Bioretention has been widely applied to mitigate urban runoff pollution. However, its net nitrogen leaching (outflow nitrogen concentration > inflow nitrogen concentration) frequently occurs and attracts more and more attention. In this paper, the variations of nitrogen leaching under different rainfall-runoff events (overall leaching) from a field bioretention were investigated based on a two-year continuous monitoring effort. The temporal course of leaching during each rainfall-runoff event (phased leaching) was analyzed by dividing the outflow into three phases in accordance with the volumes of residual water stored in the bioretention gravel and soil layers before the rain. The results indicate that: (1) The events with overall net leaching of ammonium (NH4+), nitrate (NO3−) and dissolved organic nitrogen (DON) account for 8%, 46% and 50% of the total monitored events, respectively. Overall net leaching occurs when the event mean concentration in inflow of current rain is lower than that of previous rain and the antecedent dry period (ADP) meets a certain threshold (e.g., ADP < 70 h for NO3− leaching and ADP > 24 h for DON leaching in this study); (2) The nitrogen leaching in the initial and final phase of outflow is respectively affected by the inflow of previous rain and current rain; while the nitrogen leaching in the middle phase is jointly affected by the inflow of previous rain and current rain; in addition, most NO3− leaches in the final phase, while most DON leaches in the initial phase; (3) Denitrification and assimilation in the bioretention during drying periods decreases NO3− leaching and increases DON leaching in the initial phase of outflow, respectively. This field study thus provides pertinent information that could help accurately assess and efficiently manage the risk of nitrogen leaching from a bioretention system.

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