Un-coupled effects of glyphosate on N-fixation components of agroecosystems

Authors: EPP SCHMIDT, DIETRICH - University Of Maryland; Cavigelli, Michel; Maul, Jude; Schomberg, Harry; YARWOOD, STEPHANIE - University Of Maryland

Submitted to: Applied Soil Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/28/2023
Publication Date: 3/21/2023
Citation: Epp Schmidt, D.S., Cavigelli, M.A., Maul, J.E., Schomberg, H.H., Yarwood, S.A. 2023. Un-coupled effects of glyphosate on N-fixation components of agroecosystems. Applied Soil Ecology. 278: Article 108167. https://doi.org/10.1016/j.apsoil.2023.104859.
DOI: https://doi.org/10.1016/j.apsoil.2023.104859

Interpretive Summary: Herbicides are widely used in agriculture to control weeds. While herbicides are tested for a wide range of target and non-target effects it is difficult to detect the indirect effect of an herbicide, especially if the impacts take time to develop. Evaluating the impacts of herbicides and chemicals on soil dwelling organisms is additionally challenging since most of these organisms are obscured by the soil matrix. In this study, ARS scientists, along with University of Maryland colleagues evaluated the effect of the herbicide glyphosate on the soil microbial community and its ability to capture biologically useful nitrogen, through the process of biological nitrogen fixation (BNF). The study was carried out in soybean plots within three year crop rotations managed using 1) conventional practices including tillage, 2) conventional practices with no tillage, and 3) organic practices; and also in an organic system using a 6-year crop rotation. Results show that glyphosate did not impact the total abundance of bacteria in the soil in any system, but did decrease the bacteria responsible for BNF under the conventional tilled system; this effect seemed to be mediated by soil organic matter levels. The microbial genes responsible for BNF were not affected by glyphosate in soil attached to crop roots but did affect these genes further away from the crop roots; these impacts were similar in all cropping systems. Glyphosate treatment affected soil microbial community composition only in soil attached to crop roots in the 6-year organic system. In general, effects were complex and reflect the complex nature of soil microbial communities and the environments that influence them. These results will be of interest to other scientists, the public, and policymakers concerned about possible impacts of herbicides on soils.

Technical Abstract: Glyphosate is one of the most widely used herbicides being applied to over 180 million acres of crops per year. There has been concern over the potential environmental impacts of glyphosate on specific types of microbial activities such as metal chelation, increased plant pathogenesis, and nitrogen fixation. In this study, we evaluated the effect of glyphosate on biological nitrogen fixation, soil microbial community dynamics, and soil nifH gene abundance within four types of crop management systems. The crop management systems included two under conventional management (NT is No-till and CT is Chisel till) and two under organic rotations (O3 is a 3-year rotation, and O6 is a 6-year rotation). We measured % N derived from the atmosphere (Ndfa) in soybean as a proxy for fixation, the abundance of 16S rRNA marker genes, and the nifH gene in rhizoplane (soil attached to roots) and rhizosphere (soil unattached, but under root influence) samples. We determined the response of free-living putative diazotrophs to glyphosate in the rhizoplane and rhizosphere using a combination of Q-PCR and 16S gene amplicon sequencing. We found that glyphosate led to changes in some biological compartments responsible for nitrogen transformations, but that these effects were likely governed by separate mechanisms, and did not behave in a consistant manner. Glyphosate caused a decrease in Ndfa under Chisel-till; the effect of glyphosate on Ndfa across systems was weakly negatively correlated to the amount of soil organic matter in the crop management system. The 16S rRNA gene abundance was not affected by glyphosate. Glyphosate caused a 217% increase in nifH gene abundance in the rhizosphere, and this effect was not affected by crop management system. Meanwhile, nifH gene abundance in the rhizoplane was not impacted by glyphosate. Glyphosate treatment affected community composition only in the rhizoplane of the 6-year organic system. The key finding in this report is that the assemblage of microbial taxa that responded to glyphosate in each system differed and it is likely that the subpopulations of the microbial community impacted by glyphosate do so in the context of a broader conditioning of the microbial community by soil, seasonal and cropping system effects.