Peatland drainage for agricultural purposes has transformed wetlands into major nutrient sources, but rewetting offers potential for nutrient retention and climate mitigation. We quantified hydrological and nutrient fluxes in a 14.5 ha former pump-drained fen (Strande Enge, Denmark) across 1 year before and 2 years after rewetting. Restoration involved removing pumping infrastructure and reconnecting upstream catchments with the aim to restore natural flow paths. Prior to rewetting, the site exhibited high hydraulic loading (5904 mm yr−1) dominated by groundwater inputs and acted as a strong nutrient source, with net losses of total nitrogen (TN: 123 kg ha−1 yr−1), total phosphorus (TP: 23 kg ha−1 yr−1), and total organic carbon (TOC: 1336 kg ha−1 yr−1). Following rewetting, hydraulic loading increased more than fourfold (24,258–27,160 mm yr−1), and the system shifted to a flow-through wetland dominated by surface water inputs. Despite substantially higher nutrient loads after rewetting, the fen became a large sink for TN (209–242 kg ha−1 yr−1; 21

Peatland drainage has transformed natural carbon sinks into sources of greenhouse gases and nutrient pollution, yet the nutrient fluxes from drained systems remain poorly quantified. This study presents a comprehensive three-year nutrient balance assessment of a ditch-drained riparian peatland in Vejrumbro, Denmark, prior to rewetting. The site comprises two hydrologically distinct subareas - a western groundwater-dominated zone influenced by a sand aquifer and an eastern precipitation-fed zone. Water and nutrient fluxes were quantified using high-resolution flow measurements, automated and manual water sampling, and extensive soil and groundwater characterization. The western area exhibited high nitrate loads (703 kg N ha-1 yr-1), of which 74-75% was removed, primarily via denitrification under persistently anoxic conditions. In contrast, both areas consistently released ammonium, organic nitrogen, and total phosphorus (TP), with TP exports ranging from 1.2 to 2.4 kg P ha-1 yr-1. Soil analyses revealed elevated degrees of P saturation in surface peat layers, indicating an elevated risk of P mobilization. The estimated P release risk upon rewetting (1.7 kg P ha-1 yr-1) was comparable to current losses under drained conditions, suggesting that rewetting may not exacerbate P export. These findings underscore the importance of subsurface hydrological connectivity and legacy nutrient pools in shaping nutrient dynamics and provide an empirical basis for evaluating the trade-offs of peatland rewetting in temperate lowland systems.

The rewetting of low-lying organic soils previously used for agriculture can mobilize legacy phosphorus (P), increasing the risk of eutrophication in downstream water bodies. One potential mitigation strategy is biomass harvesting, which may remove mobile P fractions from the soil. At the 44-ha, pump-drained former shallow lake Gammelgaard, biomass was harvested annually from 2020 to 2023, prior to the re-establishment of the lake in July 2023. Over this period, biomass harvesting removed a total of 701 kg P. In comparison, 254 kg of total P was exported via drainage pumping, corresponding to an average annual loss of 1.5 kg P ha- 1. Following rewetting, the P mass balance revealed an inlet load of 206 kg P and an outlet loss of 238 kg P, resulting in a net P loss from the rewetted area of 0.7 kg P ha- 1. Soil cores collected before and after 3 years of biomass harvesting period showed a reduction in the redox-sensitive P pool of 222 kg P ha- 1. This reduction exceeded the sum of harvested P and outlet P fluxes, indicating that additional processes-such as internal redistribution or transformation into more stable binding forms-likely contributed to the observed changes. Although biomass harvesting appears to significantly reduce the mobile P pool in the soil, further research is needed to clarify the extent to which it effectively mitigates P losses from rewetted wetlands.

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.

