Bioavailability and desorption characteristics of aged, nonextractable atrazine in soil

The bioavailability and desorption characteristics of nonextractable soil-a ged atrazine were investigated to examine the significance of this potentia lly large contaminant fraction in agricultural soils. Radiolabeled atrazine -applied soil was aged for 3 months and then extracted by one of three meth ods-methanol-water (4:1) at 75 degrees C, acetone, and artificial soil wate r-representing a range of extraction efficiencies. Viable microcosms were c onstructed with the dried, extracted soil to compare rates and extents of m icrobially facilitated release and mineralization with desorption rates in sterile microcosms. The most rigorously extracted soil exhibited slow desor ption over a 90-d period, but the other soil treatments showed continued sl ow sorption after reaching an initial maximum desorption concentration with in 1 to 5 d. No microbially facilitated release of the nonextractable atraz ine was detected for any of the treatments. Rates of ring-and chain-labeled atrazine mineralization declined with decreasing extractability of the soi l-associated atrazine fraction. However, the extent of biodegradation (less than 2% of the total atrazine for all extracted soil treatments) was less than the extent of abiotic desorption. Mass-balance calculations suggest th at all biodegradation occurred in the dissolved phase.