Nitrogen Retention by Soil Biota; A Key Role in the Rehabilitation of Natural Grasslands?

Environmental stress is the main cause of the decline of species diversity in low‐productive fen meadows in the Netherlands. Attempts to restore species diverse fen meadows e.g. by sod cutting frequently fail. We supposed that unsuccessful efforts are due to ignoring the impact of environmental stress on the performance of soil biota, which play a key role in N‐immobilization and keeping available‐N for primary production low. We investigated both pristine and degraded natural sites and successfully and unsuccessfully restored sites of poor and rich fen meadows. We determined plant species composition, soil chemical properties, N‐pools in soil biota, N‐mineralization rates, and N‐fluxes. In pristine rich and poor fen meadows, mineral‐N was poorly available for primary production due to a strong N‐immobilization by soil biota. Annual N‐immobilization fluxes exceeded by far the annual N‐harvest by primary production. N‐immobilization in pristine fens was higher than in degraded fens. In successfully restored rich fens, net N‐mineralization was lower and N‐immobilization higher than in the unsuccessful category. From our results, we derived the hypothesis that in degraded or in unsuccessfully restored fens the soils internal N‐balance shifted from N‐immobilization to net N‐mineralization, favoring biomass production but disadvantaging plant species diversity. N‐retention driven by an active N‐immobilizing soil biological community, is likely a decisive process for successful recovery of plant species diversity in low productive fen meadows. We recommend that restoration techniques should stimulate a functionally diverse soil fauna, as this may enhance the storage of available nutrients in the soil food web.     

Groundwater-induced redox-gradients control soil properties and phosphorus availability across four headwater wetlands, New York, USA

Hydrochemical patterns across groundwater-fed wetlands, especially carbonate and redox gradients, can influence phosphorus (P) availability by controlling its distribution among different soil pools. We explored these linkages by comparing shallow (5–20 cm) soil properties along groundwater flowpaths in two rich fens, a marl fen, and a poor fen. Organic matter content, bulk density, and total elemental content varied more with depth to underlying drift materials than with water table fluctuation, but also were influenced by groundwater discharge, which stabilized water table elevations and controlled redox conditions. Total sulfur and calcium content increased where pore-water chemistry indicated active iron and sulfate reduction. Calcium mineral dynamics, however, did not appear to influence P availability: first, calcium carbonate (CaCO₃) accounted for <2% of the soil composition, except in the marl fen where it accounted for 20–25% of the soil composition. Second, Ca-bound P pools, determined from hydrochloric extraction of wet soil samples, accounted for <25% of the inorganic soil P pool. In contrast, iron-bound P determined from bicarbonate-buffered dithionite solution, accounted for 50–80% of the inorganic soil P, and increased where there was evidence of groundwater mixing, as did P release rates inferred from incubated anion resin bags. The total carbon and phosphorus content of organic-rich soils as well as available and labile P pools were strongly correlated with pore-water iron and alkalinity concentrations. Groundwater discharge and resulting hydrochemical gradients explained significant variation in soil composition and P availability across each site. Results highlight the importance of conducting biogeochemical studies in the context of a site’s shallow geologic setting and suggest mechanisms supporting the diverse plant species unique to groundwater wetlands.     

Nutrient limitation in species-rich lowland fens

Abstract. Nitrogen, phosphorus and potassium were supplied to some Belgian fens of varying nutrient status and productivity. Plant growth in the lowest productive fen with a species-rich Caricion davallianae vegetation was strongly P-limited. N was ineffective when applied alone, but increased the effect of P-addition when applied together. Summer biomass and plant nutrient concentrations were monitored for four years, and showed partial recovery of nutrient limitation. In a more productive fen dominated by Carex lasiocarpa and in a fen meadow, nutrient limitation was less strong. N limited growth in the productive fen, and N and K were co-limiting in the fen meadow. The P-concentration in the productive fen vegetation showed a marked increase after P-fertilization, but it did not result in higher standing crop. The significance of P-limitation for the conservation of species rich low productive fens is discussed. P-limitation may be an essential feature in the conservation of low productive rich fens: because it is less mobile in the landscape than N and/or because it is an intrinsic property of this vegetation type. Plant nutrient concentrations and N:P-ratios may be used as an indication for the presence and type of nutrient limitation in the vegetation. We found N:P-ratios of 23 to 31 for a P-limited site and 8 to 15 in N-limited sites. This was in agreement with critical values from the literature: N:P > ca. 20 for P-limitation and N:P < 14 for N-limitation. Thus, this technique appears valid in the vegetation types that were studied here.     

