Long-term effects of drainage and hay-removal on nutrient dynamics and limitation in the Biebrza mires,Poland

To provide a reference for wetlands elsewhere we analysed soil nutrients and the vegetation of floodplains and fens in the relatively undisturbed Biebrza-valley, Poland. Additionally, by studying sites along a water-table gradient, and by comparing pairs of mown and unmown sites, we aimed with exploring long-term effects of drainage and annual hay-removal on nutrient availabilities and vegetation response. In undrained fens and floodplains, N mineralization went slowly (0–30 kg N ha⁻¹ year⁻¹) but it increased strongly with decreasing water table (up to 120 kg N ha⁻¹ year⁻¹). Soil N, P and K pools were small in the undisturbed mires. Drainage had caused a shift from fen to meadow species and the disappearance of bryophytes. Biomass of vascular plants increased with increasing N mineralization and soil P. Annual hay-removal tended to have reduced N mineralization and soil K pools, but it had increased soil P. Moreover, N concentrations in vascular plants were not affected, but P and K concentrations and therefore N:P and N:K ratios tended to be changed. Annual hay-removal had induced a shift from P to K limitation in the severely drained fen, and from P to N limitation in the floodplain. The low nutrient availabilities and productivity of the undisturbed Biebrza mires illustrate the vulnerability of such mires to eutrophication in Poland and elsewhere. In nutrient-enriched areas, hay removal may prevent productivity increase of the vegetation, but also may severely alter N:P:K stoichiometry, induce K-limitation at drained sites, and alter vegetation structure and composition.     

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.     

Regionalized nitrogen budgets in forest soils for different deposition and forestry scenarios in Sweden

Aim The aim of this work was to estimate on a regional scale the effects of nitrogen (N) deposition and harvest intensity on N‐budgets in forest soils as a basis for strategies of emission reduction and sustainable forest management methods. Location The calculations were applied to Sweden, a country with a managed forest area of 23 × 10⁶ ha. Methods Mass balance calculations, including N‐deposition, N‐fixation, N‐loss through harvest, and N‐leaching, were performed on a GIS platform using 5 × 5 km grids. Modelled deposition data together with spatial data obtained from the National Forest Inventory served as the basis for the calculations. Four different scenarios were run: a ‘base scenario’ involving present deposition and conventional forestry (stem harvest only); a ‘whole‐tree harvesting scenario’ with present deposition and the harvesting of stems, branches and needles; a ‘decreased deposition scenario’; and a ‘whole‐tree harvesting and decreased deposition scenario’. Results There was a sharp N‐accumulation gradient with an increase in accumulation in the direction of the south‐western part of Sweden. In the ‘base scenario’, N‐accumulation appeared in the country as a whole, apart from certain small areas in the northern part. Whole‐tree harvesting led to net losses in extensive areas located mainly in northern and central Sweden. In most parts of the country, whole‐tree harvesting combined with decreased deposition was found to result in net losses. Main conclusions The intensity of the forestry has a strong impact on the N‐budget. Conventional forestry in combination with the present deposition level results in a high net accumulation of N in the south‐western parts of Sweden and accordingly, in a risk of unwanted environmental effects such as increased N‐leaching. With whole‐tree harvesting, the N‐balance is negative in parts of Sweden, mainly in the northern and central parts. N‐fertilization may become necessary there if the present level of forest production is to be maintained.     

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.     

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.     

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     

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.     

Production,nutrient availability,and elemental balances of two meadows affected by different fertilization and water table regimes in The Netherlands

The restoration of degraded peat-grasslands is an important nature conservation goal in The Netherlands. We investigated the effects of ceased fertilization (15 years) combined with a groundwater-raised water table (6 years) on the production of the peat-grassland vegetation and soil nutrient availability in a meadow. Furthermore, we evaluated whether and how this difference between meadows affected the balances between nutrient inputs and outputs in the ecosystem. We used an adjacent fertilized meadow in which the water table followed agricultural practice as a control. Yield of the grassland vegetation was significantly lower in the wet than in the control meadow. The tissue concentrations of N, P, and K in the harvested vegetation were significantly lower, but those of Ca higher in the wet than in the control meadow. The difference between both meadows significantly affected the annual nitrification rate, but not the annual C and N mineralization rates and the annual net P and K release rates. The difference between both meadows also significantly affected the seasonal nitrification and K release rates. Season exerted a significant effect on the seasonal C and N mineralization and nitrification rates. The elemental balances and relative contributions of the balance terms to elemental inputs and outputs varied considerably with element. Annually, the wet meadow lost N, P and K, while the control meadow gained these elements. The elemental demand of the grassland vegetation in the wet meadow was met for N for a large part by mineralization and for the remainder by atmospheric deposition, for P it was in the same order of magnitude as the net soil-P release, as it was for K. It is to be expected that the soil resources of N, P and K will continue to decrease under a continued regime of ceased fertilization and a raised water table, with those of N decreasing with the same rate, of P morerapidly and of K more slowly than estimated from regressions.     

