Assessing the Effect of Foliar 13C Glyphosate Application on Rhizosphere Microbial Communities Using Stable Isotope Probing of Phospholipid Fatty Acid Biomarkers.

Research on glyphosate suggests that the use of glyphosate causes a change in soil microbial community composition which might in turn compromise plant growth and health. There is still no clear consensus on exactly how glyphosate affects the soil microbial community.

Our study was conducted to assess the effects of Glyphosate on rhizosphere microorganisms associated with Zea mays (corn) and Glycine max (soybean) grown in rhizoboxes in a greenhouse. Plant leaves were treated with two commonly used Glyphosate brands; Powermax and Cornerstone. Two soils, a conventional soil with, and a non-conventional soil without history of Glyphosate application were used. Plants were treated with ¹³C Glyphosate over eight growth periods and rhizosphere samples were collected at growth phases four and eight. The rhizosphere microbial community composition was characterized by phospholipid fatty acid (PLFA) profiling. Further, PLFA samples were analyzed for ¹³C incorporation using isotope-ratio mass spectrometry.

PLFA results have shown that Cornerstone increased the concentration of fungal PLFAs in the rhizosphere of corn treated with Cornerstone at the end of the experiment (growth phase 8). Only minor changes occurred for the remaining biomarkers.

Concomitantly, we have seen differences in ¹³C Cornerstone and Powermax incorporation for corn comparing conventional and non-conventional soils, especially at growth phase 8. Here, it appeared that Cornerstone showed higher incorporation rates into fungal PLFAs for the conventional soil. For the non-conventional soil, Powermax was incorporated to a relatively higher extent by gram negative bacteria compared to Cornerstone. Only minor micobial ¹³C Glyphosate incorporation was observed for  rhizosphere microorganisms associated with soybean. Our data suggests that foliar Glyphosate application on corn is affecting rhizosphere microorganisms.