Phosphate and glyphosate molecules compete for sorption sites in soil. The objective of this study was to quantify the impact of Olsen P concentrations in two contrasting soils on phosphate and glyphosate sorption. Soils were a sandy clay loam soil rich in iron oxides (SCL-Fe 2 O 3) and a clay loam soil rich in calcium carbonates (CL-CaCO 3). The phosphate Freundlich sorption coefficient (K f) ranged from 3 to 68 L 1/n mg 1-1/n kg-1 in the SCL-Fe 2 O 3 and from 21 to 76 L 1/n mg 1-1/n kg-1 in the CL-CaCO 3. Glyphosate sorption coefficient (K d) ranged from 293 to 1173 L kg-1 in the SCL-Fe 2 O 3 but only 99 to 141 L kg-1 in the CL-CaCO 3. Glyphosate K d and phosphate K f values decreased significantly with increasing Olsen P concentrations in both soils. Glyphosate K d values were further significantly reduced when phosphate was added to the slurry solutions, but phosphate K f values were not impacted by the presence of glyphosate in solutions. We conclude that annual phosphate fertilizer applications leave phosphate concentrations in Prairie soils to the extent that soils have a lesser capacity to retain glyphosate and phosphate that are subsequently applied, but glyphosate residues will not influence phosphate sorption.

Draining of geographically isolated (no defined inlet or outlet) freshwater mineral soil wetlands has likely converted areas that acted historically as important P sinks to sources of P. To explore the role of wetland drainage on nonpoint-source P pollution, differences in the chemical characteristics and P sorption parameters of drained and intact wetlands were investigated in a small watershed situated in the Prairie Pothole Region of southwestern Manitoba, Canada. Chemical characteristics and P sorption parameters varied across landscape positions, particularly for landscape positions that were submerged. Intact wetlands had slightly higher concentrations of organic and total P relative to drained wetlands, which is indicative of their P trapping capacity. More importantly, the maximum P sorption capacity and P buffering capacity of intact wetlands were 3.6 (1752 vs. 492 mg kg) and 17 (1394 vs. 84 L kg) times greater than those in drained wetlands. Conversely, equilibrium P concentrations and bioavailable P concentrations in drained wetlands were an order of magnitude greater than those in intact prairie wetlands. Our study suggests that intact prairie wetlands may be effective sinks for P. As a result, prairie wetlands may play an important role in mitigating nonpoint-source pollution. Conversely, our findings suggest that drained prairie wetlands are potentially a high risk for P export and should be treated as important critical source areas within prairie watersheds.

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