Mineralization and Decomposition Rates in Restored Pine Fens

Growing public interest in conserving peatlands has created a need for restoration and rapid indicators of progress in peat formation. Vegetation and hydrological indicators are commonly assessed, but changes in mineralization and decomposition rates might better indicate when peat formation is underway in restored peatlands. In Finland, we investigated differences in mineralization and decomposition in the upper peat layer of five undrained and eight drained Pinus‐dominated fens from 2006 to 2009. Forestry‐drained fens were restored in 2007 by harvesting either whole trees or only stems, and by damming and filling ditches. Before restoration, net N mineralization rate was slightly higher in the drained than in undrained fens, whereas soil pH and Betula leaf litter decomposition rate were lower. After restoration, net N mineralization rate was similar for the undrained and restored fens, except near ditches after stem harvest. Also, soil pH and decomposition rate of Betula leaf litter became similar for undrained and restored fens. We conclude that whole tree harvest is a more suitable method for peatland restoration than stem harvest and that mineralization and decomposition rates are suitable indicators for peat formation after restoration.     

Decomposition of Carex and Sphagnum litter in two mesotrophic fens differing in dominant plant species

Peatlands can be classified into fens and bogs based on their hydrology. Development of fens to bogs is accompanied by the invasion of Sphagnum species. The purpose of this study was to determine how the decomposition process in fens is influenced by the transition from a vascular plant-dominated system to a Sphagnum -dominated system. We carried out a reciprocal litter bag experiment, using litter of Carex diandra , C. lasiocarpa , Sphagnum papillosum and S. squarrosum in a fen dominated by Sphagnum species and a fen without Sphagnum . Decomposition rate and nitrogen and phosphorus dynamics of the plant litter were measured in a field experiment for two years. Decomposition rate was highest for the Carex litter types and lowest for the Sphagnum litter types. Surprisingly, decomposition rates hardly differed between the two sites. Nutrient dynamics, however, showed a clear site-effect: In the Sphagnum site net mineralization was observed for all litter types whereas in the Carex site net immobilization was observed. These results show that carbon and nutrient cycles were coupled in a different way in a Sphagnum -dominated and a Carex -dominated site, respectively. Nutrient availability and adaptation of the microbial community to nutritional and other environmental conditions may be the main regulators of carbon and nutrient cycles in these peatlands.     

The phosphorus budget of the Marsdiep tidal basin (Dutch Wadden Sea) in the period 1950–1985: importance of the exchange with the North Sea

Nutrient and carbon budgets of the western part of the Dutch Wadden Sea are based on the concept formulated by Postma (1954): import of organic compounds from the North Sea, mineralization in the estuary ans subsequently an export of dissolved inorganic nutrients to the North Sea. In this paper the phosphorus budget of the westernmost part of the Wadden Sea (Marsdiep basin) during the period 1950–1985 is considered to evaluate this concept and to investigate whether eutrophication processes have changed these P-budgets during the last decades. Most attention is paid to the importance of the exchange of P-components between the tidal basin and the open sea. The budgets are based on a general mass balance equation in which the main entries are the loadings via the freshwater sources, the net sediment-water exchange, the exchange with the North Sea, the input from the adjacent Vlie basin, and a transformation term representing the interchange between dissolved and particulate phosphorus. Averaged over the entire period 55% of the phosphate input (SRP) to the basin is delivered by the fresh water sources, and 45% by the Vlie basin. For non-SRP components these numbers are 50% and 40% respectively, while only 10% comes from the North Sea. The import from the North Sea, as calculated by our model thus seems less important than previously documented. There is a clear effect of the eutrophication during the last decades. In the first 20 years of the period investigated there was a moderate increase in the TP input to the basin, partly caused by an increasing input of non-SRP (TP minus soluble reactive phosphorus) from the North Sea. In 1970/71 almost 30% of the non-SRP loading originated from the North Sea. From approximately 1975 onwards this import of non-SRP turns into an export, while the TP-loading from the other sources inclined, until a maximum was reached in 1981. Recently a decrease in the TP-inputs has been observed. It is concluded that the recent output of phosphorus from the Marsdiep basin may have a considerable impact on the adjacent area of the North Sea.Publication no. 2 of the project Applied Scientific Research Neth Inst. for Sea Res. (BEWON).     