The potential and limitations of the Borehole Nuclear Magnetic Resonance (BNMR) technique as an in situ measurement for peatland soil characterization was tested in 163 boreholes at four selected peatlands in Denmark. The BNMR data effectively differentiated various geological units in peatland environments due to their distinct NMR responses. Moreover, field-scale variations of the porosity and pore size distribution (e.g., porosity variations within a single geological unit) were mapped to reveal possible trends reflecting geological or hydrogeological conditions in a peatland. Additionally, some of the NMR parameters were found to be correlated with peat decomposition or the degree of humification. The estimation of hydraulic conductivity (K) based on NMR data was also examined for various geological units and compared with slug test measurements. While NMR-based hydraulic conductivity estimations for sand and gyttja (fine-grained sediment with high organic matter) geological units fall within an acceptable range of error, we encountered challenges in achieving reliable estimations for peat. This study showed the potential of BNMR as a robust, rapid, and reliable in situ tool for soil characterization in peatland research.

Nitrogen (N) and phosphorus (P) losses to surface and coastal waters are still critically high across Europe and globally. Measures to mitigate and reduce these losses are being implemented both at the cultivated land surface and at the edge-of-fields. Woodchip bioreactors represent a new alternative in Denmark for treating agricultural drainage water, and the present study-based on two years of data from five Danish field-based bioreactors-determined N removal rates varying from 1.49 to 5.37 g N m(-3) d(-1) and a mean across all bioreactors and years of 2.90 g N m(-3) d(-1). The loss of phosphorus was relatively high the first year after bioreactor establishment with rates varying from 298.4 to 890.8 mg P m(-3) d(-1), but in the second year, the rates ranged from 12.2 to 77.2 mg P m(-3) d(-1). The investments and the costs of the bioreactors were larger than expected based on Danish standard investments. The cost efficiency analysis found the key issues to be the need for larger investments in the bioreactor itself combined with higher advisory costs. For the four woodchip bioreactors considered in the cost efficiency analysis, the N removal cost was around DKK 350 per kg N ($50 per kg N), which is ca. 50% higher than the standard costs defined by the Danish authorities. Based on the estimated costs of the four bioreactor facilities included in this analysis, a bioreactor is one of the most expensive nitrogen reduction measures compared to other mitigation tools.

The water quality of lakes is highly dependent on external phosphorus (P) loading. The vast external loadings from sewage and other wastewater discharge that European lakes have historically received have been dramatically reduced today by improved wastewater treatment. Gaining knowledge of the catchment characteristics that influence external P-loading should enable predictions of the achievable water quality of lakes. In this study, we tested this proposition. Data from 90 new Danish lakes show no apparent relationship between the mean summer P-concentration and the size or land use of the catchments. The external P-loading and resulting annual P-concentration were further investigated on a representative subset of 12 of the new lakes, using six methods. Three of the methods used empirical estimates of P-transport from catchments, based on the national average P-transport, runoff-dependent P-transport, and crop-dependent P-transport, and the other three methods used different empirical models tested on the lakes. External P-loading was reliably predicted by several of the methods. The predictions of the annual P-concentration were highly dependent on the inclusion of annual runoff. However, the predicted P-concentrations were generally overestimated, most pronounced for the nutrient-poor and most recently established lakes. In these lakes, internal P-loading was found to be the most important factor in predicting achievable water quality.

Floodplain wetlands in agricultural river basins provide critical ecosystem services such as nutrient retention, flood mitigation, carbon sequestration and ecological habitats and are a key component of a nature-based solution approach to restoration. In the context of the global challenge to reducing impact increasingly intensive food production on downstream ecosystems, restoration of wetlands in river floodplains offers a practical means for downstream retention and mitigation of P flux from upstream agricultural land. Data on short term, flood event related, accretion rates are, however, difficult to acquire via conventional monitoring yet such information is essential for restoration planning and scenario testing. Here, we evaluate a promising approach that applies naturally-occurring short-lived fallout radionuclide (FRN) Be-7 as a tracer to quantify sediment and, by association, particulate phosphorus retention rates in restored floodplain wetlands. Following a series of major inundation events, a restored floodplain unit was sampled to determine Be-7 inventories of the floodplain relative to an undisturbed reference site. This was undertaken in conjunction with direct measurement of sedimentation as an independent check of FRN results. Accretion rates up to 27 kg m(-2) were recorded using Be-7 for recent deposition events compared to a longer term annual average rate of ca. 6 kg m(-2) derived from Cs-137 measurements. While accretion rates varied spatially and temporally, there was excellent coherence between FRN-based measurements and direct measurements once rigorous correction for particle size effects on tracer properties had been undertaken. The study demonstrates the important contribution that FRN technology can make to support wetland management and restoration initiatives and the essential need for a systems thinking approach across the soil-sediment continuum. Such decision support tools will become increasingly important in the 21st century with growing anthropogenic pressure on aquatic ecosystems related to upstream food production, and implementation of more nature-based solutions such as restored wetlands to counteract these pressures.

