Floodplains remove nitrate from rivers through denitrification and thus improve water quality. The Danube River Basin (DRB) has been affected by elevated nitrate concentrations and a massive loss of intact floodplains and the ecosystem services they provide. Restoration measures intend to secure and improve these valuable ecosystem services, including nitrate removal. Our study provides the first large-scale estimate of the function of large active floodplains in the DRB to remove riverine nitrate and assesses the contribution of reconnection measures. We applied a nutrient emission model in 6 river systems and coupled it with denitrification and flooding models which we adapted to floodplains. The floodplains have the capacity to eliminate about 33,200 t nitrate-N annually, which corresponds to 6.5 % of the total nitrogen emissions in the DRB. More nitrate is removed in-stream at regular flow conditions than in floodplain soils during floods. However, increasing frequently inundated floodplain areas reveals greater potential for improvement than increasing the channel network. In total, we estimate that 14.5 % more nitrate can be removed in reconnected floodplains. The largest share of nitrogen emissions is retained in the Yantra and Tisza floodplains, where reconnections are expected to have the greatest impact on water quality. In absolute numbers, the floodplains of the lower Danube convert the greatest quantities of nitrate, driven by the high input loads. These estimates are subject to uncertainties due to the heterogeneity of the available input data. Still, our results are within the range of similar studies. Reconnections of large floodplains in the DRB can, thus, make a distinct contribution to improving water quality. A better representation of the spatial configuration of water quality functions and the effect of floodplain reconnections may support the strategic planning of such to achieve multiple benefits and environmental targets.

Nitrate pollution in aquatic ecosystems is still a major problem in Germany. There is a great potential to permanently remove nitrate from aquatic systems through denitrification as a relevant ecosystem function. However, the controlling factors and the dimension of the denitrification potential are still not fully understood due to the high complexity of the process. This study presents the combined assessment of potential soil denitrification rates, physical and chemical soil parameters, and hydrological parameters from six floodplains of four large German rivers, namely the Rhine, the Elbe, the Weser, and the Main. Based on multivariate statistics, results show that the denitrification potential of soil was almost solely controlled by soil pH. The lab assays showed mean soil denitrification potentials of 6.4–11.4 mg N m⁻²h⁻¹(pH < 7) and 23.0–30.5 mg N m⁻²h⁻¹(pH > 7). We contend that when upscaling these estimates to annual rates of potential denitrification, the duration of average inundation should be incorporated, as this accounts for water saturation and nutrient supply − the major controlling variables for denitrification. Results provide evidence that the denitrification potential can only be fully exploited in frequently inundated floodplains. Thus, despite favorable soil conditions for denitrification, floodplains that have suffered from anthropogenic impacts, lose their importance in nitrate removal for the river system. We conclude that pH and lateral hydrological connectivity are likely to be key factors that should be considered when estimating denitrification as an ecosystem function.

Although European coastal and riverine wetlands occupy a small share of the European land area, these ecosystems host remarkable biodiversity and provide a wide range of ecosystem services fundamental for human well-being, including clean water, recreation, food provision and climate regulation. Synthesising current knowledge across biogeographic regions, this review shows that only 12% of EU riverine and coastal wetland habitat types are in favourable condition, while the majority continue to deteriorate, mirrored by substantial declines in many wetland-dependent species. Climate-related impacts vary strongly across Europe and interact with existing human pressures such as land-use change, hydromorphological alteration, hydropower, coastal engineering and pollution, accelerating habitat and species loss and reducing the resilience of wetlands to climate change impacts. The paper calls for a rethinking of coastal and riverine wetlands as multifunctional landscapes and stresses the need for coordinated, cross-sectoral policy approaches. These wetlands are positioned not only as vulnerable systems but as pivotal Nature-based Solutions for achieving climate resilience and halting biodiversity loss, while remaining essential resources for human life support. Highlights Remaining European riverine and coastal wetlands are small and fragmented, yet vital for biodiversity and climate resilience. Only 12% of EU riverine and coastal wetland habitat types remain in favourable conservation status. Climate impacts vary regionally and interact with multiple pressures, demanding tailored adaptation strategies. Wetland multifunctionality makes them key Nature-based Solutions. Riverine and coastal wetlands must move from the policy periphery to the core of climate-biodiversity action, enabled by an integrated, cross-sectoral restoration and conservation effort.

