Display omitted
•The potential and limitations of Borehole NMR in peatland environments are studied.•Borehole NMR data from 163 boreholes across four Danish peatlands are collected.•It is found that ...peat decomposition level can be tracked by NMR data.•Correlation of common lab measurements of peat samples with NMR data is examined.•NMR-derived hydraulic conductivity estimations are evaluated against slug tests.
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.
Excess nitrogen (N) losses from intensive agricultural production are a world-wide problem causing eutrophication in vulnerable aquatic ecosystems such as estuaries. Therefore, Denmark as one of the ...most intensively farmed countries in the world has enforced mandatory regulations on agricultural production since the late 1980s. We demonstrate the outcome of the regulations imposed on agriculture by analyzing decadal trends in nitrate (NO3−) concentrations and loads in streams using 29 years of detailed monitoring data and survey information on agricultural practices at field level from five intensively cultivated headwater catchments. The analysis includes the importance of four main drivers (climate, land use, agricultural practices, and biogeophysical properties of catchments), each divided into different factors that may influence stream NO3− loads during three subperiods defined by the time of introduction of different mitigation measures: i) 1990–1998, ii) 1999–2007, and iii) 2008–2018.
Significant correlations with annual flow-weighted stream NO3− concentrations and/or loads were found for factors representing all of the four main drivers including precipitation, large scale climate fluctuations, runoff, previous year's runoff, baseflow index, number of annual frost days, agricultural area, livestock density, field N surplus, catch crop cover, manure storage capacity, method and time of manure spreading, and time of soil tillage.
Changes in the four drivers were reflected by the load-runoff (L-Q) relationships for each of the three subperiods within each of the five headwater catchments. The five catchments experienced large but catchment-specific downward shifts in the L-Q relationship attributable to changes in land use and agricultural management within the catchments. The documented large downward shifts in NO3− loads demonstrated for the five catchments (30–52%) as a consequence of mandatory regulation over a period of nearly three decades are a unique example of how agriculture can reduce its environmental impact.
Display omitted
•Agricultural practices and stream nitrate loads in Danish catchments were analyzed.•Stream nitrate loads decreased by 30–52% over 29 years.•The effect of mitigation measures is seen from shifts in load-runoff relationships.•Mitigation measures in loamy catchments have the largest impact at large runoff.•Catchment biogeophysical properties determine response to mitigation measures.
Sulphate (SO42-) concentrations in freshwaters have increased globally over the last decades even though a strong reduction in atmospheric sulphur (S) deposition has occurred across large parts of ...North America and Europe. However, the extent and effects of increased SO42- concentrations in freshwater and terrestrial ecosystems remain poorly understood regarding many aspects of ecosystem structure and functioning. Here, we review the sources of SO42- pollution, environmental impacts on freshwater ecosystems and bioremediation opportunites and we identify key knowledge gaps and future research needs. Natural sources of dissolved SO42- in freshwater ecosystems include mineral weathering, volcanic activity, decomposition and combustion of organic matter, oxidation of sulphides, and sea spray aerosols. Acid mine drainage, fertiliser leaching from agricultural soils, wetland drainage, agricultural and industrial wastewater runoff as well as sea level changes are the main direct and indirect sources of the anthropogenic SO42- input to waterbodies. Increasing SO42- concentrations in freshwater systems influence the biogeochemical processes of carbon, nitrogen and phosphorus. Similarly, iron availability can be critical in determining the adverse effects of SO42- on environmental receptors. The literature reviewed clearly demonstrates that SO42- pollution may have toxic effects on aquatic plants and animal organisms, including, among others, fishes, invertebrates and amphibians, and it may also have negative implications for human health. Bioremediation systems provide opportunities to mitigate the impacts of SO42-, but removal efficiencies range widely from 0% to 70% across treatment systems such as constructed wetlands, permeable reactive barriers and bioreactors. We conclude that examination of increased SO42- concentrations and fluxes at different spatial scales is urgently needed as the ongoing global perturbation of the S cycle is likely to be accelerated by climate change and human development activities. The adverse effects of this on freshwater organisms worldwide may prove detrimental to the future well-being of humans and ecosystems. Field-scale research to estimate the ecotoxicological effects of elevated SO42- concentrations is recommended as is widespread implementation of large-scale wetland restoration and bioremediation systems to reduce SO42- loads on freshwater ecosystems.
