Plants such as Miscanthus and switchgrass have the potential to provide large amounts of biomass that could be used for energy and fiber products. However, little is known about the effects of growing Miscanthus on soil water and nitrogen fluxes. We grew both Miscanthus and switchgrass on tile drained silty-clay loam soils in east-central Illinois, comparing soil water content and nitrogen fluxes with conventionally grown corn (fertilized with 200 kg N/ha ammonium-nitrate) and soybeans. During a two year period, soil water content was measured several times a week (from May through October) at 10 cm intervals to a depth of 1 meter. Inorganic N fluxes (nitrate and ammonium) were assessed using resin lysimeters buried at a depth of 50 cm. Because of the high productivity and deep rooting systems of the Miscanthus (15 to 25 dry tons/ha) and switchgrass (4 to 8 tons/ha), soil water content was lower in the biomass plots compared to corn or soybeans during the growing season. However, due to a lack of rainfall during the corn year comparison, the differences were small much of the time. For soil nitrogen leaching, corn and soybean production led to much higher nitrate and ammonium flux at 50 cm (30 to 50 kg N/ha/yr for corn/soybean compared to < 1 kg/ha/yr for biomass crops). These results suggest that widespread adoption of these biomass crops on tile drained corn and soybean soils in Illinois would greatly decrease N loss, but might also reduce streamflows during low flow periods in the summer and fall.