Wetland buffer zones (WBZs) are riparian areas that form a transition between terrestrial and aquatic environments and are well-known to remove agricultural water pollutants such as nitrogen (N) and ...phosphorus (P). This review attempts to merge and compare data on the nutrient load, nutrient loss and nutrient removal and/or retention from multiple studies of various WBZs termed as riparian mineral soil wetlands, groundwater-charged peatlands (i.e. fens) and floodplains. Two different soil types (‘organic’ and ‘mineral’), four different main water sources (‘groundwater’, ‘precipitation’, ‘surface runoff/drain discharge’, and ‘river inundation’) and three different vegetation classes (‘arboraceous’, ‘herbaceous’ and ‘aerenchymous’) were considered separately for data analysis. The studied WBZs are situated within the temperate and continental climatic regions that are commonly found in northern-central Europe, northern USA and Canada. Surprisingly, only weak differences for the nutrient removal/retention capability were found if the three WBZ types were directly compared. The results of our study reveal that for example the nitrate retention efficiency of organic soils (53 ± 28%; mean ± sd) is only slightly higher than that of mineral soils (50 ± 32%). Variance in load had a stronger influence than soil type on the N retention in WBZs. However, organic soils in fens tend to be sources of dissolved organic N and soluble reactive P, particularly when the fens have become degraded due to drainage and past agricultural usage. The detailed consideration of water sources indicated that average nitrate removal efficiencies were highest for ground water (76 ± 25%) and lowest for river water (35 ± 24%). No significant pattern for P retention emerged; however, the highest absolute removal appeared if the P source was river water. The harvesting of vegetation will minimise potential P loss from rewetted WBZs and plant biomass yield may promote circular economy value chains and provide compensation to land owners for restored land now unsuitable for conventional farming.
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•The efficiency of wetland buffer zones for nutrient retention was reviewed.•Organic and mineral soils as nutrient filters or sources were compared.•Processes driving phosphorus and nitrogen fluxes were described.•The indirect and direct impact of vegetation were unraveled.•Implications for wetland restoration and open research questions were specified.
Integrated buffer zones (IBZs) represent a novel form of edge-of-field technology in Northwest Europe. Contrary to the common riparian buffer strips, IBZs collect tile drainage water from ...agricultural fields by combining a ditch-like pond (POND), where soil particles can settle, and a flow-through filter bed (FILTERBED) planted with Alnus glutinosa (L.), a European alder (black alder). The first experimental IBZ facility was constructed and thoroughly tested in Denmark for its capability to retain various nitrogen (N) and phosphorus (P) species within the first three years after construction. We calculated the water and nutrient budget for the total IBZ and for the two compartments, POND and FILTERBED, separately. Furthermore, a tracer experiment using sodium bromide was conducted in order to trace the water flow and estimate the hydraulic residence time in the FILTERBEDs. The monthly average removal efficiency amounted to 10–67% for total N and 31–69% for total P, with performance being highest during the warm season. Accordingly, we suggest that IBZs may be a valuable modification of dry buffer strips in order to mitigate the adverse impacts of high nutrient loading from agricultural fields on the aquatic environment.
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.
Riparian wetlands can mitigate nutrient pollution to the aquatic environment when they serve as biogeochemically active buffer zones between arable land and water bodies. Nevertheless, as a result of ...the extensive nutrient transformation, wetlands hold a potential of atmospheric emission of greenhouse gases such as nitrous oxide (N2O). To quantify this potential, fluxes of N2O were measured over a year at 48 sub-plots located in four Danish riparian wetlands with contrasting characteristics of soil parameters and groundwater dynamics. The wetlands were hydrologically and physically relatively undisturbed, but they were all located in catchments dominated by agriculture. Individual fluxes of N2O measured using the static chamber technique ranged from −44 to 122 μg N2O–N m−2 h−1 (n = 800) while cumulative fluxes ranged from −0.25 to 0.50 g N2O–N m−2 yr−1 (n = 48), i.e., showing both uptake and emission of N2O. Modeling of the fluxes using linear mixed models revealed that ammonium in the groundwater was the only tested variable having a significant effect on N2O fluxes. Tentative maximum estimates showed that only about 2.2% of the total Danish N2O emissions could be related to freshwater wetlands (representing about 1.3% of the land area). Further, the low and frequently negative N2O fluxes (n = 294) indicated that riparian wetlands, at least under some conditions, may actually reduce atmospheric N2O pollution, although the measured N2O uptake was weak. In conclusion, riparian ecosystems with only minor disturbances are not generally to be considered as hotspots of N2O emissions in the landscape.
