transport and fate of atrazine in soil of three-, five-, and nine-year-old switchgrass (Panicum virgatum L.) RBS to that in adjacent soils cropped to a corn-soybean rotation or a grass-alfalfa pasture. Undisturbed soil columns were collected from the RBS and cropped areas within the Bear Creek watershed, near Roland, Iowa. Atrazine and bromide breakthrough curves obtained using intact soil columns under saturated conditions were described by a two-region, mobile-immobile transport model. Preferential flow of bromide and atrazine was evident in five-and nine-year-old RBS soil, but there was little difference in transport characteristics between these two RBS soils and the adjacent cropped soils. There was a trend towards an increase in dispersion coefficients between the five-and nine-year-old RBS sites, which suggests an increased degree of preferential flow with increasing RBS age. Despite similar texture and organic C contents, atrazine sorption was significantly greater in RBS soil than the adjacent cropped soil. Cropped soil degraded atrazine faster than the RBS soil. The rapid degradation of atrazine in the corn-soybean soil adjacent to the five-year-old RBS (atrazine half-life of 19 days) appeared to be due to a larger population of atrazine-degrading microorganisms. Atrazine-degrading microorganisms in the corn-soybean soil were 50,940 cells g^-1 soil compared with 2,970 cells g^-4 soil in 5-year-old RBS soil which resulted in 60 percent mineralization of [¹⁴C-UL-atrazine] in the corn-soybean soil.