In search of a hydrological explanation for vegetation changes along a fen gradient in the Biebrza Upper Basin (Poland)

The understanding of succession from rich fen to poorer fen types requires knowledge of changes in hydrology, water composition, peat chemistry and peat accumulation in the successional process. Water flow patterns, water levels and water chemistry, mineralisation rates and nutrient concentrations in above-ground vegetation were studied along a extreme-rich fen-moderate-rich fen gradient at Biebrza (Poland). The extreme-rich fen was a temporary groundwater discharge area, while in the moderate-rich fen groundwater flows laterally towards the river. The moderate-rich fen has a rainwater lens in spring and significant lower concentrations of calcium and higher concentrations of phosphate in the surface water. Mineralisation rates for N, P and K were higher in the moderate-rich fen. Phosphorus concentrations in plant material of the moderate-rich fen were higher than in the extreme-rich fen, but concentrations of N and K in plant material did not differ between both fen types. Water level dynamics and macro-remains of superficial peat deposits were similar in both fen types.We concluded that the differences observed in the moderate-rich and the extreme-rich fens were caused by subtile differences in the proportion of water sources at the peat surface (rainwater and calcareous groundwater, respectively). Development of an extreme-rich fen into a moderate-rich fen was ascribed to recent changes in river hydrology possibly associated with a change in management practices. The observed differences in P-availability between the fen types did not result in significantly different biomass. Moreover, biomass production in both fen types was primarily N-limited although P-availability was restricted too in the extreme-rich fen. Aulacomnium palustre, the dominant moss in the moderate-rich fen, might be favoured in competition because of its broad nutrient tolerance and its quick establishment after disturbance. It might outcompete low productive rich fen species which were shown to be N-limited in both fens. We present a conceptual model of successional pathways of rich fen vegetation in the Biebrza region.     

Denitrification and dinitrogen fixation in two quaking fens in the Vechtplassen area,The Netherlands

This paper reports laboratory experiments on dinitrogen fixation and denitrification for two small quaking fens (discharge fen and recharge fen) using the acetylene reduction assay and the acetylene inhibition technique, respectively.Nitrogenase activity was detected in peat muck and associated with Alnus glutinosa saplings throughout the study period (May–October 1987), whereas no activity was observed with Sphagnum species. The annual amount of dinitrogen fixed was estimated at 2.1 and 12.7 kg N/ha/y for the recharge fen and the discharge fen, respectively.Denitrification at ambient nitrate levels (0.1 ppm NO₃) was absent in the discharge fen and very low in the recharge fen (0.1 g N/g/d, or 0.3 kg N/ha/y). In nitrate-amended soil samples denitrification rates were 2 to 3 orders of magnitude higher. It is argued that in situ denitrification rates in the fens studied will depend almost entirely on the nitrate supply by precipitation. Denitrification rates associated with precipitation are estimated at 1.1 kg N/ha/y for both fens.     

Nutrient dynamics in small mesotrophic fens surrounded by cultivated land

In a typical Dutch polder landscape the effects of nutrient transport from cultivated grassland to mesotrophic fen communities were studied. In a comparative approach, biomass production and nutrient (N, P and K) uptake were determined monthly in four fens and a hayfield differeing in productivity and species composition. The interstitial ground water was sampled every two weeks for determinations of inorganic nutrient concentrations.The differences in productivity between the fens were clearly reflected in the amount of N, P and K taken up in the above-ground vegetation. N and P proved to be limiting plant growth in the fens, whereas K was the main limiting factor in the hayfield. The ground water welling up from the sandy bottom into the fens proved to be rich in ammonia (3–5 ppm). There are strong indications that this continual seepage leads to a considerable input of N into the fens but not to a higher productivity, as the ammonia is absorbed by the lowermost peat layers covering the sand.At this moment, the differences in productivity between the fens must be caused by differences in the rates of mineralization of the superficial peat layer. The degree of fixation of the floating vegetation mat, determining whether or not low water levels lead to an aerated soil top layer, is important in this respect. Within a period of decades, however, the continuous inflow of ammonia may eventually cause an increase in the productivity and a change in the species composition of the fens.     

