Submitted to: Pest Management Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/4/2016
Publication Date: 1/1/2017
Publication URL: http://handle.nal.usda.gov/10113/5661741
Citation: Jenkins, M., Locke, M.A., Reddy, K.N., McChesney, D.S., Steinriede Jr, R.W. 2017. Impact of glyphosate resistant corn, glyphosate applications, and tillage on soil nutrient ratios, exoenzyme activities, and nutrient acquisition ratios. Pest Management Science. 73:78-86. https://doi.org/10.1002/ps.4413.
Interpretive Summary: Planting corn that has been genetically engineered to resist the herbicide glyphosate and glyphosate applications to fields to control weeds has become a wide-spread management practice for implementing reduced tillage across the USA. ARS scientists in Oxford and Stoneville, MS designed and implemented a seven year-long field experiment in the Mississippi Delta to determine if this management practice would interfere in the functioning of the enzymes produced by the soil microbial community that initiate breakdown of residual plant material, a process important to maintaining soil fertility. Results of this study indicated that glyphosate applications had the potential to interfere in the balanced relation between soil carbon and essential plant nutrients nitrogen and phosphorus. Continuous long-term applications of glyphosate to control weeds appeared to affect the relation between the metabolic activities of the soils microbial community and the enzymes they produced to initiate breakdown of soil organic matter. These results appear to indicate that either glyphosate or glyphosate resistant corn isoline may have a short-lived adverse effect on the initial steps in soil organic matter/plant residue break down and long-term soil fertility.
Technical Abstract: We report results of the last two years of a 7-year (2008-2014) field experiment designed to test the null hypothesis that applications of glyphosate on glyphosate resistant corn (Zea mays L.) as a routine weed control practice under both conventional and reduced tillage practices would have no effect on soil exoenzymes involved in the initial steps of organic matter mineralization. Differences between conventional and reduced tillage systems occurred only for bulk and rhizosphere soil C and N concentrations for the two sampling years. In 2014 either glyphosate or the genetically engineered corn cultivar appeared to have affected rhizosphere nutrient status by decoupling the stoichiometric relation between N and P. Glyphosate appeared not to affect bulk or rhizosphere soil C and N balance. In both 2013 and 2014 there appeared to be no consistent connection between exoenzyme activities and microbial metabolic activities as measured by fluorescein diacetate (FDA) activity. Nutrient acquisition ratios for both bulk and rhizosphere soils indicated P limitations in 2013, and a more than sufficient presence of assimilable N. The P limitations were also apparent in 2014 for both bulk and rhizosphere soils, in contrast to balanced C and N acquisition ratios in rhizosphere soil. Stoichiometric relationships indicated few differences between glyphosate and non-glyphosate treatments except for significant negative relations between bulk and rhizosphere soil acquisition C:N and N:P ratios in 2013, and significant positive relations in 2014 for the same acquisition ratios. In 2013 the bulk and rhizosphere soil relations between microbial metabolic (FDA) activity and the exoenzyme activities appeared to indicate an ephemeral effect of glyphosate on disrupting the stoichiometric function of the exoenzymes, which was not observed in 2014. Except for apparent ephemeral effects, neither glyphosate nor the glyphosate resistant corn cultivar appeared to disrupt the function of the bulk and rhizosphere soil exoenzymes involved in organic matter mineralization of C, N, and P under either conventional or reduced tillage.