In recent decades, sulfate concentrations in many European freshwater wetlands have increased by 10-fold or more, due mainly to the use of sulfate-polluted river water to compensate for water shortage in these areas. To test the effect of sulfate enrichment, a mesocosm experiment was set up, using waterlogged soil cores, intact with vegetation, from a mesotrophic fen meadow. During sulfate addition at environmentally relevant levels (0, 2, and 4 mmol L -1 ), phosphate concentration and alkalinity of the pore water rapidly rose due to increased sulfate reduction rates. Free sulfide accumulated to levels toxic to several wetland plant species and biomass regrowth after harvesting was significantly lower on treated soils, especially for Carex species. Eventually, the concentrations of ammonium, phosphate, and potassium increased strongly in the treated soils due to reduced uptake by plants and extra mineralization. Sulfate availability was rate limiting, until the supply of readily decomposable organic matter became limited. It is argued that the significance of the observed changes in free sulfide concentrations and in the rate of nutrient mobilization should be recognized, and that these effects can be as important as direct eutrophication caused by the import of nutrients. The reported changes may severely influence the plant species composition of freshwater wetlands.

Summary 1In order to test whether the observed invasion of ombrotrophic bogs in the Netherlands by Molinia caerulea and Betula pubescens is the result of long-term high nitrogen (N) loads, we conducted a 3-year fertilization experiment with Sphagnum fallax turfs. Six different N treatments were applied ranging from 0 (control) to 4 g N m − 2 year − 1 . 2 During the experimental period, ammonium concentrations in the peat moisture remained very low due to high uptake rates by Sphagnum . Tissue N concentrations in S. fallax showed a linear response to the experimental N addition. Excess N was accumulated as N-rich free amino acids such as arginine, asparagine and glutamine, especially at N addition rates of 0.25 g m − 2 year − 1 or higher, indicating N-saturation. 3 Despite the high tissue N : P ratio (above 35), above-ground biomass production by Molinia was still stimulated at N addition rates of 4 g m − 2 year − 1 , and foliar nutrient concentrations were unaffected compared to the control. In contrast to Molinia , Betula was unable to increase its above-ground biomass. Foliar N concentrations in Betula were significantly higher at N addition rates of 4 g m − 2 year − 1 and excess N was stored in foliar arginine, making up 27% of the total N concentration. Evapotranspiration was increased at higher N addition rates due to stimulated total above-ground biomass production of the vegetation. 4 N addition at the actual Dutch deposition rate of 4 g m − 2 year − 1 stimulated the growth of Molinia in this experiment, providing evidence that the observed dominance of Molinia on ombrotrophic bogs in the Netherlands is caused by high N deposition levels. Based on the observed changes in biomass production and tissue nutrient concentrations, we assume that a long-term deposition of 0.5 g N m − 2 year − 1 , or higher, leads to undesirable changes in species composition and increased risk of desiccation.

Summary1. In many ombrotrophic bog areas the invasion of grass (e.g. Molinia caerulea ) and tree (e.g. Betula pubescens ) species has become a major problem. We investigated whether the invasion of such species is due to high atmospheric nitrogen (N) deposition by conducting a fertilization experiment. 2. The effects of experimentally increased N input on Molinia , Betula and Eriophorum vaginatum were studied in desiccated bog vegetation in Ireland, where there is relatively low background N deposition. Four different N treatments were applied for 3 years: 0 (control), 2, 4 and 8 g m − 2 year − 1 . 3. Ammonium and nitrate concentrations in the peat moisture increased at high N addition rates, leading to significantly higher carbon : nitrogen (C : N) and nitrogen : phosphorus (N : P) ratios in the top layer of the peat. The potential CO 2 production rate of the peat was not stimulated at high N addition rates due to severe acidification of the peat. 4. Despite high tissue N : P ratios (above 40), above-ground biomass production by Molinia was stimulated at high N addition rates, and foliar nutrient concentrations were unaffected. In contrast to Molinia , Betula and Eriophorum were unable to increase their above-ground biomass, probably due to P limitation. Regrowth of the lichen Cladonia portentosa was suppressed at high N addition rates. Synthesis and applications. We conclude that the invasion of bogs by Molinia andBetula is likely to be less affected by desiccation than by increased N availability. Apparently, Molinia is well adapted to P-limiting conditions, which may explain its success in regions with increased N deposition levels. The high availability of P in many Dutch bogs compared with Irish bogs, together with prolonged high N deposition levels, may explain the strong increase in both Molinia and Betula observed in the Netherlands. As long as N and P availabilities in Dutch bogs are too high to prevent invasion of Betula and/or Molinia , management measures stimulating growth of Sphagnum mosses could probably reduce the negative effects of high N deposition levels.

