Influence of earthworms (Lumbricus terrestris L.) on the dissipation and distribution of herbicides in soil

grantor: University of TorontoThe effect of earthworms on chemical, physical, and biological soil properties influenced the persistence, bioavailability, and transport of the herbicides atrazine [2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine] and metolachlor [2-chloro-'N'-(2-ethyl-6-methylphenyl)-' N'-(2-methoxy-1-methylethyl)acetamide] in soil. Herbicide applications at maximum recommended field rates had no adverse effects on earthworm biomass. [U-ring-14C]at-dzine dissipated and mineralized more rapidly in soil preincubated with earthworms than in soil that did not contain earthworms. Earthworm activity, initiated in soil following herbicide application, increased atrazine mineralization rates over 22 days in soil at 12°C, when compared to the control soil. Earthworms also increased the formation of nonextractable atrazine compounds in soil resulting in lower mineralization rates from 20 to 68 days at both 12°C and 30°C At 68 days after [U-ring- 14C]atrazine applications at 12°C incubation, total nonextractable radioactivity in soil columns with earthworms was 21% greater than that in soils without earthworms. Extractable [U-ring-14C]metolachlor decreased more rapidly in microcosms containing earthworms than in earthworm-free soil, primarily due to uptake of metolachlor residues into earthworm tissue. Earthworms feeding activity on [U-ring-14C]atrazine-sprayed corn leaves (surface application) translocated 60% of the total radioactivity below 4 cm soil depth, depositing between 1.3 * to 2.8 * more MeOH-extractable radioactivity in their burrow than in surrounding matrix soil. In soils containing no earthworms, more than 65% of the initially applied [U-ring-14C]atrazine remained in the 0 to 4 cm surface layer. The influence of earthworm-mediated processes during interstorm periods decreased preferential herbicide transport via burrow flow in subsequent rainstorms. At the end of a series of 14 and 47 mm h-1 simulated rainstorms, radioactivity in drainage from soils with biopores was between 1.9 * and 2.2 * greater in the absence than in the presence of earthworms. Metolachlor and atrazine sorbed more strongly on earthworms surface castings than on matrix soil. Burrow linings reduced [U-ring- 14C]atrazine and [U-ring-14C]metolachlor solutions, introduced in burrows, to 60% of their original concentration in the first 0 to 3 min of flow, and to 80% in 3 to 9 min of flow. The implications of the findings for weed and environmental management are discussed.Ph.D