Release of inorganic N,P and K in peat soils; effect of temperature,water chemistry and water level

In the Netherlands, fens that are fed by polluted river water are often eutrophic, whereas fens fed by calcium-rich groundwater often are mesotrophic. Differences in trophic status can not always be attributed to differences in the nutrient load of the water. In this paper we try to determine if the inflow of river water in fens, in fact, accelerates the soil nutrient release, thereby creating more eutrophic conditions (‘internal eutrophication’). For this purpose, we compared nutrient release rates (N, P and K) in soil cores fromSphagnum peat andCarex peat saturated with different media, that were artificially created to mimic the three basic water sources: polluted river water, unpolluted calcium-rich groundwater and rainwater. In addition, we studied the effect of temperature and water level on nutrient release rates. The experiments proved thatSphagnum peat released much more P and ammonium thanCarex peat. The strong site effect proved consistent throughout the water chemistry treatments, which indicates that soil quality may be the most important agent determining nutrient release rates. Nevertheless, it was established that water chemistry and water level are of significant influence on nutrient release rates in peat soils. In particular, river water stimulated P release by the peat, most notably in theSphagnum peat. P-release in both soils was only minor when the soils were incubated in clean Ca-rich groundwater. It is suggested that P release is strongly associated with soil chemical processes, and that high P release rates after incubation in river water are due to the high sulphate content of the water. The net release from the soil of ammonium, potassium and phosphate increased with increasing temperature. A freezing treatment significantly increased nutrient availability. The results of the experiments are examined in the context of hydrologic management strategies for the conservation of fens in agricultural landscapes.     

Hydrology,water chemistry and nutrient accumulation in the Biebrza fens and floodplains (Poland)

The composition, structure and above-ground biomass production of floodplain- and fen-vegetation of the Biebrza valley (N.E. Poland) are strongly correlated with water flow characteristics and water chemistry. Groundwater flow and flooding are the major conditioning factors for the vegetation in the valley.The highly productive vegetation is restricted to the dynamic floodplain where it receives nutrient-rich river water during spring floods. The non-flooded parts of the valley contain rich fen and transitional fen vegetation that have a lower biomass production. The rich fen is fed by calcareous and phosphate-poor groundwater coming from the moraines. In the transitional fen, where rainwater infiltrates, phosphate availability is large.Annual nutrient accumulation in the above-ground biomass of the floodplains is estimated to be about 8–9 § 10³ kg/km² for N and K and 1 § 10³ kg/km² for P. For the less-productive fens these figures are 60 to 70% lower. The total annual nutrient accumulation by vegetation of both floodplains and fens for the entire Biebrza valley is estimated to be about 5600 × 10³ kg N, 560 × 103 kg P and 4500 × 10³ kg K. This is high compared to the loading rates in the river near to where the Biebrza River discharges into the Narew River (N-, P- and K-loading rates are c. 900, 200 and 3000 × 103 kg/y, respectively). This implies that floodplain and fen vegetation are important sinks for nutrients, especially for N and P.This paper was presented at the INTECOL IV International Wetlands Conference in Columbus, Ohio, 1992, as part of a session organized by Prof. S. E. Jørgensen and sponsored by the International Lake Environment Committee.Corresponding Editor: J. Kvt     

Higher acidification rates in fens with phosphorus enrichment

Question: Why is bryophyte succession in eutrophicated fens faster than in natural fens? Location: Mineral‐rich fens in The Netherlands and NW Europe. Methods: Literature review on the ecology of four bryophyte species in various successional types as observed in Dutch fens. Results: Bryophyte succession in eutrophicated fens from the brown moss Calliergonella cuspidata to Sphagnum squarrosum is much faster than in natural fens with species shifts from Scorpidium scorpioides to Sphagnum subnitens. Under P‐poor conditions, the brown moss stage is stabilized as long as mineral‐rich water is supplied. This is because S. scorpioides is tolerant of rainwater, is a strong competitor and can counteract acidification to some extent while S. subnitens is intolerant to groundwater and has low growth rates and low acidification capacity. In contrast, the Sphagnum stage is stable after rapid succession from rich‐fen mosses under P‐rich conditions. Calliergonella cuspidata has suboptimal growth in rainwater, possibly due to ammonium toxicity, while the high growth rates of S. squarrosum in nutrient‐rich and highly acidic groundwater allow early establishment and rapid expansion. Conclusions: If measures to improve fen base status occur in environments of increased nutrient (P) availability, the management may not lead to the desired restoration of brown moss stages, but instead to rapid acidification by S. squarrosum.     