Display omitted
•Anthropogenic sulphate pollution of freshwater systems is an ongoing global issue.•Climate change and land use foster the perturbation of the global sulphur cycle.•Sulphate accelerates the biogeochemical turnover of carbon, nitrogen and phosphorus.•Aquatic flora and fauna can be severely impaired by sulphate pollution.•Bioremediation has proved a successful mitigation tool, but more research is required.
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.
Display omitted
•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.
Riparian lowlands act as interfaces between the streams and upland areas. This study identified and quantified local flow paths in four transects of a 26 ha Danish riparian lowland in a glacial till ...landscape. The riparian lowland was fed by drain water from the upland agricultural drainage catchments. Precipitation, stream stage, and drainage discharge into the riparian lowland were measured continuously, while groundwater hydraulic heads were measured in piezometer pipes twice per month. A water balance model was developed to quantify water fluxes leaving the riparian lowland area via evapotranspiration, leakage to a deeper aquifer, and via groundwater flow, drain flow, and overland flow to the adjacent stream. Overland flow originating from the tile drains was the main flow path in all four transects, and also fluxes to the stream via groundwater or lowland tile drains were significant in some subareas. The presence of a secondary tile drainage network within parts of the riparian lowland reduced overland flow and increased interaction with the riparian lowland soils. Area‐normalized fluxes varied greatly between transects, largely reflecting variations in hydraulic loading rate (ratio of water input rate to the area of the receiving riparian lowland). This, combined with the significance of groundwater flow and riparian lowland tile drain flow in some of the investigated transects, revealed a heterogeneous distribution of flow paths within the small headwater lowland. The rate of overland flow was highly correlated to the hydraulic loading rate, which in turn was dominated by the drainage discharge rate at the hillslope boundary.
Key Points
Overland flow, bypassing the riparian lowland, is produced when the hydraulic loading exceeds riparian lowland infiltration
Magnitudes of overland flow are correlated with the ratio of catchment area to riparian lowland area
Riparian lowland drains reduce overland flow and increase water residence times
Riparian lowlands are known to control catchment nitrogen (N) balances. This study examined the role of agricultural tile drainage systems, often present in clay till landscapes, on the transport, ...transformation, and mass balance of N species in four riparian peat lowland transects receiving agricultural tile drainage water. Monitoring of N speciation of drain, stream, and groundwater, combined with a previously established water balance, enabled the determination of N mass balances for different flow paths including groundwater, subsurface drain water, and overland flow for each piezometer transect. The type of overland flow largely affected nitrate‐N (NO3‐N) removal efficiency, as determined by the total N output from a transect relative to the NO3‐N loading (%). Infiltration and subsurface flow followed by exfiltration (short return flow) allowed an efficient removal of NO3‐N (71–94%), while direct overland flow strongly lowered NO3‐N removal (25%) in one transect. The hydraulic loading rate versus the lowland infiltration capacity determined the transport pathways and thus the resulting NO3‐N removal efficiency. For all transects there was a net export of organic N and/or ammonium, associated with in situ N release from peat decomposition, through overland flow and groundwater discharge. These exports partly counterbalanced NO3‐N removal and significantly reduced the overall total N removal for the riparian lowlands. However, the N removal efficiencies remained positive (1–56%). The study indicates that N budgets for riparian lowlands need to account for overland flow as a transport pathway for N.
Key Points
Nitrate (NO3−) removal in riparian lowlands (RLs) depends on the infiltration of NO3− into organic riparian lowland sediments.
Direct overland flow, bypassing the RL soil and sediment, decreases nitrate removal.
RLs may be sinks or sources of nitrogen (N) depending on the balance between removal of nitrate and release of ammonium and organic N.
