Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/24/2007
Publication Date: 5/2/2008
Citation: Farenhorst, A., Papiernik, S.K., Saiyed, I., Messing, P., Stephans, K.D., Schumacher, J.A., Lobb, D.A., Sheng, L., Lindstrom, M.J., Schumacher, T.E. 2008. Herbicide sorption coefficients in relation to soil properties and terrain attributes on a cultivated prairie. Journal of Environmental Quality. 37:1201-1208. Interpretive Summary: Pesticide fate models are used to assess the risk of water contamination by pesticides on a national basis. Sorption coefficients (measures of the extent to which pesticides are bound to the soil) are important variables in pesticide fate models. Accounting for the variation in pesticide sorption coefficients across landscapes could reduce uncertainties in regional-scale assessments of pesticide behavior. In these experiments, we measured the sorption coefficients of two common herbicides, 2,4-D and glyphosate, in 286 soil samples collected throughout a hilly field. Soil properties are highly variable in this field because erosion has removed topsoil from some areas of the field. Herbicide sorption coefficients are dependent on soil properties and thus were also highly variable. We found that including information about slope curvature and other factors describing the terrain could improve predictions of herbicide sorption in this field. The changes in soil properties and terrain factors had a larger impact on the sorption of a herbicide that is weakly bound by the soil (2,4-D) than for a herbicide that is strongly bound (glyphosate). These results suggest that this approach may help increase the accuracy of pesticide fate models. These results will help guide additional research to determine how including information about field terrain can improve pesticide fate models.
Technical Abstract: The sorption of the herbicides 2,4-D and glyphosate in soil was quantified for 286 surface soil samples (0-15 cm) collected in a 10 m X 10 m grid across a heavily-eroded undulating calcareous prairie landscape. At each sampling point soil organic carbon content, soil carbonate content, soil pH, tillage and water erosion rates, and selected terrain attributes and landform segments were determined. Crop yields were also recorded for four consecutive years (2000 to 2003). Soil organic carbon content (SOC) was low across the field (average 1.1% ± 0.3) in part because of the net loss of topsoil caused by high rates of water erosion (average soil loss of 13.2 Mg per ha per yr ± 22.7). The soil carbonate content was five times greater in upper-slopes compared to lower-slopes because tillage redistributed topsoil to lower slopes and incorporated calcareous subsoil materials with a low SOC into the surface layer of upper-slopes. The 2,4-D soil sorption partition coefficient was 1.8 times smaller in upper-slopes than lower-slopes. The sorption of 2,4-D by soil was significantly associated with nine terrain attributes particularly with compounded topographic index (r=0.59, P<0.001), gradient (r=-0.48, P<0.001), mean curvature (r=-0.43, P<0.001) and plan curvature (r=-0.42, P<0.001). The prediction of 2,4-D sorption was improved when selected terrain attributes were combined with soil properties (R2=0.70) relative to predictions using soil properties alone (R2=0.54) or terrain attributes alone (R2=0.58). Although this study indicates that digital terrain modeling adds to the accuracy of describing the distribution of 2,4-D sorption at the field level, the distribution of glyphosate sorption across the field was much less dependent on terrain morphology. Using digital terrain modeling to reduce uncertainties in scaling pesticide fate models from the field scale to regional levels therefore offers greater promise for herbicides such as 2,4-D that are weakly sorbed and sensitive to small changes in SOC, carbonate contents and soil pH, than for herbicides such as glyphosate that are strongly bound to soil.