Leaching sources and mechanisms of different nitrogen species from bioretention systems.

At present, nitrogen (N) leaching from bioretention systems (BRSs) has become a key issue, imposing constraints on their application, a consequence of N dynamics of both inflow and legacy N at different time scales. In this study, the distinct sources (IL: immediate leaching, FL: fast leaching, SL: slow leaching) and the principal transformation processes of different N species (i.e., NH4+, NO3- and DON) leaching originating from inflow and legacy of BRSs were firstly unveiled by various 15N species labeling (i.e., 15N-NH4+, 15N-NO3- and 15N-DON). Results indicate that: NH4+ leaching was primarily caused by FL from influent organic N and SL from influent NH4+, with mineralization being the main transformation process influencing NH4+ leaching; NO3- leaching primarily originated from SL, with the major proportion attributed to the influent organic N in SL, autotrophic and heterotrophic nitrification were the main influencing factors; DON leaching primarily originated from SL, with similar proportions coming from influent organic N, NH4+, and NO3-, inorganic N assimilation was the principal transformation process affecting DON leaching. This study provides an effective framework for apportioning the leaching sources of different N species, providing valuable insights for the implementation of both inflow and legacy N leaching control measures.