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1 In 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.sup.-2] [year.sup.-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.sup.-2] [year.sup.-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.sup.-2] [year.sup.-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.sup.-2] [year.sup.-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 [m.sup.-2] [year.sup.-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.sup.-2] [year.sup.-1], or higher, leads to undesirable changes in species composition and increased risk of desiccation. Key-words: Betula pubescens, Molinia caerulea, nitrogen deposition, ombrotrophic bog, Sphagnum fallax

Increased sulfur loads originating from polluted surface water and groundwater, and from enhanced atmospheric input, are a major threat to the biogeochemical functioning and biodiversity of freshwater wetlands. Sulfate reduction, normally playing a modest role in these systems, becomes the most important biogeocbemical process, inducing severe eutrophication and sulfide toxicity. In field enclosure experiments we observed striking differences between the responses of two freshwater marshes to sulfate. On one location sulfate addition resulted in strong phosphorus mobilization without sulfide accumulation, whereas high sediment sulfide concentrations, known to be toxic to wetland macrophytes, were reached in the other marsh without eutrophication occurring. The results could be explained by differences in groundwater iron discharge and nutrient contents of the peat sediments. Sulfate reduction rates appeared to be limited by either electron donor availability (first marsh) or electron acceptor availability (second marsh). The implications of these findings are explained in relation to freshwater wetland management.

1 Raised bogs are traditionally regarded as being solely dependent upon atmospheric influences. However, these terrestrial carbon stores may also be affected by calcareous (bicarbonate-rich) groundwater, which is often found in deeper peat layers. 2 Field data from a bog area in the Netherlands were used to select appropriate levels of bicarbonate supply for an aquarium mesocosm experiment. The effects of 0, 0.1, 0.5, 1 and 2 mmol HC[[O.sub.3].sup.-] [1.sup.-1], and l mmol HC[[O.sub.3].sup.-] [1.sup.-1] + 2 mmol S[[O.sub.4].sup.2-][1.sup.-1], on the biogeochemistry of peat monoliths, including their Sphagnum vegetation, were investigated. 3 Slightly increased alkalinity rapidly stimulated buoyancy of living Sphagnum due to higher inorganic carbon concentrations in the water layer. Moderate HC[[O.sub.3].sup.-] concentrations also stimulated buoyancy of the entire peat monoliths after 3 weeks, because of increased methane production rates. High alkalinity, however, led to rapid internal phosphate mobilization, peat disintegration and Sphagnum die-off. This detrimental effect was stronger when sulphate was supplied simultaneously. 4 The results are discussed with respect to the carbon dynamics and overall functioning of peat bogs, in relation to hydrology and hydrochemistry (groundwater influence, sulphate pollution). Keywords: bicarbonate, carbon dioxide, eutrophication, hydrology, methane, peatlands, sulphur, Sphagnum