Effects of glyphosate and foliar amendments on soil microorganisms in soybean

In 2002, 75% of soybeans (Glycine max) produced in the United States were genetically modified for glyphosate resistance or Roundup Ready (RR) varieties. Glyphosate is a systemic herbicide, which translocates and accumulates in plant roots and can be released into the root zone (rhizosphere). Glyphosate may directly or indirectly affect soil and rhizosphere microorganisms. Biostimulants and supplemental nutrient solutions may alleviate the potential adverse effects of glyphosate on soil microorganisms. The research objective was to determine effects of glyphosate on microorganisms in the rhizosphere of RR soybean receiving glyphosate alone or in combination with foliar amendments. Field and greenhouse experiments were conducted in 2002 and 2003 using the RR soybean variety DeKalb DKB38-52 grown in a disked Mexico silt loam. Roundup Ultra Max herbicide was applied in a strip block design to soybean (growth stages V4--V5) at 0.84 kg ae ha-1. At 10-days post-herbicide application, the biostimulants, PT-21 (9.2 kg ha-1) (21.0% N) and Grozyme (33.5 mL ha-1), were applied to soybean. Soil and plant samples taken 0, 5, 10, 15, 20 and 25 days after herbicide application were assayed for enzyme activity, respiration and total culturable fungi. Surface sterilized roots were selectively cultured for the fungi Fusarium spp. on Komada agar. Glyphosate increased Fusarium root colonization, especially when soil water content was not limiting. Soil respiration and enzyme activity adequately indicated effects of RR soybean on plant-microbe interactions in the rhizosphere. Glyphosate applied to RR crops on soil microorganisms could disrupt nutrient cycling, crop productivity and soil quality.