Impact of prescribed burning on the nutrient balance of heathlands with particular reference to nitrogen and phosphorus

Question: Can prescribed winter burning compensate atmospheric nutrient loads for dry heathlands? What effects does prescribed burning have on nutrient balances, particularly as regards the limiting nutrients N and P? Location: Lueneburg Heath, NW Germany. Methods: In two burning experiments (in 10/15 year old Calluna‐stands) nutrient balances (for N, Ca, K, Mg, P) were calculated by analysing nutrient inputs (atmospheric deposition, ash deposition), nutrient stores (above‐ground biomass, organic horizon) and nutrient outputs (biomass combustion, leaching). Results: Atmospheric nutrient deposition amounted to 22.8 kg.ha^‐1.a^‐1 for N and < 0.5 kg.ha^‐1.a^‐1 for P. Nutrient stores in the above‐ground biomass were 95/197 kg.ha^‐1 for N and 5/13 kg.ha^‐1 for P (first/second experiment, respectively). From these stores 90/53% (for N) and 25/14% (for P) were removed by burning. Effects of leaching on nutrient balances were low. In the first two years after burning, leaching rates of N increased by about 4/6 kg.ha^‐1, whereas leaching rates of P did not change significantly. Input/output‐ratios showed that prescribed burning leads to positive nutrient balances for N, Ca and Mg in the long term. For example, the amounts of N removed by prescribed burning are equivalent to ca. five years of atmospheric inputs. Applied in ten‐year cycles, this measure alone cannot prevent N accumulation in the long term. Conclusion: Regarding 10/15 year old Calluna‐heaths, we assume that prescribed burning cannot compensate for atmospheric N inputs, thus making long‐term changes in the nutritional state inevitable. Therefore, prescribed burning should be applied in combination with high‐intensity management measures.     

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.     

Nitrogen and phosphorus in mire plants: variation during 50 years in relation to supply rate and vegetation type

Southern Sweden has long been exposed to an increasing atmospheric nitrogen deposition. We investigated the effects of this supply on the Sphagnum mire vegetation in SW Götaland by comparing above‐ground tissue concentrations of N and P and biomass variables in five vascular plant and two Sphagnum species collected during three periods since 1955 at 81 sites representing three vegetation types, viz. ombrotrophic bog, extremely poor fen and moderately poor fen, within two areas differing in annual N deposition. The N:P ratios in the plants were rarely below 17, suggesting P as the growth‐limiting mineral nutrient. In the vascular plants both growth and concentrations of N and P were highest in the moderately poor fen sites because of a higher mineralization rate, the differences between the extremely poor fen and bog sites being smaller in these respects. In the extremely poor fen and bog sites the N concentrations were slightly higher in the area with the highest N deposition. From 1955 to 2002 the concentration of N in the Sphagnum spp. increased proportionally to the supply rate while P remained constant. In the vascular plants the concentrations of P remained constant while N showed slightly decreasing trends in the bog and extremely poor fen sites, but since the size of the plants increased the biomass content of N and P increased, too. The increased N deposition has had its greatest effects on the site types with the highest Sphagnum biomass and peat accumulation rate. The high N concentration in the Sphagnum mosses probably reduced their competitiveness and facilitated the observed expansion of vascular plants. However, the increased N deposition might also have triggered an increased mineralization in the acrotelm increasing the supply of P to the vascular plants and thus also their productivity. This may also explain the slightly higher productivity among the vascular plants in the area with the highest N deposition rate. In conclusion, it seems as the increased N deposition has directly influenced only the growth of the Sphagnum mosses and that the effects on the growth of the vascular plants are indirect.     

