Source apportionment and controlling mechanisms of nitrogen leaching from bioretention systems

Nitrogen (N) leaching has attracted concerns in bioretention systems (BRSs), while regulation of N leaching in BRSs is a challenge. In this study, N leached from BRSs with three plants (Phragmites australis, Acorus calamus L. and Cana indices L.) has for the first time been apportioned between three types of sources: immediate, fast and slow leaching based on 15N labeling. Results indicated that: fast leaching mainly comes from the previous four rainfall-runoffs, while slow leaching accounts for the largest proportion in total leaching; immediate leaching is mainly as NO3-, fast leaching mainly as NO3 - and dissolved organic nitrogen (DON), and slow leaching mainly as DON and NH4+; BRSs with different plants vary in source compositions of N leaching. Meanwhile, N fates, transformation rates, and functional genes were constrained to explain the controlling mechanisms of each source of N leaching: immediate leaching mainly relates to soil characteristics and root morphology (e.g. type, surface area and diameter) that can influence adsorption, preferential flow and plant uptake; fast leaching mainly relates to soil N transformation during the dry periods that can influence the availabilities of NO3 - and DON; slow leaching mainly relates to plant uptake and soil retention, soil N transformation and plant-soil interactions can influence the availabilities and stability of DON and NH4+. This study provides insights into the source compositions of N leaching, the long-term N legacy effect and controlling mechanisms of N leaching in BRSs-like engineering systems, which will imply future improvements in design and risk assessment of green infrastructure.