Can wetlands designed for flood attenuation efficiently remove nitrogen from agricultural runoff?

Eutrophication and flood risk are pressing issues of ecological and societal relevance. A key driver of eutrophication is the use of nitrogen (N) fertiliser in agriculture, resulting in N exports from land to water. Climate change increases the risk of flood events which can increase N exports and further worsen eutrophication. Created wetlands in agricultural areas are recognised as effective nutrient sinks, and the flood attenuating capabilities of wetlands are well established. However, the combination of these two ecosystem services in agricultural landscapes is understudied. This study examines how water flow buffering in created wetlands affects N removal and greenhouse gas (GHG) emissions. The study was performed in experimental wetlands of different designs (depth and size) subjected to intermittent or permanent flooding (gradual emptying between inflow events or a constant water level). Intermittently flooded wetlands removed less N than permanently flooded wetlands, especially during no-flow periods. The largest difference was found among deep wetlands, where the average N removal across the study was 0.19 g m^-2 d^-1 (25 %) in intermittently flooded wetlands and 0.27 g m^-2 d^-1 (38 %) in permanently flooded wetlands. No differences in the aqueous concentrations of N₂O or CH₄ were observed between wetlands with high or low water storage capacity, thus indicating similar emissions. This study highlights a risk of lowered N removal in created wetlands designed for flood attenuation. Further studies are needed on synergies and trade-offs when aiming to combine flood attenuation and N removal in created wetlands.