Variability of soil properties and 2,4-D fate at field and regional scales as affected by ecoregion, landscape position and soil depth

This study quantified variations in 2,4-D (2,4-dichlorophenoxyacetic acid) sorption and degradation as influenced by soil properties at field and regional scales. The sorption and degradation of 2,4-D was measured in soil profiles collected from upper, mid, lower and depression landscape positions at the field scale and in surface soils from upper, mid and lower landscape positions in seven ecoregions at the regional scale.; Sorption of 2,4-D varied significantly among soil horizons, as well as among slope positions in the A horizon at the field scale. Although 2,4-D sorption varied among ecoregions and among landscape positions at the regional scale, differences in sorption were generally not significant because of large variability in 2,4-D sorption within ecoregions and within landscape positions. Regardless of scale, variations in 2,4-D sorption were best predicted by regression models containing soil organic carbon content and soil pH as variables. At the field scale, carbonate content was also significant in regressions predicting 2,4-D sorption because sorption was negatively related to carbonate content. At the field scale, segmentation of soils by soil horizon and slope position improved predictions of 2,4-D sorption. At the regional scale, segmentation of soils by ecoregion and landscape position did not greatly improve predictions of 2,4-D sorption.; Compared with sorption, at the field scale, differences in total 2,4-D degradation and degradation rates among slope positions in the A horizon were small. Differences in total 2,4-D degradation and degradation rates among ecoregions and among slope positions at the regional scale were also generally not significant. The most significant differences in 2,4-D degradation occurred with soil depth, for which degradation kinetics were best described by first-order kinetics in the A horizon and by three half-order kinetics in the B and C horizons. Although sorption of 2,4-D limited the bioavailability of 2,4-D for degradation in the A horizon, predictions of 2,4-D degradation parameters were generally poor and not consistently related with 2,4-D sorption or soil properties. Segmentation of soils by ecoregion and by landscape position improved predictions of 2,4-D degradation parameters, however, regression models still had low R2 values.; Since there is only limited information on the variability of pesticide sorption and degradation in soils, single estimates of pesticide sorption and degradation parameters are used as input parameters in pesticide fate models, even for simulations assessing the environmental risks associated with pesticide use across large areas. Results from this study indicate that determining 2,4-D sorption and degradation parameters as a function of soil properties could be a better approach to obtaining estimates of pesticide input parameters for pesticide fate models. However, determining sorption parameters based on soil properties will be more accurate than determining degradation parameters based on soil properties.