Nitrate in water leaving subsurface drain (‘tile’) systems often exceeds the 10 mg-N L^-1 maximum contaminant level (MCL) set by the U.S. EPA for drinking water and has been implicated in contributing to the hypoxia problem within the Gulf of Mexico. Much of the NO₃ from agricultural lands impacting surface waters within the Midwest cornbelt is from subsurface field drainage. Because previous research shows that N fertilizer management alone is not sufficient for reducing NO₃ concentrations in subsurface drainage below the MCL, additional approaches need to be devised. We compare the efficacy of an in-field bioreactor - trenches excavated parallel to the tile and backfilled with wood chips as a carbon source to increase denitrification for removing NO₃ from tile drainage versus the NO₃ concentration in tile drainage from a control treatment (CK) consisting of a free-flowing tile installed at 1.2 m below the surface. Four replicate 30.5 x 42.7-m field plots were installed for each treatment in 1999 and a corn/soybean rotation initiated in 2000. For 2001 - 2005, the tile flow from the bioreactor plots had annual average NO₃ concentrations significantly lower than tile flow from the control treatment. Average flow-weighted NO₃ concentration in tile drainage from the control was about 22 mg-N L^-1 compared with less than 8.5 mg-N L^-1 for the bioreactor treatment. This represented an annual reduction in NO₃ mass loss of 33 kg-N ha^-1 for the bioreactor treatment.