Modelers and regulatory agencies typically use default atrazine half-life values of 60 to 120 d to predict the herbicide's transport; however, if atrazine persistence is reduced in soils exhibiting enhanced degradation, but modelers continue to use historic atrazine persistence estimates, then accurate prediction of atrazine transport in adapted soils, i.e. soils exhibiting enhanced degradation, is unlikely. The objectives of this study were to 1) screen Colorado (CO) and Mississippi (MS) atrazine-adapted and non-adapted soil for atzABC and trzN genes; 2) compare Q10 and â between adapted and non-adapted soils; and 3) compare metabolite profiles between adapted and non-adapted soils. The atzABC and (or) trzN genes were detected only in adapted soils. Atrazine's average half life in adapted soil was 10-fold lower than that of non-adapted soil and 18-fold lower than USEPA's estimate of 3 to 4 months. There is a 90% probability that Q10 is greater in adapted than non-adapted soil, and concentrations of mono-N-dealkylated metabolites of atrazine were rarely above the limit of detection in adapted soils. Environmental and agronomic implications of this study include 1) enhanced atrazine degradation and atzABC and (or) trzN genes are likely widespread across the Western and Southern corn growing regions of the United States; 2) due to the substrate specificity of the enzymes coded by atzABC and (or) trzN, concentrations of mono-N-dealkylated metabolites of atrazine will likely be lower in adapted than non adapted soil; 3) fate, transport, and risk assessment models that use historic atrazine persistence estimates as default input parameters will likely over-predict the herbicide's transport in adapted soils, and 4) soils positive for atzA and (or) trzN will likely be cross-adapted with all commercially available triazine herbicides potentially resulting in a loss of residual weed control.