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Brown moss-dominated rich fens are characterized by minerotrophic conditions, in which calcium (Ca) and iron (Fe) concentrations show large variations. We examined the relative importance of Ca and Fe in relation to the occurrence of three typical brown moss species: Scorpidium scorpioides, Scorpidium cossonii, and Hamatocaulis vernicosus. Peat chemistry was examined in 24 stands of brown moss-dominated rich fens: 12 in the Netherlands and 12 in central Sweden. Ca and Fe turned out to be important drivers of brown moss composition. Fens dominated by Scorpidium scorpioides or Scorpidium cossonii were characterized by high pore water Ca-concentrations and total soil Ca-contents, but low P-availability. In these Ca-rich, but Fe-poor fens, foliar N:P ratios of vascular vegetation exceeded 20 g g−1, indicating phosphorus (P)-limitation due to Ca-P precipitation or low P-sorption capacity due to low Fe-levels. In contrast, fens dominated by Hamatocaulis vernicosus were characterized by high pore water Fe-concentrations and total soil Fe-contents, but also relatively high P-availability. N:P ratios in these fens were below 13.5 g g−1, indicating potential nitrogen (N)-limitation. We conclude that the relative roles of Ca and Fe, as related to the geohydrological conditions present, strongly determine the brown moss composition in rich fens through their differential effects on plant P-availability. [ABSTRACT FROM AUTHOR]

Both eutrophication and SO4 pollution can lead to higher availability of nutrients and potentially toxic compounds in wetlands. To unravel the interaction between the level of eutrophication and toxicity at species and community level, effects of SO4 were tested in nutrient-poor and nutrient-rich fen mesocosms. Biomass production of aquatic and semi-aquatic macrophytes and colonization of the water layer increased after fertilization, leading to dominance of highly competitive species. SO4 addition increased alkalinity and sulphide concentrations, leading to decomposition and additional eutrophication. SO4 pollution and concomitant sulphide production considerably reduced biomass production and colonization, but macrophytes were less vulnerable in fertilized conditions. The experiment shows that competition between species, vegetation succession and terrestrialization are not only influenced by nutrient availability, but also by toxicity, which strongly interacts with the level of eutrophication. This implies that previously neutralized toxicity effects in eutrophied fens may appear after nutrient reduction measures have been taken. [Copyright &y& Elsevier]

Reviews the major environmental problems in Dutch aquatic and semi-terrestrial fens including desiccation, eutrophication, acidification, habitat fragmentation and intoxication. Positive and the negative consequences of the restoration measures taken in Dutch fens; Factors responsible for the deterioration of plant and animal communities in fens; View that the knowledge of these key processes and factors make optimal restoration and management measures possible; Call for new research.

For the conservation and restoration of biodiverse rich fens, base-rich and nutrient-poor conditions are vital. In wetlands with artificially stable surface water levels, the re-introduction of temporary water level fluctuations has been postulated to restore the acid neutralizing capacity (ANC) of fens during inundation and to reduce surface water P-input during episodes with drought. This is the first study testing this hypothesis in large-scale field manipulation experiments in rich fens with threatened rich fen mosses, rich fens with Calliergonella , and poor fens with Sphagnum . Five different experiments were conducted: 2 weeks of raised levels (+10 cm) in a floating and a non-floating fen during winter, 2 weeks of high levels in a non-floating fen during summer, and 2 weeks of lowered levels (−15 cm) in a floating and a non-floating fen during summer. For floating fens, both lowered and raised surface water levels in adjacent ditches did not show any effect on water tables, soil ANC or nutrient levels in fens. For non-floating fens, raised surface water levels led to inundation in all vegetation types, without affecting nutrient concentrations or vegetation. Although redox potentials decreased immediately in upper soils, ANC was generally not enhanced in winter due to limited infiltration into the waterlogged soils. In summer, in contrast, ANC increased because accelerated evapotranspiration led to enhanced infiltration of inundation water and higher temperatures resulted in microbial alkalinity generation. Short-term lowering of surface water levels in summer led to lower water tables in non-floating fens, but only when precipitation rates were low. Vegetation, ANC and nutrient concentrations were, however, not affected. The effectiveness of short-term surface water level fluctuation to restore ANC strongly depends on peat buoyancy, water saturation of soils, season and weather conditions. This explains why short-term inundation in winter is often inadequate, while short-term inundation in summer does increase ANC. Short-term droughts do not affect the ANC or nutrient availability. Our results are not only important for the hydrological management of fens, but also have implications for future management since short-term extreme weather events will occur more frequently due to climate change. [ABSTRACT FROM AUTHOR]

Abstract: Worldwide, fens and peat lakes that used to be peat-forming systems have become a significant source of C, N and P due to increased peat decomposition. To test the hypothesis that net nutrient mineralization rates may be uncoupled from decomposition rates, we investigated decomposition and net mineralization rates of nutrients in relation to sediment and pore water characteristics. We incubated 28 non-calcareous peat sediments and floating fen soils under aerobic and anaerobic conditions. We also tried to find a simple indicator to estimate the potential nutrient mobilization rates from peat sediments to the water layer by studying their relation with sediment and pore water characteristics in 44 Dutch non-calcareous peat lakes and ditches. Decomposition rates were primarily determined by the organic matter content, and were higher under aerobic conditions. However, highly decomposed peat sediments with low C:P and C:N ratios still showed high net nutrient mineralization rates. At Fe:PO4 ratios below 1molmol−1, PO4 mobilization from the sediment to the water layer was considerable and linearly related to the pore water PO4 concentration. At higher ratios, there was a strong linear correlation between the Fe:PO4 ratio and PO4 mobilization. Hence, measuring Fe and PO4 in anaerobic sediment pore water provides a powerful tool for a quick assessment of internal PO4 fluxes. Mobilization of mineral N was largely determined by diffusion. Total sediment Fe:S ratios gave an important indication of the amount of Fe that is available to immobilize PO4. Pore water Fe concentrations decreased at ratios <1molmol−1, whereas pore water PO4 concentrations and PO4 mobilization to the water layer increased. As PO4 mobilization rates from the sediment to the water layer contribute to almost half of the total P load in Dutch peat lakes and fens, it is of pivotal importance to examine the magnitude of internal fluxes. Dredging of the nutrient-rich upper sediment layer will only be a useful restoration measure if both the influx of P-rich water and its internal mobilization from the newly exposed, potentially more reactive peat layer are sufficiently low. [Copyright &y& Elsevier]