ARS | Publication request: Sorption and dissipation of aged metolachlor residues in eroded and rehabilitated soils

Submitted to: Pest Management Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 23, 2012
Publication Date: April 19, 2012
Citation: Cabrera, A., Papiernik, S.K., Koskinen, W.C., Rice, P.J. 2012. Sorption and dissipation of aged metolachlor residues in eroded and rehabilitated soils. Pest Management Science. 68:1272–1277. DOI: 10.1002/ps.3294.

Interpretive Summary: Land management practices affect soils in ways that may influence pesticide fate and behavior. The potential for pesticides to contamínate surface and groundwater can be assessed using models that describe pesticide movement in soil. For accurate model prediction, a better understanding is needed of the complex interactions between pesticide binding and degradation processes. In this work we determined the tendency for the herbicide metolachlor to bind to different soils collected from the same field. The results showed that under laboratory conditions, more metolachlor is bound to rehabilitated soil than to eroded soil, resulting in slower decomposition of the herbicide in soil. These studies also suggest that metolachlor, like many other pesticides, becomes more strongly bound in soil with time. Using high temperature and pressure removed more metolachlor from soil, decreasing the predicted risk of water contamination. These important results show that model predictions can be affected by the procedures used to measure herbicide binding and degradation. Metolachlor is a popular herbicide used to control weeds in corn, soybean, and sorghum. Metolachlor is one of the most heavily used herbicides in the USA and it is detected frequently in ground and surface water. The information provided by these studies contributes to a better understanding of the effects of management practices on metolachlor behavior in soil. Scientists, pesticide manufacturers, and regulators can use this information to better predict metolachlor’s effectiveness in controlling weeds and its movement in soil. With improved management of this herbicide, water contamination associated with its use can be avoided.

Technical Abstract: To accurately determine availability for offsite transport, sorption and dissipation of aged metolachlor were characterized in rehabilitated and eroded prairie soils using sequential batch slurry and accelerated solvent extraction (ASE). In the eroded upper slope, soil-landscape rehabilitation more than doubled soil organic carbon contents, resulting in greater sorption of S-metolachlor. Metolachlor dissipated more rapidly in undisturbed soil from both the upper and lower slope, perhaps due in part to decreased sorption. The elevated temperature and pressure used in ASE removed a higher proportion of metolachlor from soil using both aqueous and organic solvents. S-metolachlor Kd values for freshly-spiked soil ranged from 3 to 5 mL g-1 (manual extraction) and 6 to 9 mL g-1 (ASE); for lower slope soils, Kd values increased to 6-12 mL g-1 when samples were aged for 2 weeks. The results of these studies suggest that sorption of S-metolachlor increases with time in some soils. Residues of S-metolachlor dissipated more rapidly in undisturbed soil than in soil translocated from within the same landform. The method of extraction (manual vs ASE) affected calculated sorption coefficients, dissipation rates, and potential leaching as determined using the Groundwater Ubiquity Score (GUS) index.