Nitrogen pollution is among the most difficult to mitigate via bioretention, particularly for dissolved species like NO3−-NO2− (NOx). Bioretention soil mix (BSM) amended with compost often leaches nitrogen, making the system a nitrogen source rather than a sink. Several design modifications have been proposed to enhance nitrogen removal pathways like microbial denitrification by creating anoxic zones, but studies report varied removal rates. In this study, I evaluated nitrogen removal in bioretention systems equipped with orifice-outlet controls. While these controls primarily reduce outflow rates, it is expected that nitrogen removal would increase due to extended hydraulic residence time. The study consisted of six field-scale bioretention mesocosms with two outlet configurations (orifice/standard) and three bioretention soil mix (BSM) types (New, Mature, and Alternative). Six synthetic storms were conducted to measure total Kjeldahl nitrogen (TKN) and NOx removal performance. Continuous flow monitoring characterized the orifice-outlet performance in response to natural storm events and salt-pulse tracer testing measured the mean residence time. Additionally, soil samples were measured for TN, TOC, pH, C:N, NH4+ and NO3− and analyzed with qPCR for target genes nirK and nirS, which indicate denitrification activity. Orifice-outlets significantly increased residence times and decreased NOx effluent concentrations compared to standard-outlets. However, they also increased TKN effluent concentrations, resulting in no net benefit from a TN reduction perspective. Sizing calculations conservatively estimated the hydraulic capacity of the mesocosms, leading to orifice-engagement for only 16% of the natural storm events. Leaching in the New BSM was significantly more than in the Mature, but this gradually reduced even within this study of six storms. The Alternative BSM had the lowest effluent concentrations and therefore the best performance, but plant growth was severely limited. nirK abundances were only detected in the Alternative BSM, however results were meager compared to other studies, suggesting denitrification plays a minor role in nitrogen removal from these mesocosms.