title: Phosphate fertilizer impacts on glyphosate sorption by soil.
abstract: This research examined the impact of field-aged phosphate and cadmium (Cd) concentrations, and fresh phosphate co-applications, on glyphosate sorption by soil. Soil samples were collected in 2013 from research plots that had received, from 2002 to 2009, annual applications of mono ammonium phosphate (MAP) at 20, 40 and 80 kg P ha(-1) and from products containing 0.4, 70 or 210 mg Cd kg(-1) as an impurity. A series of batch equilibrium experiments were carried out to quantify the glyphosate sorption distribution constant, Kd. Extractable Cd concentrations in soil had no significant effect on glyphosate sorption. Glyphosate Kd values significantly decreased with increasing Olsen-P concentrations in soil, regardless of the pH conditions studied. Experiments repeated with a commercially available glyphosate formulation showed statistically similar results as the experiments performed with analytical-grade glyphosate. Co-applications of MAP with glyphosate also reduced the available sorption sites to retain glyphosate, but less so when soils already contain large amounts of phosphate. Glyphosate Kd values in soils ranged from 173 to 939 L kg(-1) under very strong to strongly acidic condition but the Kd was always <100 L kg(-1) under moderately acidic to slightly alkaline conditions. The highest Olsen-P concentrations in soil reduced Kd values by 25-44% relative to control soils suggesting that, under moderately acidic to slightly alkaline conditions, glyphosate may become mobile by water in soils with high phosphate levels. Otherwise, glyphosate residues in agricultural soils are more likely to be transported off-site by wind and water-eroded sediments than by leaching or runoff.
authors.affiliation: Department of Soil Science, Faculty of Agricultural & Food Sciences, University of Manitoba, 362 Ellis Building, Winnipeg, MB R3T 2N2, Canada. Electronic address: muniras@myumanitoba.ca.
authors.forename: Sirajum
authors.initials: S
authors.lastname: Munira
authors.affiliation: Department of Soil Science, Faculty of Agricultural & Food Sciences, University of Manitoba, 362 Ellis Building, Winnipeg, MB R3T 2N2, Canada.
authors.forename: Annemieke
authors.initials: A
authors.lastname: Farenhorst
authors.affiliation: Department of Soil Science, Faculty of Agricultural & Food Sciences, University of Manitoba, 362 Ellis Building, Winnipeg, MB R3T 2N2, Canada.
authors.forename: Don
authors.initials: D
authors.lastname: Flaten
authors.affiliation: Agriculture and Agri-Food Canada, Brandon, MB R7A 5Y3, Canada.
authors.forename: Cynthia
authors.initials: C
authors.lastname: Grant
mesh_terms: D000383:Agriculture / Q000379:methods; D002104:Cadmium / Q000032:analysis*; D005308:Fertilizers; D005998:Glycine / Q000031:analogs & derivatives* / Q000737:chemistry; D006540:Herbicides / Q000737:chemistry*; D010710:Phosphates / Q000737:chemistry*; D012987:Soil / Q000737:chemistry*; D012989:Soil Pollutants / Q000032:analysis*; D014867:Water; D000097797:Glyphosate
chemical_list: D002104:Cadmium; D005308:Fertilizers; D005998:Glycine; D006540:Herbicides; D010710:Phosphates; D012987:Soil; D012989:Soil Pollutants; D014867:Water; D000097797:Glyphosate; C024788:ammonium phosphate
keywords: Batch equilibrium experiment; Cadmium; Competitive effect; Glyphosate; Mono ammonium phosphate; Sorption distribution constant