Floodplains and coastal wetlands provide essential ecosystem services such as flood protection, water quality enhancement, carbon sequestration, and the production of raw materials and food. Despite numerous studies on wetland restoration, the complexities of restoring ecosystem functions remain challenging. Complete restoration of natural functions is rare, and therefore seldom the primary target; instead, restoration efforts typically focus on reactivating key ecological processes. This study examines the restoration of ecosystem functions in floodplains and coastal wetlands, focusing on water quality improvement. It summarizes both nitrogen and phosphorus dynamics, highlighting the potential to restore processes like denitrification and sedimentation within relatively short time frames using selected case studies from Central Europe. However, restoration success is hindered by altered soil properties, nutrient legacy, and other degradation-related constraints. Ultimately, the effectiveness of restoration endeavors depends on site-specific factors such as wetland degradation, size, hydrological connectivity, plant recovery, and ongoing global changes. This underscores the importance of considering both spatial and temporal dimensions and adopting a custom-tailored restoration strategy. Highlights Recovery of nutrient retention processes can be either rapid or delayed Nutrient legacies and soil degradation hinder ecosystem restoration. Process insights and site traits guide effective restoration efforts.

Denitrification in floodplain soils is one key process that determines the buffering capacity of riparian zones in terms of diffuse nitrate pollution. One widely used approach to measure the denitrification potential is the acetylene inhibition technique that requires fresh soil samples. We conducted experiments with air-dried soils using a time series analysis to determine the optimal rewetting period. Thus, air-dried soil samples from six different floodplain areas in Germany were rewetted for 1 to 13days to 100% water-filled pore space. We analyzed nitrogen accumulated as N₂O in the top of anaerobic flasks with and without acetylene by gas chromatography after four hours of incubation. We observed an overall optimal rewetting of at least seven days for complete denitrification. We also saw the strong influence of pH and field capacity on the denitrification product ratio; in soils with pH < 7, we hardly assumed complete denitrification, whereas the treatments with pH > 7 achieved stable values after seven days of rewetting. This advanced method provides the opportunity to carry out campaigns with large soil sample sizes on the landscape scale, as samples can be stored dry until measurements are taken.

Excess nitrogen (N) from agricultural sources is a major contributor to the water pollution of rivers in Europe. Floodplains are of tremendous importance as they can permanently remove nitrate (NO3) from the environment by releasing reactive N to the atmosphere in its gaseous forms (N2O, N-2) during denitrification. However, the quantitative assessment of this ecosystem function is still challenging, particularly on the national level. In this study, we modeled the potential of NO3-N removal through microbial denitrification in soils of the active floodplains of the river Elbe and river Rhine in Germany. We combined laboratory measurements of soil denitrification potentials with straightforward modelling data, covering the average inundation duration from six study areas, to improve an existing Germany-wide proxy-based approach (PBAe) on NO3-N retention potential. The PBAe estimates this potential to be 30-150 kg NO3-N ha(-1) yr(-1). However, with soil pH and Floodplain Status Category identified as essential parameters for the proxies, the improved PBA (PBAi) yields a removal potential of 5-480 kg N ha(-1) yr(-1). To account for these parameters, we applied scaling factors using a bonus-malus system with a base value of 10-120 N ha(-1) yr(-1). Upscaling the determined proxies of the PBAi to the entire active floodplains of the river Elbe and river Rhine results in similarly high NO3-N retention sums of similar to 7000 t yr(-1) in spite of very different retention area sizes, strengthening the argument for area availability as the primary objective of restoration efforts. Although PBAs are always subject to uncertainty, the PBAi enables a more differentiated spatial quantification of denitrification because local key controlling parameters are included. Hence, the PBAi is an innovative and robust approach to quantify denitrification in floodplain soils, supporting a better assessment of ecosystem services for decision-making on floodplain restoration.