•Undisturbed riparian wetlands soils emit low amounts of nitrous oxide (N2O).•Emission and uptake of N2O by riparian soils was detected.•Ammonium in groundwater had a positive effect on N2O emissions.•Undisturbed riparian wetlands are not hotspots for N2O emissions.
Restoration of riparian wetlands often aims at increasing the removal of nitrogen and phosphorus by re-establishing the hydrological connectivity between the stream and the surrounding floodplain. ...However, the geochemically reduced soil conditions in the newly restored area may favor the emission of greenhouse gases (GHG) such as nitrous oxide (N2O) and methane (CH4). To evaluate this risk the fluxes of N2O, CH4 and carbon dioxide from ecosystem respiration (Reco) were determined prior to and after restoration of a stream and its adjacent riparian areas. The data collected during the first year after restoration revealed spatially and seasonally variable N2O emissions ranging from 0.1 to 3.1gNm−2y−1, but no statistically significant effect of the restoration on N2O emission was observed as tested for comparable 8-month periods before and after restoration. The re-establishment of a high groundwater level (GWL) induced a significant increase in CH4 emissions (p<0.001), from -0.04 to 31.7gCm−2 at a permanently flooded, restored area during comparable 8-month periods before and after restoration. Ecosystem respiration at the restored sites decreased or remained stable after the restoration, but a decrease in Reco was also observed at a control site. According to mixed model statistical analyses both the soil temperature at 10cm depth (T(−10cm)) and GWL were apparent controllers of CH4 and Reco. Nitrous oxide emissions were related to N content in the top soil. Annual CH4 emissions the first year after restoration were comparable to those of natural riparian wetland sites and the increased CH4 emission appeared to be compensated for by a decrease in Reco, while the effect of the restoration on N2O was more uncertain–not least because of large spatial variation.
Integrated buffer zones (IBZs) have recently been introduced in the Northwestern Europe temperate zone to improve delivery of ecosystem services compared with the services associated with ...long‐established vegetated buffer zones. A common feature of all the studied IBZ sites is that tile drainage, which previously discharged directly into the streams, is now intercepted within the IBZ. Specifically, the design of IBZs combines a pond, where soil particles present in drain water or surface runoff can be deposited, and a planted subsurface flow infiltration zone. Together, these two components should provide an optimum environment for microbial processes and plant uptake of nutrients. Nutrient reduction capacities, biodiversity enhancement, and biomass production functions were assessed with different emphasis across 11 IBZ sites located in Denmark, Great Britain, and Sweden. Despite the small size of the buffer zones (250–800 m2) and thus the small proportion of the drained catchment (mostly <1%), these studies cumulatively suggest that IBZs are effective enhancements to traditional buffer zones, as they (i) reduce total N and P loads to small streams and rivers, (ii) act as valuable improved habitats for aquatic and amphibian species, and (iii) offer economic benefits by producing fast‐growing wetland plant biomass. Based on our assessment of the pilot sites, guidance is provided on the implementation and management of IBZs within agricultural landscapes.
Core Ideas
Integrated buffer zones are a novel edge‐of‐field approach within riparian zones.
Drain water and surface runoff will be trapped within a pond and charge a filter bed.
The inclusion of trees aims to provide some of the benefits of riparian forests.