Summary 1.A prerequisite for the restoration of desiccated bog remnants is rewetting the peat surface. Frequently in Europe, extensive areas are flooded in order to maximize water retention, and growth of peat mosses is often observed in the shallow zones. In deeper waters, regeneration appears to depend on whether residual peat will become buoyant and form floating rafts. 2. In order to study the initial stages of peat bog regeneration, conditions required for peat buoyancy were studied on peat monoliths collected from three cut-over bog remnants in the Netherlands. The effects of different peat quality and water chemistry on buoyancy of the monoliths, as well as growth of Sphagnum cuspidatum and nutrient availability, were followed in a glasshouse experiment. 3. Both groundwater and peat quality affected the buoyancy of the monoliths and the growth of S. cuspidatum. When groundwater containing bicarbonate (1 mmol l −1 HCO 3 -, pH 6·0) was applied, the pH of peat monoliths increased from c. 3·5 to c. 4·5 due to acid buffering. As a result, two of the peat types became more buoyant and the concentration, production and emissions of methane (CH 4 ) increased. It was concluded that the increase in CH 4 production, induced by the increased pH, was responsible for the buoyancy. ), pH was further increased to approximately pH 5·0 due to alkalinity generated by the SO 4 2-reduction process. CH 4 production, however, decreased because of interference from the SO 4 2-, as confirmed in additional incubation experiments. Phosphate concentrations, however, greatly increased in the HCO 3 -/SO 4 2-addition treatment due to the interaction between sulphide and iron phosphate precipitates. 5. In one of the peat types, treatments did not influence CH 4 production and buoyancy, most probably because of its low decomposability. The chemical characteristics of the peat, notably the concentrations of lignin and soluble phenolics as well as C:N, C:P and C:K ratios, were all higher than in the other two peat types. 6. The increase of S. cuspidatum biomass during the experiment appeared to be strongly related to the N:P ratios of the capitula, which differed considerably among the three peat types. 7. We conclude that when bog remnants are inundated the prospects for bog regeneration are largely determined by peat quality and water chemistry. Peat mats with low concentration of lignin and phenolics and low C:N ratios are most likely to become buoyant in water with a higher pH, so providing suitable environments for Sphagnum species. When peat quality is inadequate, either shallow inundation or the addition of suitable peat from elsewhere is indicated.

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Summary• The effects of different CO 2 concentrations in the acrotelm pore water on the development of Sphagnum magellanicum monoliths grown in wet conditions in the laboratory are reported here.• Decomposition processes result in much higher CO 2 concentrations in bog acrotelms than atmospheric concentrations. We therefore hypothesized that this substrate-derived carbon provides an important carbon source for Sphagnum .• S. ma gellanicum hummocks developed only when CO 2 concentrations in the acrotelm water were high. Plants in the low CO 2 treatment appeared to be carbonlimited; they had increased nutrient concentrations and decreased ratios of carbon to other nutrients. Chlorophyll concentrations were much higher in these plants, probably allowing them to harvest more CO 2 , but the allocation of carbon to structural tissues was lower, resulting in weak stems that were unable to remain upright. As a result of carbon-limitation, net biomass and height increases of the monoliths were much lower in the low CO 2 treatment.• Results indicate that, at least in wet conditions, atmospheric CO 2 alone is not sufficient to enable S. magellanicum to develop its normal vertical growth pattern. We conclude that the role of substrate-derived CO 2 as a carbon source for Sphagna is underestimated and requires reconsideration.

Summary• Peat bogs have accumulated more atmospheric carbon (C) than any other terrestrial ecosystem today. Most of this C is associated with peat moss (Sphagnum) litter. Atmospheric nitrogen (N) deposition can decrease Sphagnum production, compromising the C sequestration capacity of peat bogs. The mechanisms underlying the reduced production are uncertain, necessitating multifactorial experiments.• We investigated whether glasshouse experiments are reliable proxies for field experiments for assessing interactions between N deposition and environment as controls on Sphagnum N concentration and production. We performed a meta-analysis over 115 glasshouse experiments and 107 field experiments.• We found that glasshouse and field experiments gave similar qualitative and quantitative estimates of changes in Sphagnum N concentration in response to N application. However, glasshouse-based estimates of changes in production -even qualitative assessmentsdiverged from field experiments owing to a stronger N effect on production response in absence of vascular plants in the glasshouse, and a weaker N effect on production response in presence of vascular plants compared to field experiments.• Thus, although we need glasshouse experiments to study how interacting environmental factors affect the response of Sphagnum to increased N deposition, we need field experiments to properly quantify these effects.

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