Long-term evidence of nitrogen removal from four decades of wetland restoration in agricultural landscapes in Denmark

Abstract

Wetland restoration is increasingly used as a tool to mitigate nitrogen losses from agricultural landscapes and reduce the risk of eutrophication in aquatic ecosystems. However, the effectiveness of this measure remains highly variable and difficult to predict. In this study, data from 35 restored wetlands in Denmark were analyzed to assess the effect of wetland restoration on nitrogen removal. The results showed that these wetlands removed an average of 110 kg N ha⁻¹ yr⁻¹ for total nitrogen and 130 kg N ha⁻¹ yr⁻¹ for nitrate, although there was considerable variation between wetland sites. On average, 39 % of the incoming nitrate load was removed and 26 % of the total nitrogen. Beta regression models showed that annual TN load explained more variability in nitrate (46 %) and total nitrogen (41 %) removal than hydraulic loading rate, which accounted for 30 % and 22 %, respectively. These findings confirm the effectiveness of restored wetlands as a valuable tool for mitigating nitrogen pollution in agricultural landscapes. However, a decline in the effectiveness of newer restoration projects, potentially due to reduced nitrogen losses from agricultural fields and less stringent eligibility criteria for restoration was observed. While national upscaling confirms the importance of wetland restoration, it also highlights the need for supplementary measures to control agricultural nitrogen losses, as current efforts may not be sufficient to achieve a good ecological status in Danish coastal waters, as required by regulations.