Climate change might have profound effects on the nitrogen (N) dynamics in the cultivated landscape as well as on N transport in streams and the eutrophication of lakes. N loading from land to ...streams is expected to increase in North European temperate lakes due to higher winter rainfall and changes in cropping patterns. Scenario (IPCC, A2) analyses using a number of models of various complexity for Danish streams and lakes suggest an increase in runoff and N transport on an annual basis (higher during winter and typically lower during summer) in streams, a slight increase in N concentrations in streams despite higher losses in riparian wetlands, higher absolute retention of N in lakes (but not as percentage of loading), but only minor changes in lake water concentrations. However, when taking into account also a predicted higher temperature there is a risk of higher frequency and abundance of potentially toxic cyanobacteria in lakes and they may stay longer during the season. Somewhat higher risk of loss of submerged macrophytes at increased N and phosphorus (P) loading and a shift to dominance of small-sized fish preying upon the key grazers on phytoplankton may also enhance the risk of lake shifts from clear to turbid in a warmer North European temperate climate. However, it must be emphasised that the prediction of N transport and thus effects is uncertain as the prediction of regional precipitation and changes in land-use is uncertain. By contrast, N loading is expected to decline in warm temperate and arid climates. However, in warm arid lakes much higher N concentrations are currently observed despite reduced external loading. This is due to increased evapotranspiration leading to higher nutrient concentrations in the remaining water, but may also reflect a low-oxygen induced reduction of nitrification. Therefore, the critical N as well as P loading for good ecological state in lakes likely has to be lower in a future warmer climate in both north temperate and Mediterranean lakes. To obtain this objective, adaptation measures are required. In both climate zones the obvious methods are to change agricultural practices for reducing the loss of nutrients to surface waters, to improve sewage treatment and to reduce the storm-water nutrient runoff. In north temperate zones adaptations may also include re-establishment of artificial and natural wetlands, introduction of riparian buffer zones and re-meandering of channelised streams, which may all have a large impact on, not least, the N loading of lakes. In the arid zone, also restrictions on human use of water are urgently needed, not least on the quantity of water used for irrigation purposes.
Riparian wetlands located in agricultural catchments may often receive a high nitrate (NO3−) load because of the leaching of nutrients derived from upland farming activities. Nitrate can be removed ...in wetland soils by denitrification which is the reduction of NO3− to the gaseous forms nitrous oxide (N2O) and dinitrogen (N2). However, the release of N2O is detrimental to the environment because N2O is a potent greenhouse gas. Therefore, this study aimed at investigating the factors controlling the production of N2O and at evaluating the risk for N2O emissions from riparian wetland soils. In a laboratory setup, we simulated an upward flow of NO3− enriched groundwater through intact soil cores collected from four wetlands with contrasting soil characteristics. The results showed a rapid reduction of the NO3− fluxes, supporting the effectiveness of wetlands for removal of N. However, during the reduction of NO3− transient accumulation of N2O was observed, but the N2O concentration decreased with declining NO3− availability. In this study, the NO3− load was revealed as the only significant factor controlling both NO3− reduction and N2O production. Our results confirm the capacity of wetlands to remove large amounts of N, but it also showed that substantial emission of N2O might occur if the reduction of NO3− is not complete, a matter to be considered when diverting N rich waters toward wetlands.
► Riparian wetlands soils have the capacity to remove large amounts of nitrate (NO3−). ► High NO3− load increases the ratio between N2O production and NO3− reduction. ► Nitrous oxide (N2O) can accumulate during nitrate (NO3−) reduction in wetland soils. ► Complete NO3− reduction prevents the accumulation and the potential emission of N2O.
Shallow lakes are a key component of the global carbon cycle. It is, therefore, important to know how shallow lake ecosystems will respond to the current climate change. Global warming affects not ...only average temperatures, but also the frequency of heat waves (HW). The impact of extreme events on ecosystems processes, particularly greenhouse gas (GHG) emissions, is uncertain.
Using the world's longest‐running shallow lake experiment, we studied the effects of a simulated summer HW on the fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). The experimental mesocosms had been exposed to different temperature treatments and nutrient loading for 11 years prior to the artificial HW.
In general, there was an increase in total GHG emissions during the 1‐month artificial HW, with a significant increase in CO2, CH4 and N2O being observed in the shallow lake mesocosms. No significant effect of the HW on CO2 emissions could be traced, though, in the mesocosms with high nutrient levels. Furthermore, the data suggested that in addition to the direct effect of increased temperature on metabolic processes during the HW, biotic interactions exerted a significant control of GHG emissions. For example, at low nutrient levels, increased CO2 emissions were associated with low macrophyte abundance, whereas at high nutrient levels, decreased phytoplankton abundance was linked to increased emissions of CO2 and CH4.
In contrast to the observable heat‐wave effect, no clear general effect of the long‐term temperature treatments could be discerned over the summer, likely because the potential effects of the moderate temperature increase, applied as a press disturbance, were overridden by biotic interactions. This study demonstrates that the role of biotic interactions needs to be considered within the context of global warming on ecosystem processes.
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.
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