Formorethantwodecades,wetlandrestorationhasbeensuccessfullyappliedinDenmarkasatooltoprotectwa- tercourses from elevated nutrient inputs from agriculture, but little is known about how the fl ora and fauna re-spond to restoration. The main objective of this study was therefore to: (1) examine plant community characteristics in 10 wetland sites in the River Odense Kratholm catchment, restored between 2001 and 2011 byre-meanderingthestreamanddisconnectingthetiledrains,and(2)explorewhethertheeffectsofrestoration on plant community characteristics change with the age of the restoration. Speci fi cally, we hypothesised that plant community composition, species richness and diversity would improve with the age of the restoration andeventuallyapproachthestateofnaturalwetlandvegetation.Wefoundthattheprevailingplantcommunities couldbecharacterisedashumidgrasslands,moistfallow fi eldsandimprovedgrasslands,whereastheabundance of natural wetland plant communities (e.g., rich fens, fen-sedge beds and humid grasslands) was lower in both the recently restored as well as in older restored wetlands. Additionally, species richness and diversity did not seem to improve with the age of the restoration. We suggest that the continued high nutrient input at the re- storedsitesincombinationwithrestricteddispersalofwetlandplantspeciesmayhampertherecoveryofnatural plant communities and that the sites therefore may stay botanically poor for many decades.

Natural wetlands used to cover a significant part of the landscape, but these ecosystems have declined by >50% worldwide, and even more in Denmark and Sweden. However, since the 1980s, various policies have been implemented to restore and create wetlands. This study provides a comprehensive historical overview of policies used to stimulate the creation and restoration of wetlands in Denmark and Sweden, and also analyses what factors have facilitated participation or have been barriers for landowners. The analysis of wetlands implementation programmes in Denmark showed a change towards narrower focus on nitrogen reduction from 1998 and onwards, whereas policies in Sweden often have had a wider multifunctional purpose. In both countries, there has been a change in the compensation structure from a lump sum to annual payments, parallel to an observed increase in costs for wetlands implementation. There is still a large potential for recreating many more wetlands, and the national targets have not been reached in neither Denmark nor Sweden. Key success factors, for future wetlands implementation are sufficient compensation levels, flexible scheme designs and information-based strategies documenting relevant benefits and sustainability issues. In general, more advice and support from the state, regional and local participants, and farmers' organisations, are required to increase the participation and achieve successful and cost-efficient wetlands implementation. A collaborative and catchment-based approach holds promise, where wetland governance can serve as a platform for collaboration between policy bodies and between farmers. Additionally, politicians and decision makers need to accept the area targets presented to them when setting policy goals for wetlands implementation, and to accept that restoring and constructing wetlands requires long implementation times before results can be demonstrated.

Woodchip bioreactors are increasingly used as tools to mitigate nitrogen (N) pollution from agricultural drainage water. They consist of a basin filled with woodchip material through which N contaminated drainage water can flow. During the water transport through the filter matrix, oxygen is rapidly depleted and denitrification removes a fraction of the nitrate N present in the water. However, the N removal efficiency of the bioreactors varies significantly both across systems and seasonally. Furthermore, denitrification can also produce nitrous oxide, which is a potent greenhouse gas. Here, we investigated how variation in hydraulic residence time influenced N removal efficiency and nitrous oxide emissions at eight woodchip bioreactors of different flow designs, monitored for 2-4 years. We also characterised the relative abundance of genes involved in the N cycle at three of the bioreactors using metagenomics. Our results showed that total N removal was 17-73% of the yearly incoming N and that it was influenced by hydraulic residence time and water temperature. Nitrous oxide emissions were variable among the different bioreactors and were higher when the hydraulic residence time was less than 60 h. However, the yearly nitrous oxide release did not exceed 2.4% of the nitrate removal (on N atom basis) and the mean among the bioreactors was 0.6%. Although there were marked differences in nitrate removal and nitrous oxide emissions, there were no clear differences in the relative abundance of N-cycling genes among and within three tested bioreactors. Yet, denitrification genes greatly outnumbered genes related to dissimilatory nitrate reduction to ammonium. Overall, our study showed that all eight bioreactors were effective in removing N from agricultural drainage water and that nitrous oxide emissions were low, especially at hydraulic residence times of 60 h or more.

