Relationship of Glyphosate and Foliar Amendments to Microbial Activity in the Rhizosphere of Glyphosate-Resistant Soybean in the Clay Pan.

The rhizosphere is a crucial soil environment where plant-microbe interaction occurs. With the introduction and rapid adoption of glyphosate-resistant crops, glyphosate became the most widely used herbicide in the world. More frequent applications may accumulate high concentrations of glyphosate during a given season, thereby resulting to have impacts on soil microbial activity. Foliar amendments, such as biostimulant (Bio) and urea solution (US) may be useful in offsetting these effects. The objective of this research was to evaluate the relationship of glyphosate and foliar amendments on soil microbial activity in the rhizosphere of field- grown soybeans. Measurement of microbial activity in the rhizosphere of GR soybean was based on IAA-producing rhizobacteria (IPR), the urease assay, and C and N mineralization determinations. Total bacteria and IPR were isolated from rhizosphere of GR soybean, enumerated, and screened for IAA production using an in situ nitrocellulose membrane assay. Results revealed that effects of the interaction of herbicide and foliar amendments on IPR numbers and urease activity were significant (P < 0.016 and 0.001, respectively) for the field study. The application of Gly with Bio on GR soybean significantly increased total bacteria numbers and IPR numbers whereas the application of Gly with US on GR soybean significantly total bacteria numbers only. Results suggest that the composition of rhizobacteria including IAA-producers was modified by GR soybean to a limited extent. However, foliar amendment treatments with glyphosate may influence the bacterial communities and activities in the GR soybean rhizosphere.