Regional Assessment of Herbicide Sorption and Degradation in Two Sampling Years

Abstract

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 degrees to 60 degrees north longitude and 110 degrees to 120 degrees 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.

Sorption of four estrogens by surface soils from 41 cultivated fields in Alberta, Canada

“…Such differences in sorption coefficients are expected to influence the persistence and transport of organic chemicals in soil (Boesten and van der Linden, 1991;Farenhorst et al, 2008). The weaker K d values for soils with SOC b 10 g C kg − 1 has been highlighted in pesticide sorption studies (Wauchope et al, 2002;Gaultier et al, 2008). In our studies, estrogen K d values also varied significantly within fields, with the estrogen K d values being on average 1.3 to 1.4 times smaller in upper-slopes than lower-slopes.…”

“…Several approaches have been proposed to obtaining data on pesticide sorption parameters at the large-scale. These approaches include direct measurements, which are costly particularly when multiple pesticides are involved, or by indirect measures such as regression equations based on soil properties data (Dann et al, 2006;Gaultier et al, 2008). Such indirect approaches are also valid for estrogens but, based on r 2 values, the strength of the predictions were relatively poor for K d -estrone, K oc -17β-estradiol, K oc -equol, K oc -estriol and K oc -estrone.…”

“…2). The locations of these sampling locations are described in Gaultier et al (2008) and Cathcart et al (2008). Ecoregions constitute units with distinct regional ecological factors (i.e., climate, physiography, vegetation, soil, water, and fauna) and are an entity within a national hierarchy of ecostratification (Ecological Stratification Working Group, 1995).…”

“…Estrogen biodegradation during the experiment (Ecological Stratification Working Group, 1995;Leskiw et al, 2000;Cathcart et al, 2008;Gaultier et al, 2008). leads to underestimating K d values and hence the sterilization of laboratory equipment and soils is desired (Wolf et al, 1989). We utilized autoclaving, which is the single most commonly-used sterilization technique in batch-equilibrium sorption studies (e.g., see recent studies by Shaw and Hooker, 2008;Yang et al, 2008;Burns et al, 2009;Lee et al, 2009;Hong et al, 2008;Ren et al, 2009).…”

“…At a regional scale spanning the provinces of Alberta, Saskatchewan and Manitoba, Canada, Anderson (1979) reported that SOC increases in the order of Dark Brown b Black b Dark Gray Chernozem soil great groups. In the Province of Alberta, Gaultier et al (2008) quantified that SOC and 2,4-dichlorophenoxyacetic acid (a herbicide) sorption was significantly smaller in the Mixed Grassland ecoregion that contains Brown Chernozems than in five other ecoregions with different soil great groups. Farenhorst et al (2008) concluded that SOC and 2,4-D sorption typically increase in the order of upper b mid b lower slope positions in agricultural fields.…”