The German Baltic Sea Coast as Terrestrial-Marine Interface of Water and Matter Fluxes (Baltic TRANSCOAST).
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- Gerald Jurasinski
The ongoing climate warming is likely to increase the frequency of freeze-thaw cycles (FTCs) in cold-temperate peatland regions. Despite the importance of soil hydro-physical properties in water and carbon cycling in peatlands, the impacts of FTCs on peat properties as well as carbon sequestration and release remain poorly understood. In this study, we collected undisturbed topsoil samples from two drained lowland fen peatlands to investigate the impact of FTCs on hydro-physical properties as well as dissolved organic carbon (DOC) fluxes from peat. The soil samples were subject to five freeze-thaw treatments, including a zero, one, three, five, ten cycles (FTC0, FTC1, FTC3, FTC5, and FTC10, respectively). Each FTC was composed of 24 h of freezing (−5°C) and 24 h of thawing (5°C) and the soil moisture content during the freeze-thaw experiment was adjusted to field capacity. The results showed that the FTCs substantially altered the saturated hydraulic conductivity (K s) of peat. For peat samples with low initial K s values (e.g., < 0.2 × 10−5 m s−1), K s increased after FTCs. In contrast, the K s of peat decreased after freeze-thaw, if the initial K s was comparably high (e.g., > 0.8 × 10−5 m s−1). Overall, the average K s values of peatlands decreased after FTCs. The reduction in K s values can be explained by the changes in macroporosity. The DOC experiment results revealed that the FTCs could increase DOC concentrations in leachate, but the DOC fluxes decreased mainly because of a reduction in water flow rate as well as K s. In conclusion, soil hydraulic properties of peat (e.g., K s) are affected by freezing and thawing. The dynamics of soil hydraulic properties need to be explicitly addressed in the quantification and modelling of the water flux and DOC release from peatlands.
In light of climate change, renaturation of peatlands has become increasingly important, due to their function as carbon sinks. Renaturation processes in the Baltic Sea include removal of coastal protection measures thereby facilitating exchange processes between peatland and Baltic Sea water masses with inhabiting aquatic organisms, which suddenly face new environmental conditions. In this study, two Baltic Sea and three peatland benthic diatom strains were investigated for their ecophysiological response patterns as a function of numerous growth media, light, and temperature conditions. Results clearly showed growth stimulation for all five diatom strains when cultivated in peatland water-based media, with growth dependency on salinity for the Baltic Sea diatom isolates. Nutrient availability in the peatland water resulted in higher growth rates, and growth was further stimulated by the carbon-rich peatland water probably facilitating heterotrophic growth in Melosira nummuloides and two Planothidium sp. isolates. Photosynthesis parameters for all five diatom strains indicated low light requirements with light saturated photosynthesis at <70 µmol photons m−2 s−1 in combination with only minor photoinhibition as well as eurythermal traits with slightly higher temperature width for the peatland strains. Growth media composition did not affect photosynthetic rates.
Dissolved organic matter (DOM) plays a key role in many biogeochemical processes in peatland ecosystems, such as the formation of greenhouse gases and dynamics of plant nutrients. As sea levels are expected to rise due to global change, saltwater intrusions into coastal peatlands become more likely. To understand changes in the ecosystem that are caused by salt water intrusions, it is essential to study the effects of salt on the composition and molecular structure of the peatland DOM. In this study, DOM from different peat samples was extracted with fresh- and saltwater. Various mass spectrometrical (Py-FIMS, FT-ICR-MS), chromatographical (GC-MS, HPLC) and spectroscopical (S, N-XANES, ICP-OES) methods were used to examine molecular differences in these DOM extracts. First results show a coagulation of large molecules and an overall removal of DOM. Nevertheless, a release of certain microbially easily degradable molecule classes (e.g., carbohydrates, peptides and free fatty acids) was observed. Furthermore, the data indicates a release of ammonium along with a drop in pH, probably due to cation exchange on particles and the peat surface. This nutrient release can be expected to affect microbial and plant communities. Smallest differences in DOM composition between fresh- and saltwater extracts were found in highly decomposed peats. This suggests that protection from saltwater intrusions should be preferentially focussed on less degraded, rather than highly decomposed peatlands.
Both the frequency and intensity of drought events are expected to increase, with unresolved alterations to peatland methane cycling and the involved microbial communities. While existing studies have assessed drought effects via experimental approaches under controlled conditions, to our knowledge, no studies have examined the in-situ effects of natural drought in restored temperate fens. In this study, we used quantitative polymerase chain reaction (qPCR) and high throughput 16S rRNA gene amplicon sequencing of DNA and complementary DNA (cDNA) to determine the abundances and community structure of total and putatively active microbial communities following the 2018 European summer drought. Together with geochemical and methane flux data, we compared these results to a non-drought reference dataset. During drought, water level and methane flux rates decreased to a new recent minimum in both fens. This corresponded with pronounced shifts in porewater geochemistry. Microbial community composition in the drought year differed markedly, and was characterized by a greater relative and total abundance of aerobic methanotrophs, and, in one of the two sites, by a decrease in total methanogen abundance. In contrast to the non-drought reference years, type I methanotrophs were clearly more dominant than type II methanotrophs in both fens. cDNA sequencing confirmed the activity of type I methanotrophs during drought, with Methylomonaceae having the highest average relative abundance of bacterial cDNA transcripts. We show that changes in microbial community dynamics, porewater geochemistry, and ecosystem methane fluxes can be substantial following natural drought in restored fens, and provide the first in-situ evidence from a natural drought which suggests type I methanotroph populations are more active than type II methanotrophs in response to drought effects. Type I methanotrophs may represent a key microbial control over methane emissions in restored temperate fens subject to natural drought.
The element and water exchange between the terrestrial and the marine environment is strongly modulated by submarine groundwater discharge (SGD) on local and regional scales. In the dynamic transition zone, biogeochemical transformations may take place, that induce the fixation or liberalization of, for instance, nutrients, metals, carbon dioxide and methane. In the present communication, we report on the water and element exchange and associated biogeochemical transformation processes in front of a rewetted peatland at the margin of the southern Baltic Sea. Vertical pore water profiles were retrieved from up to 5 m long multi-port pore water samplers on a regular and event-driven base. Concentration gradients of major and redox-sensitive metals, nutrients and the stable isotope composition (H, C, S, O) of water, dissolved inorganic carbon, and sulfate was followed to characterize the mixing processes and superimposing biogeochemical transformations. Evidence for a strong control of the bottom-pore water exchange induced by exposed lithology and a high net activity of dissimilatory sulfate-reducing microorganisms in the coastal SGD-impacted sediments were found. An extraordinary storm event in early 2019 led to the partial flooding of the peatland with brackish water but also pushed Baltic Sea water into the coastal aquifers allowing to investigate the time-dependent return to previous subterrestrial ‚steady-state‘ conditions via SGD-induced freshening. Modeling indicates, that accumulation of dissolved carbonate and sulfide shortly after the storm event was not the result of short-term induced microbial activity, but the consequence of a plug-flow-type transport of diagenetically modified fluids within the aquifer. Acknowledgement: This study is supported by the DFG research training group BALTIC TRANSCOAST, DAAD, and the Leibniz IOW.