Large-scale re-establishment of wetland buffer zones (WBZ) along rivers is regarded as an effective measure in order to reduce non-point source nitrogen (N) and phosphorus (P) pollution in agricultural catchments. We estimated efficiency and costs of a hypothetical establishment of WBZs along all watercourses in an agricultural landscape of the lower Narew River catchment (north-eastern Poland, 16,444 km2, amounting to 5% of Poland) by upscaling results obtained in five sub-catchments (1087 km2). Two scenarios were analysed, with either rewetting selected wetland polygons that collect water from larger areas (polygonal WBZs) or reshaping and rewetting banks of rivers (linear WBZs), both considered in all ecologically suitable locations along rivers. Cost calculation included engineering works necessary in order to establish WBZs, costs of land purchase where relevant, and compensation costs of income forgone to farmers (needed only for polygonal WBZs). Polygonal WBZs were estimated in order to remove 11%–30% N and 14%–42% P load from the catchment, whereas linear WBZs were even higher with 33%–82% N and 41%–87% P. Upscaled costs of WBZ establishment for the study area were found to be 8.9 M EUR plus 26.4 M EUR per year (polygonal WBZ scenario) or 170.8 M EUR (linear WBZ scenario). The latter value compares to costs of building about 20 km of an express road. Implementation of buffer zones on a larger scale is thus a question of setting policy priorities rather than financial impossibility.

Nutrient losses from agricultural areas constitute a major cause for the degradation of aquatic ecosystems worldwide. In this paper, we use the intensively cultivated country of Denmark as an example to synthesise and discuss results, experiences and challenges for an optimised implementation of nutrient transport mitigation measures targeting agricultural nutrient losses to fresh and marine water. A new era in the regulation of agricultural nutrient losses to aquatic ecosystems in Denmark was initiated when the Agricultural Package was adopted in 2016 by the Danish Parliament. Danish farmers were again allowed to fertilise their crops to economic optimum and to compensate for the consequent increase in fertilisation rates and the potential negative consequences on water quality, a new nitrogen (N) and phosphorus (P) management plan was introduced. The new plan consists of measures to mitigate N losses in smaller catchments (< 15 km2) and knowledge of the N attenuation capacity of the individual catchments is used for optimisation of the implementation of mitigation measures. A suite of nutrient transport mitigation measures has been scientifically approved for use in this new regulation, and more measures are currently undergoing scientific testing. This study focuses on already approved nutrient transport mitigation measures, such as restoration of riparian wetlands, fens and swamps, re-establishment of shallow lakes, constructed wetlands (surface flow and subsurface flow), as well as measures not yet approved and still under development such as integrated buffer zones, saturated buffer zones and controlled drainage.

Diffusive losses of nitrogen and phosphorus from agricultural areas have detrimental effects on freshwater and marine ecosystems. Mitigation measures treating drainage water before it enters streams hold a high potential for reducing nitrogen and phosphorus losses from agricultural areas. To achieve a better understanding of the opportunities and challenges characterising current and new drainage mitigation measures in oceanic and continental climates, we reviewed the nitrate and total phosphorus removal efficiency of: (i) free water surface constructed wetlands, (ii) denitrifying bioreactors, (iii) controlled drainage, (iv) saturated buffer zones and (v) integrated buffer zones. Our data analysis showed that the load of nitrate was substantially reduced by all five drainage mitigation measures, while they mainly acted as sinks of total phosphorus, but occasionally, also as sources. The various factors influencing performance, such as design, runoff characteristics and hydrology, differed in the studies, resulting in large variation in the reported removal efficiencies.