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ARS Home » Midwest Area » Columbia, Missouri » Cropping Systems and Water Quality Research » Research » Publications at this Location » Publication #207642

Title: EFFECTS OF GLYPHOSATE AND FOLIAR AMENDMENTS ON ACTIVITY OF MICROORGANISMS IN THE SOYBEAN RHIZOSPHERE

Author
item MEANS, NATHAN - UNIVERSITY OF MISSOURI
item Kremer, Robert
item RAMSIER, CLIFFORD - AG SPECTRUM CO.

Submitted to: Journal of Environmental Science and Health
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/2007
Publication Date: 3/27/2007
Citation: Means, N.E., Kremer, R.J., Ramsier, C. 2007. Effects of glyphosate and foliar amendments on activity of microorganisms in the soybean rhizosphere. Journal of Environmental Science and Health Part B. 42(2):125.132.

Interpretive Summary: Soybean genetically modified (GM) for resistance to the herbicide glyphosate (Roundup®) is planted on more than 60% of the GM crop production area worldwide. Although GM soybeans and glyphosate pose little or no human health concern, their impacts on soil biology and productivity, and on the growth and health of the soybean plant itself have received little attention. The important biological processes in the root zone (rhizosphere), including nutrient transformations carried out by root-inhabiting microorganisms, have been largely neglected in environmental assessments of GM soybeans. Our objective was to determine the effects of glyphosate on microbial activity in the rhizosphere of GM soybean. We found that soil respiration (carbon dioxide released from soil) and activities of enzymes involved in carbon and nitrogen cycling increased somewhat when glyphosate was applied to GM soybean under favorable growing conditions in 2002; however, this effect was not seen during the dry year of 2003. We also applied a biostimulant that contained plant growth factors and a urea solution that delivered soluble nitrogen, in an effort to maintain or improve microbial activity in the rhizosphere. Only a few increases in activity due to application of these amendments were seen. Our study suggests that the very general measurements of respiration and enzyme activity we used in this study are not sensitive enough to detect the effects of either glyphosate or amendments on the microbial environment of the crop rhizosphere. Other selective measurements of microorganisms or functions will be examined in future studies to determine if more specific effects of glyphosate on rhizosphere and soil ecology exist. This information has important implications for scientists, extension personnel, producers, and the agricultural industry because it helps to focus investigative research aimed at explaining occasional production problems observed with GR soybean; in developing improved crop management systems for avoiding potential crop growth reductions that may be related to changes in soil ecology; and in maintaining the proper biological balance in the soil ecosystem.

Technical Abstract: A field study was conducted to determine the effects of glyphosate on microbial activity in the rhizosphere of glyphosate-resistant (GR) soybean and to evaluate interactions with foliar amendments. Glyphosate at 0.84 kg ae ha-1 was applied to GR soybean at the V4 - V5 development stages. Check treatments included a conventional herbicide tank mix (2003 study only) and no herbicides (hand-weeded). Ten days after herbicide application, a commercially available biostimulant and a urea solution (21.0% N) were applied to soybean foliage at 33.5 mL ha-1 and 9.2 kg ha-1, respectively. Soil and plant samples were taken 0, 5, 10, 15, 20 and 25 days after herbicide application then assayed for enzyme and respiration activities. Soil respiration and enzyme activity increased with glyphosate and foliar amendment applications during the 2002 growing season; however, similar increases were not observed in 2003. Contrasting cumulative rainfall between 2002 and 2003 likely accounted for differences in soil microbial activities. Increases in soil microbial activity in 2002 indicate that adequate soil water and glyphosate application acted together to increase microbial activity. Our study suggests that general soil microbial properties including those involving C and N transformations are not sensitive enough to detect effects of glyphosate on rhizosphere microbial activity. Measurements of soil-plant-microbe relationships including specific microbial groups (i.e., root-associated Fusarium spp.) are likely better indicators of impacts of glyphosate on soil microbial ecology.