Wetland ecosystems have long been recognized as vital components in maintaining ecological balance, particularly for their role in nutrient cycling and water purification. Recent comprehensive research illuminates an often underappreciated dimension of Europe’s wetlands: their critical function in nitrogen removal across river basins. Despite centuries of wetland loss primarily due to agriculture and urban development, scientists now underscore the urgent need to restore these wetlands to combat escalating nitrogen pollution threatening aquatic environments across the continent.
Historical analysis reveals that since the mid-19th century, vast areas of wetlands in Europe have been drained, predominantly to facilitate agricultural expansion. Approximately three-quarters of this loss is attributable to conversion into cropland, followed by forestry and pasture lands. This dramatic reduction in wetland cover not only diminishes biodiversity but severely curtails the natural capacity of landscapes to intercept and process excess nitrogen—a nutrient whose unchecked influx causes eutrophication in water bodies, leading to oxygen depletion and loss of aquatic life.
Building on meticulous spatial mapping, researchers have integrated data from multiple European-scale sources to quantify current wetland distribution and estimate the nitrogen removal potential of both extant and restorable wetlands. The foundation of this approach relies on the Extended Wetland Ecosystem Map developed by the European Commission, which captures a broad and hydro-ecologically informed classification scheme. This includes not just classical wetlands such as marshes, lakes, and estuaries but also seasonally flooded forests, grasslands, and agricultural wetlands like rice paddies, reflecting the complexity of wetland types under the Ramsar Convention definition.
The complexity of nitrogen removal mechanisms within wetlands defies simple characterization owing to their dependence on multiple interacting factors. These include the rates and forms of nitrogen inputs, wetland size and type, water residence times, microbial activity, and environmental conditions such as temperature and substrate availability. To navigate this complexity, researchers have employed a reduced complexity modeling framework calibrated with extensive empirical data from hundreds of wetlands globally, including a significant subset from Europe. This model allows for the estimation of nitrogen removal through first-order kinetics, linking removal efficiency to readily measurable wetland attributes like surface area and hydrological residence time.
Key to scaling these calculations is the estimation of nitrogen inputs to wetlands, which varies according to their position within river basins and associated land use in their contributing catchments. By integrating nitrogen surplus data—representing the excess nitrogen remaining after accounting for agricultural outputs and natural inputs—researchers can approximate the mass of nitrogen entering each wetland. An important parameter here is a reduction factor that represents losses through soil retention and upstream denitrification, modeled probabilistically to account for uncertainties.
The spatial resolution of the analyses leverages the HYBAS-7 watershed delineation framework, encompassing millions of wetlands and enabling nitrogen removal estimates aggregated at multiple hydrologic scales. Monte Carlo simulations are applied to propagate uncertainties in parameters such as rate constants, water residence times, catchment sizes, and nitrogen input fractions, yielding statistically robust estimates of nitrogen removal at basin and sub-basin levels.
Alongside evaluating current wetland contributions, the study models prospective restoration scenarios to guide policy and conservation efforts. Three distinct strategies shape the restoration potential: First, a Full Restoration scenario that assumes recovering all agricultural lands converted from wetlands since the 1850s; second, a CAP 4% scenario aligning restoration extent with the European Union’s Common Agricultural Policy target of dedicating 4% of arable land to non-productive features; and third, a Farmland Abandonment scenario leveraging projections of agricultural land set to be abandoned by 2040, thus minimizing conflicts with food production.
The restoration algorithm prioritizes areas with higher nitrogen surplus, aiming to allocate restoration efforts where they yield maximal nitrogen removal benefits. This spatially explicit approach ensures that restoration actions align with land-use legacies and nutrient pollution hotspots. However, limitations remain regarding hydrological connectivity among wetlands, which can modulate downstream nitrogen loads in ways not fully captured by the current modeling framework.
Nitrogen load data utilized to calibrate and validate nitrogen removal estimates derive from the well-established GREEN model, which simulates nutrient fluxes in European river basins with high spatial and temporal fidelity. Despite not explicitly modeling wetland retention, the GREEN output provides a reliable benchmark for evaluating restoration impact by reflecting observed riverine nitrogen levels post-retention in soils, rivers, and lakes.
Economic considerations form a crucial component in translating ecological insights into actionable policy recommendations. Restoration costs vary widely depending on wetland type and geographic context, but systematic reviews offer comprehensive cost estimates which, when combined with spatially modeled restoration extents, enable cost-benefit analyses. Benefits are framed in terms of ecosystem service valuations, incorporating crucial regulating functions such as flood control, water provision, and nutrient recycling. Meta-analyses suggest that the economic value of these services can significantly exceed restoration costs, providing strong financial incentives for investment in wetland revival.
The imperative for peatland restoration receives specific attention due to their disproportionate role in carbon storage and nitrogen cycling. The EU’s Nature Restoration Law (NRL) lays out ambitious targets to restore poor-condition habitats, including drained peatlands used for agriculture. The study’s modeling efforts provide baseline estimates of peatland restoration potential within these broader wetland scenarios, highlighting both opportunities and data limitations in capturing historical peatland losses.
This research not only illuminates the multifaceted benefits of wetlands for nitrogen management but also stresses their pivotal role in advancing Europe’s climate, biodiversity, and water quality goals under the Green Deal and associated policy frameworks. By quantifying restoration potentials and linking them with nutrient load reduction targets from international sea conventions, the study underscores the strategic value of wetlands in safeguarding aquatic ecosystems and coastal waters.
A critical takeaway is the nuanced understanding of wetland restoration constraints given competing land uses. By coupling restoration scenarios to existing agricultural fabric and political targets, the study presents realistic pathways for integrating ecological restoration without undermining food security and socio-economic stability. This balance will be essential as Europe navigates complex trade-offs in its environmental and agricultural policies moving forward.
Furthermore, the study’s methodological advancements in combining high-resolution wetland mapping, hydrological modeling, nitrogen budgeting, and scenario analysis represent an important step toward operationalizing wetland restoration as an ecosystem-based solution to nutrient pollution. The transparent incorporation of uncertainty and the use of Monte Carlo simulations enhance the reliability and policy relevance of the findings.
Future research avenues highlighted by the authors include refining models to incorporate temperature-dependent microbial processes, improved delineation of wetland contributing catchments, and accounting for hydrological connectivity at network scales. Such enhancements will bolster predictive capabilities and inform targeted restoration planning at fine geographic scales.
In summary, this comprehensive assessment advances the scientific and policy discourse on wetland conservation’s tangible climate and water quality benefits. It strongly supports leveraging wetland restoration as a cost-effective, multifunctional strategy to reduce nitrogen pollution across Europe’s watersheds, thereby promoting resilient aquatic ecosystems and sustainable development.
Subject of Research:
Wetland conservation and restoration for nitrogen removal in European river basins.
Article Title:
The important role of wetland conservation and restoration in nitrogen removal across European river basins.
Article References:
Bertassello, L.E., Basu, N.B., Maes, J. et al. The important role of wetland conservation and restoration in nitrogen removal across European river basins. Nat Water 3, 867–880 (2025). https://doi.org/10.1038/s44221-025-00465-0
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