Soil characteristics and herbicide sorption coefficients in 140 soil profiles of two irregular undulating to hummocky terrains of western Canada

Abstract Sensitive input parameters such as herbicide sorption coefficients are seldom quantified in sufficient spatial detail, leading to large uncertainties in pesticide fate model predictions at regional and national scales. This study examined the spatial variability of soil characteristics and herbicide sorption coefficients (Kd) in a total of 609 soil samples obtained from 140 soil profiles in two undulating to hummocky soil-landscapes of western Canada. In all soil horizons, Kd values were always the largest for glyphosate followed by atrazine then 2,4-D. In each soil-landscape, SOC, 2,4-D and atrazine Kd values (all strongly associated) numerically decreased with A > B > C mineral horizons, regardless of the segment of the slope in which the soil profile was located. SOC, 2,4-D Kd and atrazine Kd values were typically smaller in the eluvial A-horizons (Ae, Aej) than other A-horizons. In several landform elements, glyphosate Kd values were numerically greater in the B rather than the A or C mineral horizons. Glyphosate Kd values were particularly large in B-horizons with redoximorphic features such as in illuvial B-horizons enriched with clay (Btg, Btgj). When the soil properties and sorption values of similar horizons were compared across the soil-landscapes, SOC and 2,4-D Kd demonstrated greater coefficients of variation than atrazine Kd and glyphosate Kd. The strength of prediction equations for 2,4-D Kd and atrazine Kd were strong in each soil-landscape when all samples in the soil profile were included in the model. However, the strength of prediction equations by mineral horizon was generally weaker and in some cases not significant. The in-field variability of glyphosate Kd values could not be predicted from the measured soil properties data, or calculated digital terrain attributes. Terrain attributes also showed little to no correlation, or predictive strength for describing the variability of 2,4-D and atrazine sorption in the soil-landscapes.