Competitive sorption interactions of organic and inorganic chemicals in soil
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Date
2017
Authors
Munira, Sirajum
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Abstract
Agricultural soils in the Prairies encompasses 80% of Canada’s farmland that may contain a range of chemical compounds, including phosphate and cadmium from fertilizer application, glyphosate and MCPA from herbicide application and/or the antibiotic tetracycline following livestock manure application. The aim of this PhD research was to examine the competitive sorption effects of these inorganic and organic chemicals in soil. Soil samples were collected in 2013 from two research sites that had received repeated annual application (2002-2009) of mono ammonium phosphate at 20, 40 and 80 kg P ha-1 with cadmium added at low, medium or high rates. A series of batch equilibrium experiments were conducted to quantify sorption of phosphate, glyphosate, MCPA and tetracycline. Results showed that cadmium concentrations in soil had no significant effect on sorption of glyphosate. Sorption of glyphosate significantly decreased with increasing phosphate concentrations in soils regardless of whether phosphate levels arose from repeated applications in the field, or from fresh applications in the laboratory. Similarly, sorption of phosphate significantly decreased with increasing phosphate concentrations in soils, suggesting that the capacity of soils to retain phosphate or glyphosate was reduced resulting from previous (repeated) phosphate fertilizer applications. The effect of phosphate on reducing phosphate and glyphosate sorption was greater in an acidic soil that had high Fe/Al-oxides than in slightly alkaline calcareous soil. Analytical-grade glyphosate and commercially-available glyphosate formulation, Roundup Ultra2, behaved similarly for their sorption pattern. Phosphate sorption was not significantly impacted when Roundup Ultra2 was added to soil slurries in the laboratory. Repeated application of phosphate in the field had no significant impact on MCPA and tetracycline sorption, but fresh addition of phosphate in the laboratory significantly reduced MCPA and tetracycline sorption. Phosphate addition significantly increased desorption of glyphosate, MCPA and tetracycline but the impact was numerically very small. Thus, phosphate had a greater impact on sorption than desorption of the three organic chemicals. The presence of MCPA significantly reduced glyphosate sorption and increased desorption only when MCPA was added at high amounts and no phosphate was added. Overall, the competitive effect of phosphate on glyphosate sorption was strongest among the chemicals studied.
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Keywords
Glyphosate, MCPA, tetracycline, phosphate, batch equilibrium experiment, competitive effects, sorption, desorption.