Geochemical tracers have been indicated that large volumes of groundwater discharge to the sea/oceans through diffusive and advective processes, however locations as causes of this submarine groundwater discharge (SGD) are still under continuous investigation. The present study reports results of a combination of geochemical and geophysical surveys carried out in the urbanized Wismar Bay (WB), southern Baltic Sea. Areas of elevated activities of radium and radon isotopes suggested the influence of SGD on WB when compared to the adjacent coastal areas. The acoustic profiles show that the bottom sediments in the central bay are under local impact of excavation, and the shallow geologic units have been changed. The impact of the excavation reduced the sediment thickness of the aquitard promoting freshwater discharge to surface marine sediments. Geochemical porewater data from the central part of the bay allowed the identification of the freshening layers. The water isotope composition of porewaters at this site is close to the local meteoric water line defined for the southern part of the German Baltic Sea , suggesting a discharge of relatively modern freshwaters. The results obtained so far indicated that artificial infrastructures, like sediment channeling, may ease the hydrological connection between coastal aquifer and coastal bottom water increasing SGD at coastal urbanized areas. The investigations are supported by the DAAD, DFG RTS Baltic TRANSCOAST, KiSnet project, BONUS SEAMOUNT, FP7 EU Marie Curie career integration grant, DAM-MFG, and IOW.
Submarine groundwater discharge (SGD) is an important pathway for water and materials within the land-ocean transition zone that can impact coastal environments and marine life. Although research from sandy shorelines has rapidly advanced in recent years, there is very little understanding of coastal areas characterized by a low hydraulic conductivity, such as carbon-rich coastal peatlands. The objective of this study was to determine the magnitude and location of terrestrial SGD to be expected from a non-tidal low-lying coastal peatland located along the Baltic Sea and to understand the controlling factors using numerical modeling. We employed the HYDRUS-2D modeling package to simulate water movement under steady-state conditions in a transect that extends from the dune dike-separated rewetted fen to the shallow sea. Soil physical properties, hydraulic gradients, geological stratifications, and topography were varied to depict the range of properties encountered in coastal peatlands. Our results show that terrestrial SGD occurs at the study site at a flux of 0.080 m2 d−1, with seepage rates of 1.05 cm d−1 (upper discharge region) and 0.16 cm d−1 (lower discharge region above submerged peat layer). These calculated seepage rates compare to observations from other wetland environments and SGD sites in the Baltic Sea. The groundwater originates mainly from the dune dike—recharged by precipitation and infiltration from ponded peatland surface water—and to a lesser extent from the sand aquifer. The scenario simulations yielded a range of potential SGD fluxes of 0.008–0.293 m2 d−1. They revealed that the location of terrestrial SGD is determined by the barrier function of the peat layer extending under the sea. However, it has little impact on volume flux as most SGD occurs near the shoreline. Magnitude of SGD is mainly driven by hydraulic gradient and the hydraulic conductivity of peat and beach/dune sands. Anisotropy in the horizontal direction, aquifer and peat thickness, and peatland elevation have little impacts on SGD. We conclude that SGD is most probable from coastal peatlands with high water levels, large Ks and/or a dune dike or belt, which could be an essential source for carbon and other materials via the SGD pathway.
Peatlands serve as important ecosystems since they store a substantial fraction of global soil carbon. Through draining the internal biogeochemical processes may be changed impacting the transformation of stored carbon and plant material. Pristine peatlands are primarily associated with methanogenic and iron-cycling conditions, however, minor sulfur cycling may contribute to carbon mineralization in these ecosystems depending on the amount of atmospheric sulfur deposition and accumulation. In near coastal peatlands the element budget may be altered through natural or artificial flooding by brackish/marine waters. When introducing sulfate-bearing solutions, the concentrations of electron acceptors for anaerobic mineralization or organic matter increase when compared to fresh water conditions. The investigated area is planned to be flooded by Baltic Sea coastal waters in the near future. Here we present results from a study from a drained peatland located in the southern part of the Baltic Sea. In the past the area was agriculturally used as grassland. Soil cores were retrieved along a transect perpendicular to the coast line for (isotope) biogeochemical analyses of pore water and solid phases. Analyses included the CNS composition of soils, and dissolved major elements, nutrients, sulphide, trace metals and stable isotopes of water, DIC, and sulfate (H, O, C, S). Furthermore, acid-extractions of metals were carried out to identify zones of dissolution and formation of authigenic phases. For quantification of microbial sulphate reduction rates (SRR) additional cores were retrieved and SRR were measured in whole-core incubations. The pore water isotopic composition is close to the local meteoric water line at the German Baltic Seas coast line. Concentration and stable isotope composition of DIC indicate mineralization of C3 type organic matter. Pore water trace metals content indicates the importance of anaerobic mineralization for release of metals into the pore and surface waters.
Land-ocean interactions in the coastal zone (LOICZ) are of particular interest regarding the exchange of water and elements, like nutrients, carbon, sulfur, and metals. Processes impacting groundwater fluxes at these boundaries belong to the still unsolved problems in hydrology (Blöschl et al., 2019). Stable isotope signatures (H, C, O, S), major and trace element contents in surface waters of a rewetted coastal peatland were investigated to understand the impact of storm-induced flooding by brackish seawater on hydrology and biogeochemical element cycling. The study area is the Hütelmoor, a wetland located at the coastline of the southern Baltic Sea. The area is characterized by a continuous release of fresh water to the Baltic Sea via submarine groundwater discharge (Jurasinski et al., 2018). Surface water is partly drained to a nearby river, but the introduction of brackish waters into the peatland is typically precluded by a small dune and limited to storm-induced flooding events. In the present study, the spatially distributed composition of surface waters was investigated briefly after a flooding event. The results are compared with previous campaigns without actual salt water impact. Conservative elements and water isotopes demonstrate the importance of seasonal variations due to varying evapotranspiration during pre-flood times and allow for a quantification of mixing processes in the post-flood waters. The impact of soil respired CO 2 , and/or the mineralization of organic matter or methane on the surface waters is indicated by a shift of the C isotope composition of DIC towards lighter data. The S and O isotopic composition of dissolved sulfate indicates an impact by solutions modified by net microbial sulfate reduction on pre-flood surface waters and a potential oxidation of reduced sulfur species in post-flooding solutions. Previous flooding events already impacted element cycling in the peatland's past and are also reflected by a sulfidization of peat layers (Fernández-Fernández et al., 2017) and the observation of local areas with enhanced dissolved concentrations in the central part of the peatland. The study is supported by DFG during GK Baltic TRANSCOAST, DAAD, and Leibniz IOW.
