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 Hord 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 14C-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. 14CO2 evolution was monitored to determine mineralization with time, and soil was extracted and combusted to determine residual 14C-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 14C in surface soil to 2 and 3% of 14C applied in deep soil. Stepwise regression indicated that rate of degradation of both herbicides in these soils could be estimated from sorption, NH+4 or NO-3 + NO-2, 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.