ALACHLOR AND ATRAZINE DEGRADATION IN A NEBRASKA SOIL AND UNDERLYING SEDIMENTS

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Abstract
Degradation reduces ground water contamination from soil-mobile herbicides, but degradation rates vary among soils and with depth, Alachlor and atrazine degradation were determined in Herd silt loam (fine-silty, mixed, mesic Pachic Haplustolls) surface soil (0-15 cm), subsurface soil (45-120 cm), and underlying sediment (150-240 cm) from a terrace of the Platte River near Shelton Nebraska. Herbicide solution containing C-14-ring-labeled alachlor or atrazine was added at 100 or 1000 ng g(-1) to soil adjusted to -50 kPa water content and incubated at 22 degrees C up to 200 days. (CO2)-C-14 evolution was monitored to determine mineralization with time, and soil was extracted and combusted to determine residual C-14-labeled herbicide and bound residue formation. Respective first-order half-lives of alachlor and atrazine ranged from 8 and 11 days in surface soil to 49 and 248 days in deep soil. Soil-bound (unextractable) residue from alachlor and atrazine typically increased with time and ranged from 54 and 46% of applied C-14 in surface soil to 2 and 3% of C-14 applied in deep soil. Stepwise regression indicated that rate of degradation of both herbicides in these soils could be estimated from sorption, NH;: or NO3- + NO2- and orthophosphate content. Although atrazine is usually persistent, with little or no mineralization of the s-triazine ring, enhanced degradation was observed in two of the surface soils, where more than 60% of the herbicide was mineralized in 28 days. Lower deethylatrazine concentrations in soil exhibiting enhanced atrazine degradation suggested transformation to more labile polar degradates. A decrease in the polar degradation products and bound residue fractions as mineralization increased indicated their utilization during atrazine degradation in soil.