The spatial variability of soil properties plays an important role in water and carbon cycles in peatlands. The objectives of this study were to analyze the spatial variation of hydro-physical properties of peat soils and to establish pedotransfer functions (PTFs) to estimate the hydraulic properties of peat using readily available soil properties. We selected three study sites, each representing a different state of peat degradation (natural, degraded and extremely degraded). At each site, 72 undisturbed soil cores were collected from 5 m by 5 m grid cells in an area of 35 m by 40 m. The saturated hydraulic conductivity (Ks), soil water retention curves, total porosity, macroporosity (pore diameter >30 μm), bulk density and soil organic matter content (SOM) were determined for all sampling locations. The Van Genuchten (VG) model parameters (θs , α, and n) were optimized using the RETC software package. A strong positive correlation between macroporosity and Ks was observed irrespective of the degradation stage of the peat. However, the relationships between macroporosity and Ks differed between the natural and the drained peatlands. Adding macroporosity to the PTFs substantially improves the prediction of Ks as well as VG parameters. Results show that the soil physical and hydraulic properties (e.g. Ks and VG model parameters) exhibit different levels of spatial heterogeneity depending on the peat degradation stage. The geostatistical analysis suggests that the spatial dependence of soil hydro-physical properties varies depending on the considered property as well as land management (e.g. drainage). Bulk density and SOM are spatially dependent, whereas Ks and macroporosity are spatially independent if the peat is severely degraded. In conclusion, the peat degradation stage plays an important role and should be generally considered in the spatial analysis of peatlands. The obtained semivariograms may serve as a basis for 2D and 3D hydrological modelling as well as peatland restoration studies.
The German Baltic Sea coastline is characterized by sea-land transitions zones, specifically coastal peatlands. Such transition zones exhibit highly fluctuating environmental parameters and dynamic gradients that affect physiological processes of inhabiting organisms such as microphytobenthic communities. In the present study four representative and abundant benthic diatom strains [Melosira nummuloides, Nitzschia filiformis, Planothidium sp. (st. 1) and Planothidium sp. (st.2)] were isolated from a Baltic Sea beach and three peatlands that are irregularly affected by Baltic Sea water intrusion. Ecophysiological and cell biological traits of the strains were investigated for the first time as function of light, temperature and salinity. The four strains exhibited euryhaline growth over a range of 1-39 S A , surpassing in situ salinity of the respective brackish habitats. Furthermore, they showed eurythermal growth over a temperature range from 5 to 30 • C with an optimum temperature between 15 and 20 • C. Growth rates did not exhibit any differences between the peatland and Baltic Sea strains. The photosynthetic temperature optimum of the peatland diatom isolates, however, was much higher (20-35 • C) compared to the Baltic Sea one (10 • C). All strains exhibited light saturation points ranging between 29.8 and 72.6 µmol photons m −2 s −1. The lipid content did not change in response to the tested abiotic factors. All data point to wide physiological tolerances in these benthic diatoms along the respective sea-land transitions zones. This study could serve as a baseline for future studies on microphytobenthic communities and their key functions, like primary production, under fluctuating environmental stressors along terrestrial-marine gradients.
Fens belong to the most threatened ecosystems in Europe. Maintaining a high water table through rewetting is an effective measure to rehabilitate many of their ecosystem functions. However, the impact of meteorological conditions such as vapor pressure deﬁcit (VPD) and precipitation on water tables is still unclear for rewetted fens. Here, we compare the impact of meteorological factors on water table dynamics in a drained and a rewetted fen, using multiple regression with data from continuous high-resolution (temporal) water level monitoring and weather stations. We ﬁnd that an increase in the daily mean VPD causes a higher drop in the water table at the drained and degraded fen compared to the rewetted fen. Precipitation contributes to recharge, causing the water table to rise higher at the drained site than at the rewetted site. We attribute the differential inﬂuence of meteorological conditions on water table dynamics to different soil speciﬁc yield values (i.e., water storage capacity) largely driven by lower water table position at the drained site. Our study underlines the importance of understanding how and why water tables in peatlands vary in response to meteorological factors for management decisions (e.g., rewetting). Continuous monitoring of water table and vegetation development in rewetted fen peatlands is advisable to ensure long-term success especially under climate change conditions and associated drought events.
Eight benthic diatom taxa (Actinocyclus octonarius, Melosira moniliformis, Halamphora sp. 1, Halamphora sp. 2, Navicula perminuta, Navicula phyllepta, Nitzschia dubiiformis, Nitzschia pusilla) were isolated from sediments sampled in the southern coastal brackish Baltic Sea and established as unialgal cultures. The coastal shallow water sampling area lies close to a fen peat site (Hütelmoor) and both are connected through an underground peat layer, which might facilitate organic matter and nutrient fluxes along the terrestrial-marine gradient. The photosynthetic performance of these diatoms was measured at different photon fluence rates (0–1200 μmol photons m–2 s–1, always recorded at 20°C) and different temperatures (5–40°C, always measured at saturating ∼270 μmol photons m–2 s–1), resulting in light saturation points between 32 and 151 μmol photons m–2 s–1 and maximum net primary production rates of 23–144 μmol O2 mg–1 Chl a h–1. None of the species showed severe photoinhibition, and hence all displayed a high photo-physiological plasticity. Photosynthetic oxygen evolution and respirational oxygen consumption between 5 and 40°C revealed eurythermal traits for half of the studied taxa as photosynthetic efficiency was at least 20% of the maximum values at the extreme temperatures. The remaining taxa also indicated eurythermal characteristics, however, photosynthetic efficiency of at least 20% was at a narrower temperature range [5 (10) °C to 30 (35) °C]. Species-specific optimum temperatures for photosynthesis (15–30°C) were always lower compared to respiration (25–40°C). Actinocyclus octonarius and Nitzschia dubiiformis were grown in different defined media, some enriched with Hütelmoor water to test for possible effects of organic components. Hütelmoor water media stimulated growth of both diatom species when kept in a light dark cycle. Actinocyclus octonarius particularly grew in darkness in Hütelmoor water media, pointing to heterotrophic capabilities. The benthic diatoms studied are characterized by high photo-physiological plasticity and a broad temperature tolerance to maintain high primary production rates under wide environmental fluctuations. Organic carbon fluxes from the Hütelmoor into the Baltic Sea may support mixo- and/or heterotrophic growth of microphytobenthic communities. These are essential traits for living in a highly dynamic and variable shallow water environment at the coastal zone of the Baltic Sea.
