The effect of water velocity on nitrate removal in vegetated waterways

The extended networks of canals and ditches in agricultural landscapes provide high buffer capacity towards nitrogen (N) excess. Their N mitigation potential depends on several biotic and abiotic factors, among which water velocity is poorly explored and generally omitted from the parameterization of this remarkable ecosystem service. The present work reports new insights on the role of flow velocity in regulating N removal via denitrification in sediments colonized by Phragmites australis. Denitrification was investigated in outdoor mesocosms in the presence and absence of P. australis and over a small range of flow velocity (0-6 cm s^-1) typical of low-gradient water bodies. Simultaneous measurements of NO₃^- consumption and N₂ production based on analyses of N₂:Ar by Membrane Inlet Mass Spectrometry were undertaken. Vegetated sediments were found more efficient in converting NO₃^- to N₂ via microbial-mediated denitrification (27-233 mmol N m^-2 d^-1) than bare sediments (18-33 mmol N m^-2 d^-1). Vegetation provides multiple interfaces, i.e. in the rhizosphere and on epiphytic biofilms, that support the development and activity of bacterial communities responsible for NO₃^- dissipation. NO₃^- removal and denitrification rates exhibited one order of magnitude raise when water velocity passed from 0 to 6 cm s^-1 in vegetated sediments. Indeed, in slow-flow vegetated waterways denitrification may be physically limited and the increase of water velocity enhances the rate of NO₃^- supply through the diffusive boundary layer, thereby promoting its consumption and loss from the system. Water velocity should be taken into account as a key factor for management and restoration actions aimed at maximizing the NO₃^- buffer capacity of low-flow drainage networks.