Urban runoff is a leading cause of water pollution and urban flooding. Nitrogen, a common contaminant in urban runoff, cannot be removed efficiently by current stormwater treatment best management practices. In this study, biochar was evaluated as an amendment of stormwater treatment media to enhance nitrogen removal from urban runoff and improve the hydraulic performance of stormwater treatment facilities. A commercial wood biochar pyrolyzed at 550℃ was added at 4% (w/w) to a standard bioretention medium (soil-mix: 4% saw dust, 88% sand, 8% clay). Laboratory ammonium sorption experiments demonstrated that biochar increased ammonium sorption at typical stormwater concentrations (2 mg/L) by a factor of ten, total porosity by 16.6%, and water retention at most matric potentials. Sorptive removal of nitrate by biochar in laboratory column experiments was negligible. To evaluate performance of biochar-amended media in the field, two pilot-scale bioretention cells (91 cm dia., 1.2 m deep) were constructed in parallel, one filled with a biochar-amended medium and the other with a control medium. Three synthetic storm events containing a bromide tracer and nitrate pollutant were infiltrated into each bioretention cell over a five month period. The biochar-amended cell reduced NO3-N concentrations by 30.6-84.7%, while the control cell only reduced NO3-N by -6-43.5%, depending on the storm. Data from bromide tracer tests and water content sensors installed in each cell indicate that the biochar-amended medium increased residence time by 1.6h and water retention by 15%, similar to what was predicted from laboratory experiments. The marked removal of nitrate by biochar in the field tests was not due to sorption. Further laboratory studies suggested that biochar may exhibit a battery-like behavior, storing electrons in the absence of nitrate in between storm events and subsequently released them for microbial nitrate reduction when nitrate enters during a storm. The hypothesis for the enhanced denitrification is being tested in our laboratory.