A comparison of fens in natural and artificial landscapes

Fens depend on inputs of groundwater or surface water. In Western Europe especially soligenous fens, receiving groundwater, are threatened by human hydrological intervention. We demonstrate the impact of artificial versus natural hydrologies on such fens by comparing 3 case areas: the Biebrza valley (reference) and the Gorecht and Vecht river plains (both reclaimed and drained). The patterns found in the fairly undisturbed Biebrza area suggest local water quality is governed by a strong regional groundwater flow emerging in the fen near the valley margins and seeping through it down to the river. Hence water quality gradients are smooth: there is little variation in water type over large distances. The pattern is determined by the natural geomorphology. In the reclaimed Vecht and Gorecht river plains large differences exist at short distance. Regional water flow from the adjacent ridges into the plains is weak here and governed primarily by water management (polders and pumping wells). However, the relations between specific water types and fen species and communities in this artificial pattern are quite similar to those found in the natural landscape. Low-productive rich fens are fed by calcium-rich and base-rich, nutrient-poor groundwater in both cases. While conservation of such rich fens is served best by maintaining the natural groundwater flow, some opportunities for restoration with an artificial hydrology are discussed.     

Comparison of nitrogen and phosphorus fluxes in some European fens and floodplains

Question: How do nitrogen and phosphorus budgets and balances differ between eutrophic fens and floodplains in western Europe and fens and floodplains in Poland, where we expect less eutrophication to occur? Location: Wetlands along the rivers Dommel (The Netherlands), Zwarte Beek (Belgium) and Biebrza (NE Poland). Methods: Assessment of external input and output fluxes as well as net N‐mineralization rates. Annual N‐ and P‐balances were estimated by the sum of all external input and output fluxes: atmospheric deposition, input of dissolved matter by flooding, input of sediment by flooding, input by groundwater, output by leaching, output by hay‐making and for N also input by N₂‐fixation. For N we also estimated net annual N‐availability for plant growth, i.e. the N‐budget, which includes net mineralization in soil. Results: The studied wetland sites had a negative balance, which means that nutrients are depleted but only if mown annually, except for the Dutch/Belgian fens which had an equilibrium N‐balance and the Polish fen which had an equilibrium P‐balance. For the N‐budget it appeared that atmospheric deposition added significantly to the budget of Dutch/Belgian fens and N‐mineralization added significantly to fen and floodplain budgets, except for the Polish fens. Mineralization dominates the N‐budget of the western European floodplains. Hay‐making is the most important output pathway, particularly if practised annually. It seems to diminish N‐enrichment in the Dutch fens and floodplains. Conclusions: We conclude that western European fens and floodplains as well as Polish floodplains have a significant positive N‐budget indicating that there is a surplus of N for plant growth. In the Polish fens this is less due to low atmospheric deposition and lower N‐mineralization rates. The latter is associated with less drying out of the studied Polish ecosystems in summer. Our approach, although an approximate quantification, is helpful for assessing priorities focused on nutrient management.     

