Enhancing Nitrogen Removal in European River Basins: The Crucial Role of Wetland Conservation and Restoration as Nature-Based Solutions

Over the past century, the nitrogen pollution problem has grown concomitantly with population growth, intensified agricultural practices, and a warming climate. In Europe, the presence of excess nitrogen in the environment has already exceeded safe planetary boundaries, posing a threat to Earths water supply and biodiversity.While considerable efforts are now focused toward mitigating this problem through increased regulatory measures on wastewater treatment plants and implementation of better agricultural management practices, there is a growing interest in the use of wetlands as nature-based solutions (NBS) to improve water quality and, in particular, to reduce nitrogen loading to downstream water.Despite these benefits, wetlands are among the most degraded ecosystem in Europe, having experienced significant shrinkage over the past centuries, now constituting only one-third of their 1700 extent. This decline is largely attributed to agricultural expansion on drained productive wetland soils, while also contributing to increased nitrogen pollution from excess use of fertilizers.To address these issues, starting from nitrogen surplus data and current wetland extent at European scale we estimate with a physically-based model that current removal potential of wetlands is about 1113 101 kt of N per year (~6.5% of total N surplus in European soil). The significance of wetlands in water quality remediation is underscored by the fact that this nitrogen would otherwise enter the river network and, subsequently, the sea. Given that the current riverine loads in EU watersheds amount to about 2730 kt N per year, the loss of current wetlands would increase this figure by over 40%, with detrimental consequences for the status of surface waters and the eutrophication of coastal areas.We propose a set of restoration scenarios, along with the associated costs, for the restoration of wetlands that have been drained for agricultural purposes. Our analysis aligns with the objective of the Nature Restoration Law, requiring EU member states to implement effective restoration measures to cover at least 20% of the EUs land and sea areas by 2030. We show that by restoring 2.6% of EU land (equivalent to 20% of historical wetlands), we could nearly double the current nitrogen uptake (2108 187 kt of N per year), and significantly improve riverine water quality by reducing more than 30% of their loads to the sea. In addition, wetland restoration will offer a wide array of ecosystem co-benefits from flood prevention and carbon sequestration to provision of critical habitat for specialized flora and fauna.