Impact of sheep grazing on nutrient budgets of dry heathlands

Questions: What effect does sheep grazing have on the nutrient budgets of heathlands? Can grazing compensate for atmospheric nutrient loads in heathland ecosystems? What are the conclusions for heathland management? Location: Lüneburg Heath, NW Germany. Methods: During a one-year grazing experiment (stocking rate 1.1 sheep/ha) nutrient balances for N, Ca, K, Mg and P were calculated by quantifying input rates (atmospheric deposition, sheep excrement) and output rates (biomass removal, leaching). Results: Atmospheric nutrient deposition amounted to 22.8 kg.ha⁻¹.a⁻¹ for N and < 0.2 kg.ha⁻¹.a⁻¹ for P. Sheep excrement increased the inputs for N and P by ca. 3.5 and 0.2 kg.ha⁻¹.a⁻¹, respectively. Grazing reduced N- and P-stores in the above-ground biomass by 25.6 and 1.9 kg.ha⁻¹.a⁻¹, respectively. N-and P-losses via leaching amounted to 2.2 and < 0.2 kg.ha⁻¹.a⁻¹. Output:input ratios for P were high, indicating that grazing severely affected P-budgets of heaths. Conclusions: Our results suggest that sheep grazing has the potential to compensate for atmospheric nutrient loads (particularly for current N deposition rates). However, in the long term the combination of elevated N-deposition and P-loss due to grazing may cause a shift from N-(co-) limited to more P-(co-) limited plant growth. To counteract an aggravation of P-deficiency in the long term, grazing may be combined with management measures that affect P-budgets to a lesser extent (e.g. prescribed burning).     

Importance of sediment deposition and denitrification for nutrient retention in floodplain wetlands

Questions: Various floodplain communities may differ in their relative abilities to influence water quality through nutrient retention and denitrification. Our main questions were: (1) what is the importance of sediment deposition and denitrification for plant productivity and nutrient retention in floodplains; (2) will rehabilitation of natural floodplain communities (semi‐natural grassland, reedbed, woodland, pond) from agricultural grassland affect nutrient retention? Location: Floodplains of two Rhine distributaries (rivers Ussel and Waal), The Netherlands. Methods: Net sedimentation was measured using mats, denitrification in soil cores by acetylene inhibition and bio‐mass production by clipping above‐ground vegetation in winter and summer. Results: Sediment deposition was a major source of N and P in all floodplain communities. Highest deposition rates were found where water velocity was reduced by vegetation structure (reedbeds) or by a drop in surface elevation (pond). Sediment deposition was not higher in woodlands than in grassland types. Denitrification rates were low in winter but significantly higher in summer. Highest denitrification rates were found in an agricultural grassland (winter and summer) and in the ponds (summer). Plant productivity and nutrient uptake were high in reedbeds, intermediate in agricultural grasslands, ponds and semi‐natural grasslands and very low in woodlands (only understorey). All wetlands were N‐limited, which could be explained by low N:P ratios in sediment. Conclusions: Considering Rhine water quality: only substantial P‐retention is expected because, relative to the annual nutrient loads in the river, the floodplains are important sinks for P, but much less for N. Rehabilitation of agricultural grasslands into ponds or reedbeds will probably be more beneficial for downstream water quality (lower P‐concentrations) than into woodlands or semi‐natural grasslands.     

The relationships of vegetation to surface water chemistry and peat chemistry in fens of Alberta,Canada

The relationships between vegetation components, surface water chemistry and peat chemistry from 23 fens in boreal Alberta, Canada, substantiate important differences along the poor to rich fen gradient. Each of the three fen types have their own characteristic species. The extreme-rich fens are characterized by Calliergon trifarium, Drepanocladus revolvens, Scirpus hudsonianus, S. cespitosus, Scorpidium scorpioides, and Tofieldia glutinosa. Moderate-rich fens are characterized by Brachythecium mildeanum, Carex diandra, Drepanocladus vernicosus, D. aduncus, and D. polycarpus. Poor fens are characterized by Carex pauciflora, Drepanocladus exannulatus, Sphagnum angustifolium, S. jensenii, and S. majus. Moderate-rich fens have fewer species in common with poor fens than with extreme-rich fens, while species richness is highest in the moderate-rich fens and lowest in poor fens. Variation in vascular plant occurrence appears to be more associated with nutrient levels, while bryophytes are more affected by changes in acidity and mineral elements. Based on chemical criteria, the three fen types are clearly separated by surface water pH, calcium, magnesium, and conductivity, but are less clearly differentiated by the nitrogen and phosphorus components of the surface waters. Moderate-rich fens are chemically variable both temporally and spatially, whereas poor fens and extreme-rich fens are more stable ecosystems. Whereas components of alkalinity-acidity are the most important factors that distinguish the three fen types in western Canada, nutrient concentrations in the surface waters generally do not differ appreciably in the three fen types.     

