Multiple chemical forms of nitrogen in urban storm water make its management challenging. Three types of bioretention systems were constructed in 2015 with loamy sand as filter media, including a conventional freely drained bioretention (CB), a modified bioretetion incorporated a submerged zone (MB1), and a modified bioretention incorporating a submerged zone with woodchips addition (MB2). This study investigated the role of vegetation, the use of submerged zone and carbon addition in achieving co-optimized dissolved nitrogen removal in bioretention systems. Twelve bioretention columns were monitored over a 12-month period of dosing with synthetic storm water under varying hydrology and nitrogen loading rates. All the studied bioretention systems could achieve very good ammonia removal (more than 95%) at an average inflow ammonia concentration of (5.45±2.21) mg·L . The filter media sorption, nitrification and plants uptake were the main removal pathways for incoming ammonia. The effluent nitrate concentration