|Lerch, Robert - Bob|
Submitted to: Journal of Environmental Quality
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/14/2003
Publication Date: 11/30/2003
Citation: LIN, C.H., LERCH, R.N., GARRETT, H.E., JOHNSON, W.G., JORDAN, D., GEORGE, M.F. THE EFFECT OF FIVE FORAGE SPECIES ON TRANSPORT AND TRANSFORMATION OF ATRAZINE AND BALANCE (ISOXAFLUTOLE) IN LYSIMETER LEACHATE. JOURNAL OF ENVIRONMENTAL QUALITY. 2003. V. 32. P. 1992-2000. Interpretive Summary: Grass buffers are intended to reduce agricultural contaminant loads to surface waters and to reduce contaminant leaching to ground water. However, their ability to reduce herbicide leaching or to enhance herbicide degradation has not been thoroughly evaluated. In this study, we applied the weed killers atrazine and isoxaflutole (trade name, Balance) at levels that would be expected in runoff to five different grass species (orchardgrass, tall fescue, Timothy, smooth bromegrass, and switchgrass). We then measured the amount of the herbicides and their metabolites (i.e., break down products) that leached through 3 ft of soil following natural rainfall events. Balance represents a new strategy for chemical weed control; it must be activated by conversion in soil to a diketonitrile (DKN) metabolite, which is the actual herbicide. Our results showed that Balance was, in fact, highly unstable, degrading to the DKN metabolite in less than 24 hours, regardless of grass species. The DKN metabolite leached more readily than atrazine or atrazine metabolites. Grasses did not enhance Balance degradation, but they did decrease leaching of its metabolites. Grasses significantly enhanced atrazine degradation, which led to proportionally more leaching of less toxic atrazine metabolites. Overall, grasses did not reduce total atrazine (atrazine + metabolites) leaching, but they do help to detoxify atrazine. Orchardgrass, Timothy, and smooth bromegrass enhanced atrazine degradation the most. These results indicated that herbicide leaching and degradation in grass buffers were dependent upon both the herbicide and the forage species. This research will benefit landowners and state and federal agencies in the design of more effective grass buffer strips to reduce herbicide contamination of ground and surface waters.
Technical Abstract: Grass riparian buffer strips are recognized as one of the most effective bioremediation approaches to mitigate the transport of agricultural chemicals from croplands. A lysimeter study with six different ground covers (bare ground, orchardgrass, tall fescue, timothy, smooth bromegrass, and switchgrass) was established to evaluate the effect of forage grasses on the fate and transport of atrazine (ATR) and Balance (isoxaflutole; IXF) in leachate. The results suggested that total Balance (parent + metabolites) showed higher mobility than atrazine and its metabolites. Differences in the timing of transport reflected the rapid degradation of IXF to the more soluble, stable and biologically active diketonitrile (DKN) metabolite in the system. Although grass treatments did not promote the hydrolysis of DKN, they significantly reduced the total quantity in the leachate through enhanced evapotranspiration. Grass treatments significantly enhanced ATR degradation in the leachates and soils, especially through N-dealkylation, but they did not reduce total ATR transported in the leachate. Leachate from the orchardgrass lysimeters contained the highest proportion of ATR metabolites. Timothy and smooth bromegrass treatments also displayed a significant increase in ATR metabolites in leachate. Grass treated lysimeters showed significantly higher microbial biomass carbon than bare ground. For ATR treatments, the proportion of metabolites in the leachate strongly correlated with the elevated soil microbial biomass carbon in forage treatments. In contrast, DKN degradation was poorly correlated with soil microbial biomass carbon, suggesting that DKN degradation is an abiotic process. These results indicated that herbicide transport and degradation in grass buffer systems were dependent upon the herbicide chemistry and the forage species employed.