Eutrophication of natural water bodies is moderated by transformation of nitrate (NO₃⁻) in riparian wetlands, which serve as filters of infiltrating drain water from upland agricultural areas. The ...present study comprised field observations, laboratory experiments and metagenomic studies to describe NO₃⁻ removing transformation pathways and interactions with the cycling of iron (Fe) in a temperate riparian wetland soil profile down to 1 m depth. Water samples from piezometers showed a distinct plume of riparian wetland soil profile down to 1 m depth. Water samples from piezometers showed a distinct plume of NO₃⁻ in the subsurface soil where agricultural drain water was infiltrating. However, within a distance of few meters in the water flow direction, NO₃⁻ was depleted from the percolating water. Sampling and analyses of soil from the active zone of the biogeochemical NO₃⁻ removal showed that denitrifying enzyme activity was ~ tenfold higher in the upper 0–25 cm than in the lower 25–100 cm. Yet, net transformation of NO₃⁻ was substantial also at 25–100 cm when assayed with relatively undisturbed soil samples and by ¹⁵N tracer techniques in soil slurries. Transformation pathways of dissimilatory 3 was substantial also at 25–100 cm when assayed with relatively undisturbed soil samples and by 15 N tracer techniques in soil slurries. Transformation pathways of dissimilatory nitrate reduction to ammonium and anaerobic ammonium oxidation were identified, but were quantitatively minor as compared to denitrification. Heterotrophic denitrification and denitrification mediated by oxidation of ferrous iron, Fe(II), were identified as important processes in the wetland soil. The latter was substantiated by geochemical observations, by rates of NO₃⁻ depletion in slurry incubations with added FeCl 2, and by identification of microorganisms with known capacity of NO₃⁻ reduction coupled to Fe (II) oxidation (Acidovorax sp.). The transformation pathway of iron-mediated NO₃⁻ reduction could involve biotic and abiotic reactions, and N₂O, which is a potent greenhouse gas, was a major product of the process. It remains to be seen under field conditions if N₂O emission hotspots are linked to specific sites of dynamic NO₃⁻ reduction coupled to Fe(II) oxidation. process. It remains to be seen under field conditions if N 2 O emission hotspots are linked to specific sites of dynamic NO-3 reduction coupled to Fe (II) oxidation.
Peatlands cover only 3–4% of the Earth’s surface, but they store nearly 30% of global soil carbon stock. This significant carbon store is under threat as peatlands continue to be degraded at alarming ...rates around the world. It has prompted countries worldwide to establish regulations to conserve and reduce emissions from this carbon rich ecosystem. For example, the EU has implemented new rules that mandate sustainable management of peatlands, critical to reaching the goal of carbon neutrality by 2050. However, a lack of information on the extent and condition of peatlands has hindered the development of national policies and restoration efforts. This paper reviews the current state of knowledge on mapping and monitoring peatlands from field sites to the globe and identifies areas where further research is needed. It presents an overview of the different methodologies used to map peatlands in nine countries, which vary in definition of peat soil and peatland, mapping coverage, and mapping detail. Whereas mapping peatlands across the world with only one approach is hardly possible, the paper highlights the need for more consistent approaches within regions having comparable peatland types and climates to inform their protection and urgent restoration. The review further summarises various approaches used for monitoring peatland conditions and functions. These include monitoring at the plot scale for degree of humification and stoichiometric ratio, and proximal sensing such as gamma radiometrics and electromagnetic induction at the field to landscape scale for mapping peat thickness and identifying hotspots for greenhouse gas (GHG) emissions. Remote sensing techniques with passive and active sensors at regional to national scale can help in monitoring subsidence rate, water table, peat moisture, landslides, and GHG emissions. Although the use of water table depth as a proxy for interannual GHG emissions from peatlands has been well established, there is no single remote sensing method or data product yet that has been verified beyond local or regional scales. Broader land-use change and fire monitoring at a global scale may further assist national GHG inventory reporting. Monitoring of peatland conditions to evaluate the success of individual restoration schemes still requires field work to assess local proxies combined with remote sensing and modeling. Long-term monitoring is necessary to draw valid conclusions on revegetation outcomes and associated GHG emissions in rewetted peatlands, as their dynamics are not fully understood at the site level. Monitoring vegetation development and hydrology of restored peatlands is needed as a proxy to assess the return of water and changes in nutrient cycling and biodiversity.
Artificial drainage of agricultural fields represents a major flow path way of both water and nutrients which may contribute to eutrophication issues in the recipient waters. Several studies have ...shown that riparian lowlands (alluvial plains, wetlands, meadows), if present, may act as buffer zones with high nutrient retention capacities. To assess the fate of water and nutrient flow in riparian lowlands in tile drained catchments, it is essential to know the locations of tile drainage outlets as sources of nutrient input. Using a thermal infrared (TIR) remote-sensing survey, we identified potential tile drainage outlets in a riparian lowland. We also applied a normalized differentiated vegetation index (NDVI) approach to illustrate how tile drainage outlets can be identified with free broadband RGB-NIR data. The positions of identified outlets were validated in the field by visual observation. Our study finds that TIR remote sensing is a strong tool when assessing the sources of water input. NDVI is also applicable, however the background values are very variable making the outlets difficult to locate. The results can be applied in studies of water movement and solute transport via tile drainage as well as model studies where knowledge of input areas through tile drainage is of great importance.
PDF
