Combined use of cold-season and warm-season macrophytes in floating constructed wetlands to increase nitrogen removal in the early cold season

To synchronously decrease the risk of secondary pollution from decomposing litter of the widely planted warm-season macrophytes (WSMs) and improve nitrogen (N) removal in the early cold season, a strategy of planting both cold-season macrophytes (CSMs) and WSMs (in the form of litter, WSML) in a single unit of constructed wetlands (CWs) was tested in pilot-scale floating CWs. The interaction between CSMs and WSML as well as the pathway of N removal was examined. CSMs stimulated carbon source release but inhibited N and phosphorus (P) release from WSML, while WSML inhibited CSM growth, resulting in a decrease in N and P absorption by CSMs. The average removal efficiencies (REs) of total nitrogen (TN), NH4+-N and NO3--N were 2.0%, 8.8% and 14.9% for the WSML-only treatment and 25.8%, 90.3% and 25.7% for the CSM-only treatment, respectively. In contrast, the combined CSM and WSML treatment obtained average REs of 44.8%, 94.7% and 48.9% for TN, NH4+-N and NO3--N, respectively. WSML was not only the source of NH4+-N but also the possible pathway of its removal by adsorption. CSM absorption is an important pathway for NH4+-N and NO3--N removal, especially the former. Therefore, planting both CSMs and WSMs in a single unit is recommended to reduce the risk from the current overreliance on WSMs for most CWs. (C) 2020 Elsevier Ltd. All rights reserved.