Effects of enhanced nitrogen deposition and phosphorus limitation on nitrogen budgets of semi-natural grasslands

Increased reactive atmospheric N deposition has been implicated in floristic changes in species‐rich acidic and calcareous grasslands, but the fate of this pollutant N in these ecosystems is unknown. This paper reports the first analysis of N budgets and N fluxes for two grasslands in the White Peak area of Derbyshire, one of the most heavily N‐polluted locations in the UK. N fluxes were monitored in lysimeter cores (retaining the original turfs) taken from field plots of unimproved acidic and calcareous grasslands that had received (in addition to ambient N deposition) simulated enhanced N deposition treatments of 3.5 and 14 g N m^?2 yr^?1 for 6 years. The influence of reducing phosphorus limitation was assessed by factorial additions of P. Seasonal leached losses of nitrate, ammonia and organic N were monitored in detail along with estimates of N removal through simulated grazing and gaseous losses through denitrification and volatilization. The rates of N fluxes by these pathways were used to create N budgets for the grasslands. Both grasslands were found to be accumulating much of the simulated additional N deposition: up to 89% accumulated in the calcareous grassland and up to 38% accumulated in the acidic grassland. The major fluxes of N loss from these grasslands were by simulated grazing and leaching of soluble organic N (constituting 90% of leached N under ambient conditions). Leached inorganic N (mainly nitrate) contributed significantly to the output flux of N under the highest N treatment only. Loss of N through ammonia volatilization accounted for less than 6% of the N added as simulated deposition, while denitrification contributed significantly to output fluxes only in the acidic grassland during winter. The implications of the results for ecosystem N balances and the likely consequences of N accumulation on these grasslands are discussed.     

Nutrient dynamics in small mesotrophic fens surrounded by cultivated land

Summary Release of inorganic N and P in the organic soils of three small quaking fens in The Netherlands was studied by means of in situ incubation of the peat soil in plastic bottles. One of the fens had higher biomass production and lower species richness than the other two. The former fen is located in an area with downward groundwater percolation, whereas the latter fens are situated in an area of upward seepage of groundwater rich in calcium and bicarbonate.Mineralization of N proved to be slow in the low-productive fens, and 6 times faster in the highly productive fen. In the latter fen the amount of N mineralized during a certain period exceeded by far the amount accumulated in that period in the above-ground parts of the vegetation.The release of inorganic P was extremely slow in the two low-productive fens and rapid in the highly productive fen. The release rates were not related to the total P content of the soils. The slow P release in the low-productive fens may be due to the continuous inflow of groundwater rich in Ca, Al and Fe compounds.It can be concluded that the soil is a potentially large source of inorganic N and P in quaking fens. The importance of the soil versus other N and P sources is greater in nutrient-rich fens.     

Nutrient concentrations in mire vegetation as a measure of nutrient limitation in mire ecosystems

Abstract. The above-ground standing crop and nutrient concentrations in plant material were examined in 45 stands of mire vegetation in the Biebrza peatland, Poland. The stands included flood-plains, rich fens, transitional fens and bogs. The pattern in nutrient concentrations in the above-ground plant material resembled the pattern in nutrient concentrations in peatwater and peat which had been investigated in an earlier study. Concentrations of N were quite uniform along the gradient. P-concentrations were highest in the transitional fen. Critical nutrient concentrations were defined on the basis of a review of nutrient concentrations in plant material from peatlands in which a fertilization experiment had been carried out. Defined critical values for phanerogams were: 13-14 and 0.7 mg/g dry wt for N and P respectively. Concentrations lower than these values indicate deficiency. P/N ratios ≥ 0.07 indicate N-deficiency and P/N ratios ≤ 0.04 — 0.05 indicate P-deficiency. According to these values the Biebrza fens and bogs appear to be primarily deficient in N. The growth of the flood-plain vegetation does not appear to be restricted by nutrients.