Hydro-physical properties of peat influence the partitioning of rainfall into infiltration versus runoff, determine water flow and solute transport patterns, and regulate the carbon and nitrogen cycles in peatlands. Compared with mineral soils, our understanding of hydraulic properties of peat soils is limited, especially of the temporal dynamics of peat properties. A dataset of peat subsidence as well as the bulk density (BD) change rate following artificial drainage was assembled from the literature. The collected data cover a time period of up to 272 years of land drainage for forestry and agriculture in boreal and temperate climate zones. The results show that the subsidence rate and BD change rate, and hydro-physical properties of peat can be estimated based on land drainage duration and land use. The most severe shift in pore structure of peat occurs within the first 20 years of land drainage. Peatland drainage reduces macroporosity with pore diameter greater than 50 μm, but increases the volume of pores < 5 μm. In the long term, peat thickness loss is responsible for more than 80% of water storage loss. In conclusion, the derived functions between subsidence rate, BD change rate, and drainage duration provide a new approach to estimate the hydro-physical properties of peat (pore structure, saturated hydraulic conductivity, specific yield, and soil water storage) on a centennial-scale. The derived hydro-physical parameter values can be used for long-term hydrological modelling, especially if measured hydraulic parameters of peat are not available.
(download final version at https://authors.elsevier.com/a/1bjcsB8ccoKaV) Precipitation is a key factor affecting shallow water table fluctuations. Although the literature on shallow aquifers is vast, groundwater response to precipitation in peatlands has received little attention so far. Characterizing groundwater response to precipitation events in differently managed peatlands can give insight into ecohydrological processes. In this study we determined the groundwater level response rate following precipitation events at a drained and a rewetted fen to characterize the effect of rewetting on hydrological buffer capacity. Multiple regression analysis revealed that groundwater table in the rewetted fen has a two times lower rate of response to precipitation events of a given intensity, compared to that of the drained fen, even after adjusting for antecedent groundwater levels. Thus, the rewetted fen delivers a better hydrological buffer function against heavy precipitation events than the drained fen. We found that for the depths at which the groundwater interacts with incoming precipitation, the peat of the rewetted fen has a higher specific yield causing groundwater to rise slower compared to the response at the drained fen. A period of 20 years of rewetting was sufficient to form a new layer of organic material with a significant fraction of macropores providing storage capacity. Long-term rewetting has the potential to create favorable conditions for new peat accumulation, thereby altering water table response. Our study has implications for evaluating the success of restoration measures with respect to physical functions of percolation fens.
Nitrous oxide (N2O) is approximately 265 times more potent than carbon dioxide (CO2) in atmospheric warming. Degraded peatlands are important sources of N2O. The more a peat soil is degraded, the higher the N2O-N emissions from peat. In this study, soil bulk density was used as a proxy for peat degradation to predict N2O-N emissions. Here we report that the annual N2O-N emissions from European managed peatlands (EU-28) sum up to approximately 145 Gg N year−1. From the viewpoint of greenhouse gas emissions, highly degraded agriculturally used peatlands should be rewetted first to optimally reduce cumulative N2O-N emissions. Compared to a business-as-usual scenario (no peatland resetting), rewetting of all drained European peatlands until 2050 using the suggested strategy reduces the cumulative N2O-N emissions by 70%. In conclusion, the status of peat degradation should be made a pivotal criterion in prioritising peatlands for restoration.
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Although the majority of coastal sediments consist of sandy material, in some areas marine ingression caused the submergence of terrestrial carbon-rich peat soils. This affects the coastal carbon balance, as peat represents a potential carbon source. We performed a column experiment to better understand the coupled flow and biogeochemical processes governing carbon transformations in submerged peat under coastal fresh groundwater (GW) discharge and brackish water intrusion. The columns contained naturally layered sediments with and without peat (organic carbon content in peat 39 +/- 14 wt%), alternately supplied with oxygen-rich brackish water from above and oxygen-poor, low-saline GW from below. The low-saline GW discharge through the peat significantly increased the release and ascent of dissolved organic carbon (DOC) from the peat (δ13CDOC − 26.9‰ to − 27.7‰), which was accompanied by the production of dissolved inorganic carbon (DIC) and emission of carbon dioxide (CO2), implying DOC mineralization. Oxygen respiration, sulfate (SO42− ) reduction, and methane (CH4) formation were differently pronounced in the sediments and were accompanied with higher microbial abundances in peat compared to sand with SO42− -reducing bacteria clearly dominating methanogens. With decreasing salinity and SO42− concentrations, CH4 emission rates increased from 16.5 to 77.3 μmolm−2 d−1 during a 14-day, low-saline GW discharge phase. In contrast, oxygenated brackish water intrusion resulted in lower DOC and DIC pore water concentrations and significantly lower CH4 and CO2 emissions. Our study illustrates the strong dependence of carbon cycling in shallow coastal areas with submerged peat deposits on the flow and mixing dynamics within the subterranean estuary.
