Submitted to: Geoderma
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/3/2009
Publication Date: 3/25/2009
Publication URL: hdl.handle.net/10113/29892
Citation: Farenhorst, A., McQueen, D.A.R., Saiyed, I., Hilderbrand, C., Li, S., Lobb, D.A., Messing, P., Schumacher, T.E., Papiernik, S.K., Lindstrom, M.J. 2009. Variations in Soil Properties and Herbicide Sorption Coefficients with Depth in Relation to PRZM (Pesticide Root Zone Model) Calculations. Geoderma. 150:267-277. Interpretive Summary: Little experimental data is available on how herbicide behavior in soil changes across small distances. Soil erosion results in a large variation in soil properties that affect herbicide fate and transport. We evaluated the retention of the commonly-used herbicides: 2,4-D and glyphosate in soils from three landform elements (eroded upper slope, deposition zone, and eroded waterway) in a strongly eroded agricultural field. For 2,4-D, herbicide retention by soil was most strongly influenced by variations in soil organic carbon concentrations. In contrast, retention of the herbicide glyphosate was predominantly controlled by variations in soil pH and clay content. The pesticide root zone model (PRZM) predicted that glyphosate would be immobile in soils even under an extreme rainfall scenario. The model predicted that 2,4-D would be much more mobile in soil. The model output was particularly sensitive to input values of herbicide retention, which depended strongly on soil properties. When values determined for the deposition zone were replaced by those measured in the eroded upper slope, the predicted amount of 2,4-D leached to a depth of 15 cm depth increased by 29,081% under an actual rainfall scenario. These results suggest that herbicide transport may be vastly different in different portions of the same field. Pesticide fate models such as PRZM are being used in policy analyses at large scales. These results indicate that to strengthen pesticide leaching predictions, data on herbicide retention in different landform elements and for different depths in soil should be included. These results will be useful to those who develop and use pesticide fate models to improve predictions of pesticide transport in eroded hilly landscapes common throughout the prairie region.
Technical Abstract: There are few experimental data available on how herbicide sorption coefficients change across small increments within soil profiles. Soil profiles were obtained from three landform elements (eroded upper slope, deposition zone, and eroded waterway) in a strongly eroded agricultural field and segmented into 2-cm intervals. Soil samples were analyzed for soil organic carbon content (SOC), soil pH, soil carbonate content, CEC, soil texture, bulk density, 2,4-D [2,4-(dichlorophenoxy) acetic acid] or glyphosate [N-phosphonomethylglycine] sorption by soil (Kd), and 2,4-D or glyphosate sorption per unit soil organic carbon (Koc). Considering all soil profiles, 2,4-D Kd values ranged from 0.12 to 2.61 L/kg and were most strongly influenced by variations in SOC. In contrast, glyphosate Kd values ranged from 19 to 547 L/kg and were predominantly controlled by variations in soil pH and clay content. The pesticide root zone model (PRZM) predicted that glyphosate would be immobile in soils even under an extreme rainfall scenario of 384 mm at one day after herbicide application. In contrast, for 2,4-D, PRZM predicted that up to 6% of the applied herbicide would move to a depth of 15 cm under an actual rainfall scenario. PRZM output was particularly sensitive to input values of Kd, relative to input values of soil properties. The greatest change to PRZM outputs occurred when Kd values of toeslope profiles were replaced by those measured in knoll profiles, when the amount of 2,4-D leached to a depth of 15 cm increased by 29,081% (from 0.09 to 26.17 g/ha) under an actual rainfall scenario. We conclude that, when pesticide fate models such as PRZM are being used in policy analyses at larger scales, data on Kd values in different landform elements and at the soil horizon level could be important for strengthening pesticide leaching predictions.