title: Impact of glyphosate-resistant corn, glyphosate applications and tillage on soil nutrient ratios, exoenzyme activities and nutrient acquisition ratios.
abstract: BACKGROUND
  We report results of the last two years of a 7 year field experiment designed to test the null hypothesis: applications of glyphosate on glyphosate-resistant (GR) and non-resistant (non-GR) corn (Zea mays L.) under conventional tillage and no-till would have no effect on soil exoenzymes and microbial activity.
  
  
  RESULTS
  Bulk soil (BS) and rhizosphere soil (RS) macronutrient ratios were not affected by either GR or non-GR corn, or glyphosate applications. Differences observed between exoenzyme activities were associated with tillage rather than glyphosate applications. In 2013, nutrient acquisition ratios for bulk and rhizosphere soils indicated P limitations, but sufficient assimilable N. In 2014, P limitations were observed for bulk and rhizosphere soils, in contrast to balanced C and N acquisition ratios in rhizosphere soils. Stoichiometric relationships indicated few differences between glyphosate and non-glyphosate treatments. Negative correlations between C:P and N:P nutrient ratios and nutrient acquisition ratios underscored the inverse relation between soil nutrient status and microbial community exoenzyme activities.
  
  
  CONCLUSIONS
  Inconsistent relationships between microbial community metabolic activity and exoenzyme activity indicated an ephemeral effect of glyphosate on BS exoenzyme activity. Except for ephemeral effects, glyphosate applications appeared not to affect the function of the BS and RS exoenzymes under conventional tillage or no-till. Published 2016. This article is a U.S. Government work and is in the public domain in the USA.
authors: National Sedimentation Laboratory, Water Quality and Ecology Research Unit, USDA - Agriculture Research Service, Oxford, MS, USA.
authors: Michael B
authors: MB
authors: Jenkins
authors: National Sedimentation Laboratory, Water Quality and Ecology Research Unit, USDA - Agriculture Research Service, Oxford, MS, USA.
authors: Martin A
authors: MA
authors: Locke
authors: Crop Production Systems Research Unit, USDA - Agriculture Research Service, Stoneville, MS, USA.
authors: Krishna N
authors: KN
authors: Reddy
authors: National Sedimentation Laboratory, Water Quality and Ecology Research Unit, USDA - Agriculture Research Service, Oxford, MS, USA.
authors: Daniel S
authors: DS
authors: McChesney
authors: National Sedimentation Laboratory, Water Quality and Ecology Research Unit, USDA - Agriculture Research Service, Oxford, MS, USA.
authors: R Wade
authors: RW
authors: Steinriede
mesh_terms: D000383:Agriculture / Q000379:methods; D002244:Carbon / Q000032:analysis; D005998:Glycine / Q000031:analogs & derivatives* / Q000494:pharmacology; D053209:Herbicide Resistance; D009584:Nitrogen / Q000032:analysis; D010571:Pest Control / Q000379:methods; D010758:Phosphorus / Q000032:analysis; D058441:Rhizosphere; D012987:Soil / Q000737:chemistry*; D012988:Soil Microbiology; D003313:Zea mays / Q000187:drug effects / Q000235:genetics*; D000097797:Glyphosate
chemical_list: D002244:Carbon; D005998:Glycine; D009584:Nitrogen; D010758:Phosphorus; D012987:Soil; D000097797:Glyphosate
keywords: corn; exoenzymes; glyphosate; rhizosphere; soil; tillage