Over the past century, mires and peatlands have faced a wide range of degradation by artificial drainage, making them one of the most threatened ecosystems in Europe. However, restoration of drained peatlands has gained much importance over the last three decades, mostly due to the multiple ecosystem services they provide such as carbon storage, habitat provision and water flow regulation. Although there has been an increased focus on such ecosystems, spatial research on hydrophysical soil properties following rewetting in coastal mires is lacking. Therefore, the objectives of the study were to understand the spatial structures of hydrophysical properties of organic soils and spatial patterns of organic matter accumulation in relation to soil surface microtopography. Soil organic matter content (SOM) and hydraulic conductivity (Ks) of topsoils (0-28 cm), along with soil textures of the underlying mineral substrate, were investigated in a rewetted non-tidal coastal flood mire (Baltic Sea). The results indicate that the organic horizon with its relatively low Ks acts as a hydrological barrier to infiltration. Soil organic matter content (SOM), Ks and soil surface microtopography are all spatially auto-correlated within 100, 87 and 53 m, respectively. Bivariate Moran's I revealed a positive but weak spatial correlation between SOM and Ks and a moderately strong negative spatial correlation between SOM and soil surface microtopography. A map of SOM was generated using simple kriging, which predicts higher SOM in the centre of the ecosystem, at lower elevations; and lower SOM at the edges of the study area, at higher elevations. Local depressions in the centre of the ecosystem provide a wetter and therefore more anaerobic environment, thereby decreasing carbon mineralisation rates and enabling peat accumulation. The low hydraulic conductivity of the degraded peat in the presence of lower micro-elevations in the centre of the ecosystem is likely to increase the residence time of floodwater and thus may enhance (new) peat accumulation. Thus, we conclude that, for the restoration of non-tidal coastal mires where flooding events are not as frequent, Ks and soil surface microtopography are even more important factors to consider than for tidal systems.
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Cyanobacteria and associated heterotrophic bacteria hold key roles in carbon as well as nitrogen fixation and cycling in the Baltic Sea due to massive cyanobacterial blooms each summer. The species specific activities of different cyanobacterial species as well as the N- and C-exchange of associated heterotrophic bacteria in these processes, however, are widely unknown. Within one time series experiment we tested the cycling in a natural, late stage cyanobacterial bloom by adding 13C bi-carbonate and 15N2, and performed sampling after 10 min, 30 min, 1 h, 6 h and 24 h in order to determine the fixing species as well as the fate of the fixed carbon and nitrogen in the associations. Uptake of 15N and 13C isotopes by the most abundant cyanobacterial species as well as the most abundant associated heterotrophic bacterial groups was then analysed by NanoSIMS. Overall, the filamentous, heterocystous species Dolichospermum sp., Nodularia sp., and Aphanizomenon sp. revealed no or erratic uptake of carbon and nitrogen, indicating mostly inactive cells. In contrary, non-heterocystous Pseudanabaena sp. dominated the nitrogen and carbon fixation, with uptake rates up to 1.49 ± 0.47 nmol N h-1 l-1 and 2.55 ± 0.91 nmol C h-1 l-1. Associated heterotrophic bacteria dominated the subsequent nitrogen remineralization with uptake rates up to 1.2 ± 1.93 fmol N h-1 cell -1, but were also indicative for fixation of di-nitrogen.
The impact of synoptic scale and mesoscale variability on the Lagrangian residence time (LRT) of the surface water in the Bay of Gdańsk was investigated using the results from an eddy-resolving model. The computed LRT of 53–60 days was up to four times longer than the estimated flushing time reported by Witek et al. (2003). The highest residence times were those of Puck Bay and near the coast, shallower than 50 m water depth, especially during the winter. These sites also had the highest annual mean in LRT. During the summer, when the level of biological activity is high, the LRT distribution was very heterogeneous and patchy, possibly due to the dynamics of varying eddy field and to variable wind forcing. Long-term run tracking of the inflowing water from the Vistula River (VR) showed a broad spectrum of tracer distribution. The potential impact of a much higher LRT on the near-coastal nitrogen cycle, coastal filter function and genetic differentiation is discussed, and the consequences for coastal zone management are considered. Since residence time is the most important factor regulating nutrient cycling, the incorporation of residence time into the Marine Strategy Framework Directive descriptors would result in an improved, unbiased evaluation of good environmental status.
Peat soils are heterogeneous, anisotropic porous media. Compared to mineral soils, there is still limited understanding of physical and solute transport properties of fen peat soils. In this study, we aimed to explore the effect of soil anisotropy on solute transport in degraded fen peat. Undisturbed soil cores, taken in vertical and horizontal direction, were collected from one drained and one restored fen peatland both in a comparable state of soil degradation. Saturated hydraulic conductivity (Ks) and chemical properties of peat were determined for all soil cores. Miscible displacement experiments were conducted under saturated steady state conditions using potassium bromide as a conservative tracer. The results showed that (1) the Ks in vertical direction (Ksv) was significantly higher than that in horizontal direction (Ksh), indicating that Ks of degraded fen peat behaves anisotropically; (2) pronounced preferential flow occurred in vertical direction with a higher immobile water fraction and a higher pore water velocity; (3) the 5% arrival time (a proxy for the strength of preferential flow) was affected by soil anisotropy as well as study site. A strong correlation was found between 5% arrival time and dispersivity, Ks and mobile water fraction; (4) phosphate release was observed from drained peat only. The impact of soil heterogeneity on phosphate leaching was more pronounced than soil anisotropy. The soil core with the strongest preferential flow released the highest amount of phosphate. We conclude that soil anisotropy is crucial This article is protected by copyright. All rights reserved. in peatland hydrology but additional research is required to fully understand anisotropy effects on solute transport.
Abstract. In the context of the DFG research training Group Baltic Transcoast the scalar transport of discharged submarine groundwater and the included tracer substances will be investigated. The research focus is the mixing of ground water under the influence of waves in shallow coastal water. The waves will be generated by a Piston type wave generator and can be varied with respect to the specific wave parameters. In the water channel a permeable sea bed model is installed which allows the intrusion of a florescent tracer fluid into the measurement area. The transport of this tracer in the water column is affected by the wave motion and will be analysed using optical measurement technology. The methods used are on the one hand the Particle Image Velocimetry, to determine the velocity field in the measurement area. On the other hand, simultaneous Laser Induced Florescence will be used to obtain the concentration field. Thus, the relation between the unsteady velocity field and the mixing of the tracer can be determined quantitatively.