Germination and seedling survival in fens undergoing succession

We studied mechanisms of vegetation change in fens subject to succession from open water to floating mats and finally herbaceous rich-fens. Earlier research showed that these systems are characterized by transient seed banks. Our main question was whether seedlings of later successional fen stages are already present in earlier stages, remaining subordinate in the vegetation until conditions become suitable for them. If, however, conditions during succession change in a way that only a limited set of species can survive as seedlings during each of the successional stages, no seedling bank will exist. The transient character of the seeds would then imply that seeds will not germinate and will subsequently die and that seeds that have germinated in the “wrong” stage will not become established. We hypothesized that: (1) germination and seedling survival of fen species are significantly better in the successional fen stage for which these species are characteristic, (2) as a consequence no seedling banks occur in these fens. In a field experiment, seeds of five characteristic fen species in the standing vegetation of three successional fen stages i.e. raft fen, quaking fen and rich fen were sown in each of these stages in a turf pond in the Tienhoven area, The Netherlands. Germination and seedling survival were measured over two growing seasons together with environmental variables. Germination was higher in the “own” stage for all species groups as was survival for quaking fen species and rich fen species. For both these stages, percentage of germination and survival of four out of five characteristic species were significantly higher in the “own” stage. Germination and survival can be considered stage-dependent and it was concluded that seedling banks do not exist in these fens. Site-specific environmental variables act as a sieve and differentiate on species presence already during early life history stages. We found clues that the environmental sieve acts at the level of nutrient availability, tolerance for high sulphide concentration and light climate. Because of the transient seed bank and absence of a seedling bank in these fen wetlands, successful establishment of species necessitates a continuous dispersal of characteristic species until the environmental conditions permit establishment. This also implies that species of the whole successional sere should be present within dispersal distance.     

Effects of stream flooding on the distribution and diversity of groundwater‐dependent vegetation in riparian areas

1. Effects of the frequency and duration of flooding on the structural and functional characteristics of riparian vegetation were studied at four sites (n = 80, 50 × 50 cm, plots) along medium‐sized naturally meandering lowland streams. Special focus was on rich fens, which – due to their high species richness – are of high priority in nature conservation. 2. Reed beds, rich fens and meadows were all regularly flooded during the 20‐year study period, with a higher frequency in reed bed areas than in rich fen and meadow areas. In rich fens, species richness was higher in low frequency flooded areas (≤3 year^?1) than in areas with a high frequency of flooding (>3 year^?1) or no flooding, whereas species richness in reed beds and meadows was unaffected by flood frequency. 3. The percentage of stress‐tolerant species was higher in low intensity flooded rich fen areas than in high intensity and non‐flooded areas, indicating that the higher species richness in low frequency flooded rich fens was caused by competitive release. We found no indication that increased productivity was associated with high flooding frequencies. 4. We conclude that the restoration of morphological features in stream channels to increase the flooding regime can be beneficial for protected vegetation within riparian areas, but also that groundwater discharge thresholds and critical levels for protected vegetation should be identified and considered when introducing stream ecosystem restoration plans.     

Permafrost conditions in peatlands regulate magnitude,timing, and chemical composition of catchment dissolved organic carbon export

Permafrost thaw in peatlands has the potential to alter catchment export of dissolved organic carbon (DOC) and thus influence downstream aquatic C cycling. Subarctic peatlands are often mosaics of different peatland types, where permafrost conditions regulate the hydrological setting of each type. We show that hydrological setting is key to observed differences in magnitude, timing, and chemical composition of DOC export between permafrost and nonpermafrost peatland types, and that these differences influence the export of DOC of larger catchments even when peatlands are minor catchment components. In many aspects, DOC export from a studied peatland permafrost plateau was similar to that of a forested upland catchment. Similarities included low annual export (2–3 g C m^?2) dominated by the snow melt period (~70%), and how substantial DOC export following storms required wet antecedent conditions. Conversely, nonpermafrost fens had higher DOC export (7 g C m^?2), resulting from sustained hydrological connectivity during summer. Chemical composition of catchment DOC export arose from the mixing of highly aromatic DOC from organic soils from permafrost plateau soil water and upland forest surface horizons with nonaromatic DOC from mineral soil groundwater, but was further modulated by fens. Increasing aromaticity from fen inflow to outlet was substantial and depended on both water residence time and water temperature. The role of fens as catchment biogeochemical hotspots was further emphasized by their capacity for sulfate retention. As a result of fen characteristics, a 4% fen cover in a mixed catchment was responsible for 34% higher DOC export, 50% higher DOC concentrations and ~10% higher DOC aromaticity at the catchment outlet during summer compared to a nonpeatland upland catchment. Expansion of fens due to thaw thus has potential to influence landscape C cycling by increasing fen capacity to act as biogeochemical hotspots, amplifying aquatic C cycling, and increasing catchment DOC export.     

