotrophs or saprotrophs that also have been observed on wheat roots in different cropping systems and described as hub taxa [21,28,30].…”

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.…”