Sea-level rise coupled with land subsidence from wetland drainage exposes increasingly large areas of coastal peatlands to seawater intrusion. Seawater contains high concentrations of sulfate (SO 4 2−), which can alter the decomposition of organic matter thereby releasing organic and inorganic solutes from peat. In this study, a flow-through reactor system was used in order to examine the transport of SO 4 2− through peat as well as its effect on solute release. Moderately-decomposed fen peat samples received input solutions with SO 4 2− concentrations of 0, 100, 700, and 2,700 mg L −1 ; sample effluent was analyzed for a variety of geochemical parameters including dissolved organic carbon (DOC), dissolved inorganic carbon (DIC) and total dissolved nitrogen (TDN) as well as the concentrations of major cations and anions. The input solution remained anoxic throughout the experiment; however, no signs of a pronounced SO 4 2− reduction were detected in the effluent. SO 4 2− transport in the fen peat resembled non-reactive bromide (Br −) transport, indicating that in the absence of SO 4 2− reduction the anion may be considered a conservative tracer. However, slightly elevated concentrations of DOC and TDN, associated with raised SO 4 2− levels, suggest the minor desorption of organic acids through anion exchange. An increased solute release due to stimulated decomposition processes, including SO 4 2− reduction, was observed for samples with acetate as an additional marine carbon source included in their input solution. The solute release of peats with different degrees of decomposition differed greatly under SO 4 2−-enriched conditions where strongly-decomposed fen peat samples released the highest concentrations of DOC, DIC and TDN.
In Baltic TRANSCOAST we study the physical, biogeochemical, and biological processes at the land-ocean interface. The coastal zone is heavily impacted by various human activities as well as by geomorphological and climatic processes – on both the land and the sea side. Land-sea interactions at low lying coastal areas that are often dominated by peatlands, and are a common feature along the Baltic Sea coast, are not well understood. The core hypothesis of Baltic TRANSCOAST is that the shallow sea and the terrestrial peatland have a mutual impact on each other with far-reaching consequences for water and energy fluxes, matter cycling and the biota. The interdisciplinary research focuses on the significance of flooding frequency and duration on biogeochemical processes by concentrating the investigations on three comparison sites. We are assessing how changing boundary conditions (such as climate and land use) affect the hydrology, biota and biogeochemical processes in coastal wetlands and the adjacent marine ecosystem.
Carbon (C) and nitrogen (N) release from peatlands are closely related to water management and soil degradation. However, peat degradation has not been explicitly accounted for when estimating national greenhouse gas inventories. Here, we assembled a comprehensive dataset covering European, Russian and Canadian peatlands and introduced soil bulk density (BD) as a proxy for peat degradation to estimate nitrous oxide (N2O) and dissolved organic carbon (DOC) release. The results show that physical and biogeochemical properties of peat are sensitive to soil degradation. The BD is superior to other parameters (C/N, pH) to estimate annual N2O emissions and DOC pore water concentrations. The more a peat soil is degraded, the higher the risk of air/water pollution in peaty landscapes. Even after rewetting, highly degraded soils may exhibit high N2O release rates. The estimated annual N2O-N emissions from European, Russian and Canadian degraded peatlands sum up to approximately 81.0 Gg. The derived BD-based functions can assist in computing global matter fluxes from peatlands.
- Haojie Liu
- Bernd Lennartz
Over the past two decades, great efforts have been made to restore coastal wetlands through the removal of dikes, but challenges remain because the effects of flooding with saline water on water quality are unknown. We collected soil samples from two adjacent coastal fen peatlands, one drained and diked, the other open to the sea and rewetted, aiming at assessing the mobility and export of various compounds. Microcosm experiments with constant flow-through conditions were conducted to determine the effluent concentrations of dissolved organic carbon (DOC), ammonium (NH 4 +), and phosphate (PO 4 3−) during saline-fresh water cycles. Sodium chloride (NaCl) was used to adjust salinity (saline water, NaCl concentration of 0.12 mol L −1 ; fresh water, NaCl concentration of 0.008 mol L −1) and served as a tracer. A model analysis of the obtained chloride (Cl −) and sodium (Na +) breakthrough curves indicated that peat soils have a dual porosity structure. Sodium was retarded in peat soils with a retardation factor of 1.4 ± 0.2 due to adsorption. The leaching tests revealed that water salinity has a large impact on DOC, NH 4 + , and PO 4 3− release. The concentrations of DOC in the effluent decreased with increasing water salinity because the combination of high ionic strength (NaCl concentration of 0.12 mol L −1) and low pH (3.5 to 4.5) caused a solubility reduction. On the contrary, saline water enhanced NH 4 + release through cation exchange processes. The PO 4 3− concentrations, however, decreased in the effluent with increasing water salinity. Overall, the decommissioning of dikes at coastal wetlands and the flooding of once drained and agriculturally used sites increase the risk that especially nitrogen may be leached at higher rates to the sea.
Baltic Transcoast ist ein durch die Deutsche Forschungsgemeinschaft finanziertes Graduiertenkolleg. Unser Hauptziel ist die Ausbildung von Experten für den Küstenraum. Wir untersuchen die deutsche Ostseeküste als terrestrisch-marine Schnittstelle für Wasser- und Stoffflüsse. Seit Januar 2016 laufen insgesamt 13 Doktroarbeiten, die Angang 2019 fertig gestellt werden sollen. Ab Hanuar 2019 wird die zweite Kohorte an Doktorandinnen und Doktoranden ihre dreijährige Arbeit aufnehmen.
The rewetting of drained peatlands alters peat geochemistry and often leads to sustained elevated methane emission. Although this methane is produced entirely by microbial activity, the distribution and abundance of methane-cycling microbes in rewetted peatlands, especially in fens, is rarely described. In this study, we compare the community composition and abundance of methane-cycling microbes in relation to peat porewater geochemistry in two rewetted fens in northeastern Germany, a coastal brackish fen and a freshwater riparian fen, with known high methane fluxes. We utilized 16S rRNA high-throughput sequencing and quantitative polymerase chain reaction (qPCR) on 16S rRNA, mcrA, and pmoA genes to determine microbial community composition and the abundance of total bacteria, methanogens, and methanotrophs. Electrical conductivity (EC) was more than 3 times higher in the coastal fen than in the riparian fen, averaging 5.3 and 1.5 mS cm−1, respectively. Porewater concentrations of terminal electron acceptors (TEAs) varied within and among the fens. This was also reflected in similarly high intra- and inter-site variations of microbial community composition. Despite these differences in environmental conditions and electron acceptor availability, we found a low abundance of methanotrophs and a high abundance of methanogens, represented in particular by Methanosaetaceae, in both fens. This suggests that rapid (re)establishment of methanogens and slow (re)establishment of methanotrophs contributes to prolonged increased methane emissions following rewetting.
