VARIATION IN ATRAZINE MINERALIZATION RATES IN RELATION TO AGRICULTURAL MANAGEMENT PRACTICE

Atrazine (6-chloro-N-etllyl-N ''-(1-methylethyl)-1,3,5-triazine-2,4-di amine) mineralization rates were measured in surface soils with differ ent agricultural management practices at the Ohio Management Systems E valuation Area (Ohio MSEA) site in Piketon, OH, Two management practic es were studied at this site: (i) a continuous-corn (Zea maps L.) plot (CC) receiving annual application of atrazine, and (ii) a crop-rotati on plot (CR) with corn-soybean [Glycine max (L.) Merr.]-wheat (Triticu m aestivum L.) and hairy vetch [Vicia villosa (L.) Roth] with reduced use of atrazine during corn years. Soil collected from a riparian zone (RZ) near the site served as an herbicide-free reference soil, The ag ricultural site (both CC and CR) had received atrazine applications fo r about 25 yr before this study, Uniformly ring-labeled C-14-atrazine was added to the soil samples in biometer flasks and (CO2)-C-14 evolut ion was measured by scintillation counting of alkaline trapping soluti on. Sterile controls showed no evolution of (CO2)-C-14. Within 30 d of incubation at 22 +/- 2 degrees C, about 80% of the initial radioactiv ity was evolved as (CO2)-C-14 in CC soils collected during different s easons of the year. Parallel samples fi om CR showed 15 to 30% atrazin e mineralization while samples from RZ showed 3 to 7% (CO2)-C-14 evolu tion within 80 d of incubation, Prior amendment of soil samples from t he three sites with 1 mg kg(-1) atrazine accelerated subsequent minera lization measured after 90 d of incubation, indicating enhanced activi ty of indigenous microorganisms. The CC mineralization data suggest th at a single annual atrazine application sustains an active microbial c ommunity throughout the year.