Long-Term Trends in Nitrogen Removal by an Aridland Constructed Treatment Wetland

Cities are increasingly pursuing more sustainable and resilient infrastructure. The increased use of Urban Ecological Infrastructure (UEI), including constructed treatment wetlands (CTW), may be particularly important for aridland cities with scarce water resources. In this paper we document eight years of nitrogen (N) dynamics in an aridland CTW in Phoenix, Arizona, USA, where N removal must be balanced by the trade-off of atmospheric water losses. We have documented a "biological tide", wherein transpiration-driven water loss is actively replaced by a slow movement of surface water into the marsh from adjacent open water areas. Our analysis combined long-term water budget data with nitrogen budgets for the vegetated marsh and the entire CTW system. The objective was to demonstrate how the biological tide enhanced N uptake in this aridland CTW. We attributed roughly 50% of the annual N uptake by the vegetated marsh to new water entering via the biological tide. Thus, while it seems counter-intuitive to design aridland CTWs to optimize transpirational water losses, our data suggested that careful design of the plant community and spatial configuration of vegetated marsh versus open water may enhance both the biological tide and N removal efficiency.