Understanding the influence of salt on the release and transformations of dissolved organic matter (DOM) is crucial for developing management strategies for coastal wetlands. We hypothesize that salt causes distinct changes in certain compound classes of DOM, distinguishable from the impact of other factors such as peat origin and decomposition degree. Therefore, the DOM composition was investigated in freshwater and saltwater extracts of nine peat samples of varying origin and decomposition degree. Samples were analyzed for carbon and nitrogen concentrations and pH values, and organic matter composition by temperature-resolved pyrolysis-field ionization mass spectrometry (Py-FIMS). The data revealed larger abundances of phenols/lignin monomers, peptides, carbohydrates, (mainly heterocyclic) N-compounds/nitriles and alkylaromatics in the saltwater extracts. The thermal release curves in Py-FIMS indicated a precipitation of fractions of alkylaromatics, N-compounds/nitriles and phenols/lignin monomers and a release of lipids, alkylaromatics and free fatty acids n-C16 to n-C34 in the saltwater extracts. Coagulation/precipitation, cation exchange and, for free fatty acids, the formation of micelles seem to be key mechanisms of the salt impact. Due to the stronger direct and indirect influences of saltwater on less decomposed peat an adapted management should be directed to preferentially protect rather pristine peatlands from saltwater intrusions that can be expected as a result of rising ocean levels under climate change.
Coastal zones connect terrestrial and marine ecosystems forming a unique environment that is under increasing anthropogenic pressure. Rising sea levels, sinking coasts, and changing precipitation patterns modify hydrodynamic gradients and may enhance sea–land exchange processes in both tidal and non-tidal systems. Furthermore, the removal of flood protection structures as restoration measure contributes locally to the changing coastlines. A detailed understanding of the ecosystem functioning of coastal zones and the interactions between connected terrestrial and marine ecosystems is still lacking. Here, we propose an interdisciplinary approach to the investigation of interactions between land and sea at shallow coasts, and discuss the advantages and the first results provided by this approach as applied by the research training group Baltic TRANSCOAST. A low-lying fen peat site including the offshore shallow sea area on the southern Baltic Sea coast has been chosen as a model system to quantify hydrophysical, biogeochemical, sedimentological, and biological processes across the land–sea interface. Recently introduced rewetting measures might have enhanced submarine groundwater discharge (SGD) as indicated by distinct patterns of salinity gradients in the near shore sediments, making the coastal waters in front of the study site a mixing zone of fresh- and brackish water. High nutrient loadings, dissolved inorganic carbon (DIC), and dissolved organic matter (DOM) originating from the degraded peat may affect micro- and macro-phytobenthos, with the impact propagating to higher trophic levels. The terrestrial part of the study site is subject to periodic brackish water intrusion caused by occasional flooding, which has altered the hydraulic and biogeochemical properties of the prevailing peat soils. The stable salinity distribution in the main part of the peatland reveals the legacy of flooding events. Generally, elevated sulfate concentrations are assumed to influence greenhouse gas (GHG) emissions, mainly by inhibiting methane production, yet our investigations indicate complex interactions between the different biogeochemical element cycles (e.g., carbon and sulfur) caused by connected hydrological pathways. In conclusion, sea–land interactions are far reaching, occurring on either side of the interface, and can only be understood when both long-term and event-based patterns and different spatial scales are taken into account in interdisciplinary research that involves marine and terrestrial expertise.
Coastal low-lying areas along the southern Baltic Sea provide good conditions for coastal peatland formation. At the beginning of the Holocene, the Littorina Sea transgression caused coastal flooding, submergence and erosion of ancient coastlines and former terrestrial material. The present Heiligensee and Hütelmoor peat deposits (located near Rostock in Northern Germany) were found to continue more than 90 m in front of the coastline based on on-and offshore sediment cores and geo-acoustic surveys. The seaward areal extent of the coastal peatland is estimated to be around 0.16-0.2 km 2. The offshore boundary of the former peatland roughly coincides with the offshore limit of a dynamic coast-parallel longshore bar, with peat deposits eroded seawards. While additional organic-rich layers were found further offshore below a small sand ridge system, no connection to the former peatlands can be established based on 14 C age and C/N ratios. The preserved submerged peat deposits with organic carbon contents of 37% in front of the coastal peatland Heiligensee and Hütelmoor was radiocarbon-dated to 6725 ± 87 and 7024 ± 73 cal yr BP, respectively, indicating an earlier onset of the peatland formation as presently published. The formation time of the peat layers reveals information about the local sea level rise. The local sea level curve derived from our 14 C-dated organic-rich layers is in general agreement to nearby sea level reconstructions (North Rügen and Fischland, Northern Germany), with differences explained by slightly varying local isostatic movements.
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The rewetting of drained peatlands alters peat geochemistry and often leads to sustained elevated methane emission. Although this methane is produced entirely by microbial activity, the distribution and abundance of methane-cycling microbes in rewetted peatlands, especially in fens, is rarely described. In this study, we compare the community composition and abundance of methane-cycling microbes in relation to peat porewater geochemistry in two rewetted fens in northeastern Germany, a coastal brackish fen and a freshwater riparian fen, with known high methane fluxes. We utilized 16S rDNA high-throughput sequencing and quantitative polymerase chain reaction on 16S rDNA, mcrA, and pmoA genes to determine microbial community composition and the abundance of total bacteria, methanogens, and methanotrophs. Electrical conductivity was more than three times higher in the coastal fen than in the riparian fen, averaging 5.3 and 1.5 mS cm−1, respectively. Porewater concentrations of terminal electron acceptors varied within and among the fens. This was also reflected in similarly high intra- and inter-site variations of microbial community composition. Despite these differences in environmental conditions and electron acceptor availability, we found a low abundance of methanotrophs and a high abundance of methanogens, represented in particular by Methanosaetaceae, in both fens. This suggests that rapid re/establishment of methanogens and slow re/establishment of methanotrophs contributes to prolonged increased methane emissions following rewetting.
Constructed wetlands (CWs) provide favorable conditions for removing nitrate from polluted agricultural runoff via heterotrophic denitrification. Although the general operability of CWs has been shown in previous studies, the suitability of peat soils as a bed medium for a vertical flow through a system for nitrate attenuation has not been proven to date. In this study, a mesocosm experiment was conducted under continuous flow with conditions aiming to quantify nitrate (NO3−) removal efficiency in degraded peat soils. Input solution of NO3− was supplied at three different concentrations (65, 100, and 150 mg/L). Pore water samples were collected at different depths and analyzed for NO3−, pH, and dissolved N2O concentrations. The redox potential (Eh) was registered at different depths. The results showed that the median NO3-N removal rate was 1.20 g/(m2·day) and the median removal efficiency was calculated as 63.5%. The nitrate removal efficiency was affected by the NO3− supply load, flow rate, and environmental boundary conditions. A higher NO3− removal efficiency was observed at an input NO3− concentration of 100 mg/L, a lower flow rate, and higher temperature. The results of pore water pH and NO3− and N2O levels from the bottom of the mesocosm suggest that N2 is the dominant denitrification product. Thus, degraded peat soils showed the potential to serve as a substrate for the clean-up of nitrate-laden agricultural runoff.
