Particle-facilitated pesticide leaching from differently structured soil monoliths
Received for publication September 19, 2008. The leaching of soil particles and surface applied 14C-labeled glyphosate and pendimethalin from intact soil columns (height: 50 cm; diameter: 30 cm) were investigated, and the relative significance of particle-facilitated pesticide transport was quantified. Investigations were performed with a recently plowed (four columns) and an untilled (five columns) sandy loam soil. Leaching was driven by three irrigation events (15 mm h–1; 2 h each). Samples of the leachate were filtered immediately (within 1.5 minutes) using 20 nm filters, and the 14C-pesticide content was determined for filtered and unfiltered samples. Pesticide leaching was driven by preferential water flow in macropores. For the plowed structure, 68 ± 10% of the leached glyphosate (average of 6 events ± std.) was bound to particles whereas significantly less glyphosate was bound to particles in leachate from minimally disturbed columns (17 ± 12%). Thus, the results suggest that soil structure affected the mode of transport of glyphosate. It is likely that glyphosate sorbed strongly when applied on recently plowed soil (Kd = 503 L kg–1 for the soil), and that it could be mobilized and transported independently of soil particles more easily when applied on the minimally disturbed soil covered in part with crop residues (Kd < 1 L kg–1 for straw). Significantly less amounts of soil particles were leached from minimally disturbed (119–247 mg) than from recently plowed (441–731 mg) columns. The significance of particle-facilitated pendimethalin leaching could not be accurately quantified due to disagreement between control measurements based on both 14C-activity and chemical analyses.