Restoring wetlands strategically could reduce nitrogen flowing to the sea with minimal impact on agriculture

Nitrogen is a leading pollutant in European surface waters, posing a significant risk to ecosystems and public health. A nutrient which causes eutrophication in aquatic ecosystems, its chief sources are chemical fertilisers, livestock manure, wastewater, and deposition of nitrogen from our atmosphere¹. The impacts are widespread and severe, causing coastal dead zones, algal blooms, loss of biodiversity, and contributing to rising costs for water treatment when polluted water bodies are deemed unsafe for use as drinking water.

There is a pressing need to reduce the amount of nitrogen entering inland and coastal waters across the EU. At EU level, a third of rivers, lakes, coastal and transitional waters, and 81% of marine waters were reported as eutrophic in the last report on the implementation of the Nitrates Directive. This is one of numerous directives protecting water across Europe, with specific frameworks focusing on urban wastewater, marine strategy, and inland, transitional, coastal surface, and groundwaters. European seas also fall under various international conventions that mandate the protection of the marine environment from threats such as pollution.

Nature-based solutions (NBSs) – fostered by the European Green Deal – offer a potential way to combat polluting nutrient loss from European soils while maintaining benefits of soil fertility for agriculture. Wetlands are one such NBS, which act as natural water filters, removing excess nutrients and protecting downstream environments. Studies of individual wetlands and catchments in Europe, the US and China have suggested wetlands are effective at retaining nitrogen, offering benefits for water quality downstream.

A new study now provides a Europe-wide perspective. It explores the role of wetlands in removing nitrogen from river basins across the EU, also assessing how conservation and restoration of wetlands – many of which have been drained, dredged, or filled for agricultural or urban development over many decades – could help meet water quality targets. The researchers look at how much nitrogen is removed by existing wetlands in European river basins; how their loss might impact nitrogen loads; and how managers can strategically restore wetlands to remove more nitrogen and improve water quality.

The study drew on datasets on nitrogen surplus and mapping of 2.6 million wetlands in the 27 EU Member States plus Albania, Bosnia and Herzegovina, Macedonia, Norway, Switzerland, Serbia and the United Kingdom. The researchers then developed a range of scenarios for wetland restoration that balanced the competing objectives of water quality improvement and agricultural sustainability.

The modelling shows that existing European wetlands intercept 1,092 kilotonnes of nitrogen per year, representing 6.7% of current landscape-level nitrogen surplus (the difference between nitrogen inputs and outputs – the amount of excess nitrogen that could pollute waterways or accumulate in soil). Given the average levels of nitrogen discharged into European seas by rivers (4,239 kilotonnes per year in 2014–2018), this amount suggests that riverine nitrogen loads to the sea could be 25% higher in the absence of existing wetlands. The researchers estimate that, without current wetlands, nitrogen loads into the Danube, Elbe, Wisla and Oder river basins could increase by 20%, 29%, 33% and 38%, respectively.

The study’s spatial mapping shows a correlation between wetland loss and nitrogen surplus hotspots, especially in areas where agriculture has become more intensified (thereby increasing fertiliser use, irrigation, and livestock density). While there remains substantial variability across the EU depending on land-use and agricultural practices, this highlights the importance of wetland restoration in such hotspots. The regions with the highest nitrogen removal rates were also areas of high wetland density, such as parts of the United Kingdom, Netherlands, Ireland, and Belgium; the lowest removal rates were seen in southern Europe, where both wetland density and nitrogen surplus were comparatively low.

When exploring wetland conservation scenarios, the study concludes that wetland restoration is a cost-effective nature-based solution. Restoring 27% of wetlands historically drained for agriculture, targeted in high nitrogen input areas, could reduce nitrogen loads into European seas by up to 36%, they found - but with potential costs for agricultural productivity. A more efficient strategy would be to focus on wetland restoration on farmland projected to be abandoned by 2040, yielding a 22% load reduction and enabling major rivers such as the Rhine, Elbe and Vistula to meet water quality targets with minimal agricultural impact.

Results vary by river basin; for instance, restoring wetlands on 4% of arable land is enough to meet full water quality targets in the Elbe and Oder basins, but only 70% of the target in the Wisla, Rhine and Meuse basins. Achieving good ecological status would therefore require additional measures to reduce sources of nitrogen – such as minimising fertiliser use and promoting good agricultural practices (both priorities of the European Commission’s Zero Pollution Action Plan).

Achieving a reduction in nitrogen load to the sea by 36% across the EU (the most ambitious scenario above) could be done by restoring wetlands on 3.2% of the total land area of the EU, the analysis finds – which would also contribute to the target of restoring ecosystems in 20% of EU land and sea area by 2030, as laid out in the European Commission’s Nature Restoration Regulation (NRR). In areas without especially high nitrogen surpluses, just 2.4-7.4% of drained peatlands would need restoration to meet water quality goals, the study indicates.

The researchers conclude that safeguarding existing wetlands is fundamental to current water quality and provides a foundation for future restoration efforts. Then, such efforts must be spatially targeted within specific river basins to achieve targets – and different environmental goals – most effectively. Action should focus on areas of high nitrogen surplus, and those likely to be abandoned in the near future. The researchers propose that their mapping could support dialogue between policymakers and other key stakeholders, to enable coordinated planning to enhance water system resilience. They also suggest that policy such as the NRR and Common Agricultural Policy is applied synergistically to achieve multiple objectives (climate mitigation, biodiversity conservation, nutrient reduction) via wetland restoration.

(PM European Commission)

Further information: environment.ec.europa.eu