The behaviors of inorganic nitrogen species in three types of bioretention columns under an intermittently wetting regime were investigated. The mean NH4+—N, NO3−—N and total N (TN) removal efficiencies for the conventional bioretention column (Col. T1) are 71%, 1% and 41%, for layered bioretention column with less permeable soil layer (Col. T2) the efficiencies are 83%, 84% and 82%, and for the bioretention column with submerged zone (Col. T3) the values are 63%, 31% and 53%, respectively. The best nitrogen removal is obtained using Col. T2 with relatively low infiltration rate. Adsorption during runoff dosing and nitrification during the drying period are the primary NH4+—N removal pathways. Less permeable soil and the elevated outlet promote the formation of anoxic conditions. 30%–70% of NO3−—N applied to columns in a single repetition is denitrified during the draining period, suggesting that the draining period is an important timeframe for the removal of NO3−—N. Infiltration rate controls the contact time with media during the draining periods, greatly influencing the NO3−—N removal effects. Bioretention systems with infiltration rate ranging from 3 to 7 cm/h have a great potential to remove NO3−—N.
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