Soilborne bacteria with novel metabolic abilities have been linked with enhanced atrazine degradation and complaints of reduced residual weed control in soils with an s-triazine use history. However, no field study has verified that enhanced degradation reduces atrazine's residual weed control. The objectives of this study were to (1) compare atrazine persistence and prickly sida density in s-triazine-adapted and nonadapted field sites at two planting dates; (2) utilize original and published data to construct a diagnostic test for identifying s-triazine-adapted soils; and (3) develop and validate an s-triazine persistence model based on data generated from the diagnostic test, i.e., mineralization of ring-labeled ¹⁴C-s-triazine. Atrazine half-life values in s-triazine-adapted soil were at least 1.4-fold lower than nonadapted soil and 5-fold lower than historic estimates (60 d). At both planting dates atrazine reduced prickly sida density in the nonadapted soils (P ≤ 0.0091). Conversely, in the s-triazine-adapted soil, prickly sida density was not different between no atrazine PRE and atrazine PRE at the March 15 planting date (P  =  0.1397). A lack of significance in this contrast signifies that enhanced degradation can reduce atrazine's residual control of sensitive weed species. Analyses of published data indicate that cumulative mineralization in excess of 50% of C₀ after 30 d of incubation is diagnostic for enhanced s-triazine degradation. An s-triazine persistence model was developed and validated; model predictions for atrazine persistence under field conditions were within the 95% confidence intervals of observed values. Results indicate that enhanced atrazine degradation can decrease the herbicide's persistence and residual activity; however, coupling the diagnostic test with the persistence model could enable weed scientists to identify s-triazine-adapted soils, predict herbicide persistence under field conditions, and implement alternative weed control strategies in affected areas if warranted.

Nomenclature: Atrazine; prickly sida, Sida spinosa L.