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TECHNICAL REPORTS

Organic Compounds in the Environment

Regional Assessment of Herbicide Sorption and Degradation in Two Sampling Years

^a Dep. of Soil Science, Univ. of Manitoba, Winnipeg, MB, Canada R3T 2N2
^b Alberta Agriculture (AF), Resource Science Branch, Edmonton, AB, Canada T6H 5T6

^* Corresponding author (farenhor{at}ms.umanitoba.ca).

Received for publication November 1, 2007. Sorption and degradation of the herbicide 2,4-D [2,4-dichlorophenoxyacetic acid] were determined for 123 surface soils (0 to 15 cm) collected in 2002 and in 2004 between 49° to 60° north longitude and 110° to 120° west latitude in Alberta, Canada. The soils were characterized by soil organic carbon content (SOC), pH, electrical conductivity, soil texture, cation exchange capacity, carbonate content, and total soil microbial activity. The 2,4-D sorption coefficients, Kd and Koc, were highly variable with coefficients of variation of 89 and 59%, respectively, at the provincial scale. Both Kd and Koc were well described by regression models with SOC and soil pH as variables, regardless of scale. Surprisingly, variations in 2,4-D mineralization were much smaller than variations in sorption. Variability in total 2,4-D mineralization was particularly low, with a coefficient of variation of only 7% at the provincial scale. Average 2,4-D half-lives in ecoregions ranged from 1.7 to 3.5 d, much lower than the field dissipation half-life of 10 d reported for 2,4-D in general pesticide property databases. Regression models describing degradation parameters were generally poor or not significant because 2,4-D mineralization was only weakly associated with measured 2,4-D sorption parameters and soil properties. As such, regional variations in herbicide sorption coefficients should be measured or calculated based on soil properties, to assign distinct pesticide fate model input parameters when estimating 2,4-D off-site transport at the provincial scale. Spatial variations in herbicide degradation appear less important for Alberta as 2,4-D half-lives were similar in soils across the province. The rapid mineralization of 2,4-D is noteworthy because 2,4-D is widely used in Alberta and perhaps adaptation of soil microbial communities allowed for accelerated degradation regardless of soil properties or the extent of 2,4-D sorption by soil.

Abbreviations: AP, Aspen Parkland • BT, Boreal Transition • CEC, cation exchange capacity • CV, coefficient of variation • EC, electrical conductivity • FG, Fescue Grassland • k, first order mineralization rate constant • Kd, sorption coefficient • Koc, soil organic carbon sorption coefficient • L, lower • M, mid • MG, Mixed Grassland • MMG, Moist Mixed Grassland • M_Texp, total 2,4-D mineralization at end of experiment • PL, Peace Lowland • RDA, redundancy analysis • SOC, soil organic carbon content • t, soil half-life • U, upper