Under steady-state conditions, multi-isotope profiles in marine surface sediment reflect the redox condition, transformation rates and fluxes of dissolved species in the sediments and across the sediment-water interface. Most of these processes in nature, however, take place under non-steady state conditions and the top sediment below oxygenated bottom waters are commonly under the impact of transport processes. Sedimentation events, macro zoo-benthos, wave actions or even fishing activity could affect the top parts of shallow sediments. The consequences of sediment disturbance on the isotope profiles is fundamental for understanding the relationship between redox condition, biogeochemical processes and fluxes of elements with the development of biogeochemical stable isotope signatures and proxies in natural ecosystems. We present here a sediment ecosystem model, incorporating oxic respiration, sulfate reduction, and sulfide re-oxidation. In this study, the dynamics of downcore profiles of DI13C, 34SO4 and S18O4 in pore waters are used to identify the impact of sediment disturbance on the dynamics of biogeochemical processes with a focus on the carbon and sulfur cycles. Two extreme cases of sediment disturbance upon early diagenesis were considered: 1) sediment and pore water mixing, and 2) only pore water mixing. The average oxygen uptake rate, and sulphate and DIC flux are dependent of the frequency and depth of the disturbance, and an overall enhancement of the element flux was estimated. The onsequence for the developed of different relationships between C, S and O isotope signatures is demonstrated. The study is supported by BMBF during SECOS II project and Leibniz IOW.
Bioturbation of sediments by burrowing organisms plays a key role in the functioning of the coastal ecosystems. Burrowing is considered an energetically expensive activity, yet the energy costs of burrowing and the potential impacts of multiple stressors (such as salinity stress and wave action) on bioenergetics and burrowing performance of marine bioturbators are not well understood. We investigated the effects of mechanical disturbance and salinity stress on the burrowing behavior, aerobic capacity and energy expense of digging in a common marine bioturbator, the soft clam Mya arenaria from the Baltic Sea (control salinity 15). M. arenaria showed large individual variability in the burrowing efficiency, with an average of ∼7% of the body energy reserves used per burial. Clams with higher mitochondrial capacity and lower energy expenditure per burial showed higher endurance. Acclimation for 3-4 weeks to low (5) or fluctuating (5-15) salinity reduced the burrowing speed and the number of times the clams can re-bury but did not affect the mitochondrial capacity of the whole body or the gill. Acclimation to the fluctuating salinity shifted the predominant fuel use for burrowing from proteins to lipids. Our data indicate that the reduced burrowing performance of clams under the salinity stress is not due to the limitations of energy availability or aerobic capacity but must involve other mechanisms (such as impaired muscle performance). The reduction in the burrowing capacity of clams due to salinity stress may have important implications for survival, activity and ecological functions of the clams in shallow coastal ecosystems.
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In peat soils, decomposition and degradation reduce the proportion of large pores by breaking down plant debris into smaller fragments and infilling inter-particle pore spaces. This affects water flow and solute migration which, in turn, influence reactive transport processes and biogeochemical functions. In this study we conducted flow-through reactor experiments to investigate the interplay between pore structure and solute transport in samples of undegraded and degraded peat collected in Canada and Germany, respectively. The pore size distributions and transport parameters were characterised using the breakthrough curve and two-region non-equilibrium transport model analyses for a non-reactive solute. The results of transport characterisation showed a higher fraction of immobile pores in the degraded peat with higher diffusive exchanges of solutes between the mobile and immobile pores associated with the dual-porosity structure. The rates of steady-state potential nitrate reduction were compared with pore fractions and exchange coefficients to investigate the influence of pore structure on the rates of nitrate reduction. The results indicated that the degraded peat has potential to provide the necessary boundary conditions to support nitrate removal and serves as a favourable substrate for denitrification, due to the nature of its pore structure and its lower organic carbon content compared to undegraded peat.
There is a limited understanding of solute transport properties of degraded peat soils as compared to mineral substrates. A lower organic matter (OM) content is often the result of peat degradation and mineralization following artificial drainage. In this study, we aimed at deducing changes in solute transport properties of peat soils differing in OM content. Miscible displacement experiments were conducted on 70 undisturbed soil columns with OM contents ranging from 11 to 86 wt% under saturated steady state conditions using tritium and bromide as conservative tracers. Measured breakthrough curves (BTCs) were subjected to model analysis employing three different approaches: single porosity model (SPM), mobile and immobile model (MIM), and two flow region model (TFR). The results indicated that: (i) non-equilibrium solute transport processes are common in peat soils, (ii) the TFR model improved predictions of BTCs with heavy tailing or two peaks; (iii) applied tracers, tritium and bromide, were retarded in peat soils with higher OM content; and (iv) pronounced preferential flow mainly occurred in peat soils with lower OM content. This type of strong preferential flow had a small ratio of measured to fitted pore water velocity and a greater ratio of velocities (v A /v B >3.0) in the fast and slow transport region as obtained from the TFR model. We conclude that shallow groundwater resources are more likely to become polluted in drained and degraded fen peats that are used for agricultural purposes.
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Soil type is an important factor defining terrestrial ecosystems and plays a major role for the movement of solutes and cycling of nutrients and carbon. This paper focuses on the effect of peat complex dual-porosity structure on nitrate reduction, with the main objective to show how this process is controlled by pore-scale mass transfer and exchange of nitrate between mobile and immobile pore fractions. A mesocosm experiment was conducted where input solutions of bromide (Br⁻) and nitrate (NO3⁻) were continuously supplied downward into 40 cm depth of peat. Br⁻ and NO3⁻ breakthrough curves were used to constrain transport parameters and nitrate reduction rates in the peat depth profile. The vertical distribution of potential nitrate reduction rates were compared with depth distributions of partitioning mobile-immobile pores and the exchange coefficient between the pores. The results showed that an increase of immobile pore fractions with depth increases the common interface surface area between mobile and immobile pores which constitutes to a more pronounced exchange between the two transport domains and enhances the nitrate reduction. Hence, the pore structure with mobile-immobile pore fractions and exchange rate of solutes between mobile and immobile phases play a major role in nitrate reduction in peat soils.
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