Phosphorus retention and movement across an ombrotrophic-minerotrophic peatland gradient

An understanding of the mechanisms controlling nutrient availability and retention in and across ecosystems allows for a greater understanding of the role of nutrients in maintaining ecosystem structure and function. To examine the underlying mechanisms of phosphorus (P) cycling in northern peatlands, we compared the retention and movement of P across a natural hydrologic/pH gradient in nine peatlands by applying as a light rain an in situ tracer amount of ³²PO₄ ^–3 to track changes in P pools (vegetation, soil, microbial) over 30 days. The ³¹P concentrations of available P, microbial P, and the root P at 10–20 cm did not differ across the gradient, although total soil P and aboveground vegetation P content (g m^–2) increased from bog to rich fen. Total retention of ³²P in the first 24 hours of application was greatest in the bogs and intermediate fens (90–100%) and was very low (20–50%) in the rich fens. Retention of P in the different pools was dependent on the type of peatland and changed with time. In the first 24 hours in the bogs and intermediate fens, the microbial pool contained the largest amount of ³²P, but by the seventh day, the aboveground vegetation contained the largest amount. In the rich fen, the recovered ³²P was almost equally divided between the aboveground vegetation and the litter layer with very little in other pools. Therefore, although bogs and intermediate fens have a small total P pool, they have similar P availability to rich fens because of rapid cycling and efficient retention of P.     

Hydrological landscape settings of base‐rich fen mires and fen meadows: an overview

Question: Why do similar fen meadow communities occur in different landscapes? How does the hydrological system sustain base‐rich fen mires and fen meadows? Location: Interdunal wetlands and heathland pools in The Netherlands, percolation mires in Germany, Poland, and Siberia, and calcareous spring fens in the High Tatra, Slovakia. Methods: This review presents an overview of the hydrological conditions of fen mires and fen meadows that are highly valued in nature conservation due to their high biodiversity and the occurrence of many Red List species. Fen types covered in this review include: (1) small hydrological systems in young calcareous dune areas, and (2) small hydrological systems in decalcified old cover sand areas in The Netherlands; (3) large hydrological systems in river valleys in Central‐Europe and western‐Siberia, and (4) large hydrological systems of small calcareous spring fens with active precipitation of travertine in mountain areas of Slovakia. Results: Different landscape types can sustain similar nutrient poor and base‐rich habitats required by endangered fen meadow species. The hydrological systems of these landscapes are very different in size, but their ground water flow pattern is remarkably similar. Paleoecological research showed that travertine forming fen vegetation types persisted in German lowland percolation mires from 6000 to 3000 BP. Similar vegetation types can still be found in small mountain mires in the Slovak Republic. Small pools in such mires form a cascade of surface water bodies that stimulate travertine formation in various ways. Travertine deposition prevents acidification of the mire and sustains populations of basiphilous species that elsewhere in Europe are highly endangered. Conclusion: Very different hydrological landscape settings can maintain a regular flow of groundwater through the top soil generating similar base‐rich site conditions. This is why some fen species occur in very different landscape types, ranging from mineral interdunal wetlands to mountain mires.     

Potential role of seed banks and understory species in restoring quaking fens from floating forests

Quaking fens and floating forests in the Westbroek Polder, the Netherlands, have seed banks with an estimated mean total seed density of 920 and 690 seeds m^-2, respectively. Fifty nine species were found in two or more quaking fens of which 39 were also found in their seed banks. Fifty species were found in two or more floating-forests and 31 of these were also found in their seed banks. The seed banks of fens and forests had 36 species in common. The seed banks of fens and forests were more similar (Spearman rank order correlation coefficients of 0.64 on the basis of percent presence and 0.61 on the basis of mean seed density in the seed bank and mean species cover in the vegetation) than were the seed banks and vegetation of either fens or forests (0.46 and 0.46 for fens; 0.26 and 0.23 for forests).When the tree canopy is removed from a floating forest to try to restore it to a quaking fen, it is predicted that a total of 47 species from the floating-forest understory will persist. Eleven of these species are not present in fens. Fifteen additional species can become established potentially from the forest seed banks. Sixteen species found in fens, however, are predicted to be missing from cleared sites. Of these 16 species, 7 are uncommon aquatic-and fen species.     

