Components of N-fixation system response to glyphosate are un-coupled in agroecosystems

Abstract

Glyphosate, introduced 50 years ago, is one of the most widely used herbicides. There are lingering concerns over the potential environmental impacts of glyphosate, prompting many studies. Many of these studies have isolated specific types of glyphosate effects – such as on N-fixation rate, or microbial community dynamics – but have not examined how glyphosate effects behave in relationship to one another. In this study, we evaluated the effect of glyphosate on biological N-fixation, soil microbial community dynamics, and soil nifH gene quantity within four types of crop management systems. Our hypothesis was that if farm management mediates the effects of glyphosate, then the effects of glyphosate on different N-fixing components of crop management systems should have coordinated effects; when one component is impacted, the others should be as well. We measured % N derived from the atmosphere (Ndfa) in soybean as a proxy for fixation, the quantity of 16S rRNA marker genes, and the nifH gene in Near soil (soil attached to roots) and Far soil (soil unattached, but under root influence) samples. We determined the response of putative diazotrophs to glyphosate in the Near and Far soil using a combination of Quantitative-Polymerase Chain Reaction (QPCR) and 16S gene amplicon sequencing. 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 found that glyphosate led to changes in some components, but that these effects were likely governed by separate mechanisms and did not behave in a coordinated manner. Glyphosate caused a decrease in Ndfa under CT; the effect of glyphosate on Ndfa across systems was weakly negatively correlated to the amount of soil organic matter in the crop management systems. The 16S rRNA gene abundance was not affected by glyphosate. Glyphosate caused a 217 % increase in nifH gene abundance in the rhizosphere, but this effect was not affected by crop management system. Glyphosate treatment affected community composition only in the Near soil of O6. The assemblage of taxa that responded to glyphosate in each system differed and it is possible that dynamics in the population of 16S sequence variants is decoupled from the selective pressure of glyphosate treatment in soils.