Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 8/15/2006
Publication Date: 10/6/2006
Citation: Kremer, R.J., Means, N.E. 2006. Rhizosphere ecology and glyphosate-resistant soybean. Glyphosate and Potassium Nutrition Symposium, N. Lorenz and R.P. Dick (ed.), p.12-29.
Technical Abstract: The interaction of glyphosate-resistant (GR) soybean with the ecology of plant-associated microbial populations has been the subject of environmental assessments in response to the public concern regarding the ecological compatibility of transgenic crops. Changes in the dynamics of soil and rhizosphere microbial community structure may lead to functional consequences with either positive or negative effects on plant growth and long-term effects on the soil ecosystem. However, the impacts of GR cropping systems on the productivity and economic yields of crops in the year of production or in subsequent rotation may be of equal importance as ecological impacts, yet have not been widely researched. Our objectives of this research overview are to illustrate the relationship of GR soybean to changes in root- and soil-associated microorganisms based on research conducted during 1997-2006. We suggest that this information provides a framework for understanding how microbial relationships might be altered under GR cropping systems and contribute to productivity problems reported by farmers and agricultural industry representatives. The ultimate goal is to aid in developing management and remediation strategies for addressing altered microbial activity that may be associated with widespread use of GR technology. Glyphosate applied to GR soybean consistently affected root-associated Fusarium. Populations often increased 2-5 times within five days after glyphosate application and continued to increase during the growing season. Fusarium populations on roots and in rhizosphere soil generally did not increase in GR soybean receiving either no herbicide or "conventional" tank-mix herbicides. Fusarium populations did not increase on a non-transgenic soybean (‘Williams 82’) with or without a tank-mix herbicide in 2005. Fusarium colonization of the soybean root surface was directly correlated with Fusarium populations in the rhizosphere soil. Cultures isolated from roots were primarily members of the Fusarium oxysporum complex and F. solani complex. Consequences of increased Fusarium may include alteration in soil microbially-mediated processes such as decomposition and increases in potential pathogenic effects of certain Fusarium spp. on soybean and other rotation crops. We subsequently found that glyphosate released in root exudates of glyphosate-treated GR soybean coupled with increased exudation of carbohydrates and amino acids from GR roots stimulated growth and colonization of Fusarium spp. in the GR soybean rhizosphere. Nodulation by Bradyrhizobium japonicum was higher on Williams 82 soybean compared with GR soybean across all herbicide treatments. However, a biostimulant (‘Grozyme Z-93’) applied to GR soybean after glyphosate application increased nodulation. Populations of rhizobacteria able to produce the plant-growth-regulator indole-3-acetic acid (IAA) increased on GR soybean with either glyphosate, biostimulant, or their combined applications. Mn-oxidizing bacteria in the rhizosphere also increased on GR soybean compared with Williams 82. We also examined several ‘general microbial ecology’ parameters, which did not yield measurable responses to treatment. For example, soybean cyst nematode populations and reproduction on GR soybean roots were not affected consistently by glyphosate. Substrate-induced respiration and selected enzyme activities in rhizosphere soils were only slightly affected by glyphosate treatment. Results to date show that Fusarium colonization of roots and rhizosphere soil of GR soybean increased with glyphosate application; glyphosate exuded into the rhizosphere in combination with high concentrations of carbohydrates and amino acids may favor high root colonization by Fusarium; plant-growth-regulator (i.e., IAA) concentration balance in the rhizosphere may be affected by glyphosate and may affect colonizatio