Relict occurrences of boreal brown-moss quaking rich fens in the Carpathians and adjacent territories

Quaking rich fens dominated by boreal semi-aquatic brown-mosses such as Scorpidium scorpioides and Calliergon trifarium are extremely rare in the Carpathians. These fens harbour endangered species persisting at few localities in the region. However, their phytosociological classification has not been sufficiently solved yet, because they lack Sphagnum species as well as calcicole species characteristic for the Caricion davallianae alliance. A recent pan-European synthesis on fen vegetation suggests that these fens belong to the Stygio-Caricion limosae alliance (boreal rich fen vegetation). The isolated occurrence of this alliance southward of the boreal zone and outside the Alps is rather exceptional and might represent a relict from an early post-glacial period. In this study, we compared phytosociological data for the Stygio-Caricion limosae alliance between Northern Europe and the Carpathians plus adjacent regions (the Bohemian Massif, the Dinaric Alps) using NMDS and cluster analysis. We found that the species composition of brown-moss quaking rich fens in Central and Southeastern Europe corresponds well with that in Northern Europe, confirming their assignment to Stygio-Caricion limosae. We further reconstructed the potential past distribution of the alliance in Czech Republic and Slovakia using available floristic and macrofossil data. Macrofossil data suggest that this vegetation type had been much more common in Central Europe and that today it persists only in ancient fens, showing the long-term stability of environmental conditions. The main causes of its present-day rarity are Middle-Holocene woodland phases in fens and recent water table decreases caused by anthropogenic deterioration of the water regime in the landscape.     

Impact of local- and regional-scale restoration measures on a vulnerable rich fen in the Naardermeer nature reserve (the Netherlands)

Restoration of rich fens is commonly attempted through local-scale measures, such as removal of sod or blockage of ditches. However, regional-scale restoration measures, that aim to re-establish the original hydrology in which rich fens developed, might have a more long-lasting effect. We investigated the effect of local- and regional-scale restoration measures on a vulnerable rich fen in the Naardermeer nature reserve in the Netherlands. We compared water quality and vegetation composition of the fen before and after the restoration measures, almost 30 years apart. Overall rich fen species increased and although this indicates the desired increased supply of fresh mineral-rich groundwater to the fen, continued succession towards poor fen vegetation has not been prevented in the entire fen. Despite sod layer removal, we observed an increase in a Polytrichum-dominated vegetation in patches that are primarily fed by rainwater. Our findings confirm results from a previous study which showed that brackish palaeo-groundwater is still contributing substantially to the water balance of the fen, especially in periods of precipitation deficit. We conclude that the local- and regional-scale restoration measures have been successful in increasing the abundance of rich fen species in parts of the fen. However, considering the pressures of climate change and high atmospheric N-deposition on the fen, it is uncertain whether rich fen species can be sustained in quite nutrient-poor conditions in the future. Therefore, there is a need for continued management that keeps the nutrient-poor and mineral-rich conditions of the fen intact.

     

Sulphate reduction and calcite precipitation in relation to internal eutrophication of groundwater fed alkaline fens

Although in Europe atmospheric deposition of sulphur has decreased considerably over the last decades, groundwater pollution by sulphate may still continue due to pyrite oxidation in the soil as a result of excessive fertilisation. Inflowing groundwater rich in sulphate can change biogeochemical cycling in nutrient-poor wetland ecosystems. Incoming sulphate loads may induce internal eutrophication as well as the accumulation of dissolved sulphide, which is phytotoxic. We, however, argue that upwelling sulphate rich groundwater may also promote the conservation of rare and threatened alkaline fens, since excessive fertilisation and pyrite oxidation also produces acidity, which invokes calcite dissolution, and increased alkalinity and hardness (Ca²⁺ + Mg²⁺) of the inflowing groundwater. Our observations in a very species-rich wetland nature reserve show that sulphate is reduced and effectively precipitates as iron sulphides when this calcareous and sulphate rich groundwater flows upward through the organic soil of the investigated nature reserve. Furthermore, we show that sulphate reduction coincides with an increase in alkalinity production, which in our case results in active calcite precipitation in the soil. In spite of the occurring sulphate reduction we found no evidence for internal eutrophication. Extremely low phosphorous concentration in the pore water could be attributed to a high C:P ratio of soil organic matter and co-precipitation with calcite. Our study shows that seepage dependent alkaline fen ecosystems can be remarkably resilient to fertilisation and pyrite oxidation induced groundwater quality changes.