Saturday, 15 July 2006

Relationship of Glyphosate Application and Foliar Amendment on IAA-Producing Bacteria and Urease Activity in the Rhizosphere of Glyphosate-Resistant Soybean.

Su-Jung Kim, Dept of Soil, Environmental, and Atmospheric Sciences, Univ of Missouri-Columbia, 302 ABNR Bldg. School of Natural Resources, Columbia, MO 65211 and Robert J. Kremer, USDA-ARS, Cropping Systems and Water Quality Research Unit, 302 ABNR Building, Columbia, MO 65211-7250.

Increased use of glyphosate-resistant crops has raised concerns regarding the potential environmental impact of glyphosate. Furthermore, foliar-applied chemicals as well as herbicides may impact soil microbial ecology. Biostimulants (products containing plant hormones) and liquid fertilizers affect soil microorganisms by providing additional nutrients or growth factors that alter metabolic activity and improve crop growth and productivity. Grozyme® and PT-21® are foliar amendments used to increase crop yield. Metabolites that have been implicated in growth-suppressive activity include hydrogen cyanide, phytohormones such as Indole-3-Acetic Acid (IAA), and unidentified phytotoxins. The objectives of this study were to describe changes in bacterial populations which synthesize IAA and urease activity in the rhizosphere of glyphosate-resistant soybean (Glycine max, ‘Roundup Ready'). This study was conducted at Bradford Agronomy Center of the University of Missouri-Columbia. RoundUp (RU) herbicide was applied to main plots at the recommended growth stage (pre-bloom) following the suggested label rate. Split-plots 10 days after the RU application were treated with foliar amendments of biostimulant or urea (21%) solution. Soybean roots and associated soil were collected from the outer rows of each plot immediately prior to RU application and 10, 20, and 30 days after RU application. The results showed that the populations of IAA-producing bacteria in overall RU treatments decreased. Urease activity 30 DAT significantly decreased in RU compared with urea solution. These results indicate that the herbicide glyphosate in association with certain foliar amendments may influence the rhizosphere bacterial populations which synthesize IAA and also may alter microbial enzyme activity.

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