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1. Water levels in areas with intensive agriculture have often been strictly controlled for decades. Recently, more natural fluctuating water levels have been propagated to improve the ecological quality of wetlands in these areas. This study investigated the effects of water levels on protected base-rich mesotrophic fens during winter and summer. 2. We used a mesocosm approach to simulate water level fluctuations under winter and summer conditions (light and temperature) to test the effects of water level on the biogeochemistry and vegetation of two highly endangered fen types with the brownmosses Scorpidium scorpioides and Hamatocaulis vernicosus as dominant mosses. 3. Both species showed decreased photosynthetic capacity at lower water levels. These levels also resulted in decreased alkalinity due to oxidation processes in the moss layer. High water levels led to increased pH and alkalinity, due to reduction processes and infiltration of baserich water. 4. For the P-limited S. scorpioides mesocosms, high water levels did not lead to P mobilization or to significant changes in biomass production, P uptake and N:P ratios. However, for the mesocosms with H. vernicosus, where P limitation was not obvious, high water levels did result in P mobilization and increased P availability due to iron reduction. The lower P mobilization for S. scorpioides appeared to be related to lower total soil P content and higher Fe:P and Ca:P ratios. Although high water levels resulted in the accumulation of ammonium under winter conditions, this did not happen under summer conditions, making ammonium toxicity unlikely. 5. Synthesis and applications. Our results suggest that low water levels could hamper protected brownmosses in rich fens, especially during the summer. In contrast, inundations may have a positive effect on rich fens by increasing the alkalinity. However, inundations may lead to eutrophication due to internal P mobilization in soils with a high total P content and low Fe:P ratio. Therefore, we recommend that soils with high total P content and low Fe:P ratios should not be flooded.

In regions with intensive agriculture, water level fluctuation in wetlands has generally become constricted within narrow limits. Water authorities are, however, considering the re-establishment of fluctuating water levels as a management tool in biodiverse, base-rich fens ('rich fens'). This includes temporary inundation with surface water from ditches, which may play an important role in counteracting acidification in order to conserve and restore biodiversity. Inundation may result in an increased acid neutralizing capacity (ANC) for two reasons: infiltration of base-rich inundation water into peat soils, and microbial alkalinity generation under anaerobic conditions. The main objectives of this study were to test whether short-term (2 weeks) summer inundation is more effective than short-term winter inundation to restore the ANC in the upper 10 cm of non-floating peat soils, and to explain potential differences. Large-scale field experiments were conducted for five years in base-rich fens and Sphagnum-dominated poor fens. Winter inundation did not result in increased porewater ANC, because infiltration was inhibited in the waterlogged peat and evapotranspiration rates were relatively low. Also, low temperatures limit microbial alkalinity generation. In summer, however, when temperature and evapotranspiration rates are higher, inundation resulted in increased porewater Ca and HCO3- concentrations, but only in areas with characteristic rich fen bryophytes. This increase was not only due to stronger infiltration into the soil, but also to higher microbial alkalinity generation under anaerobic conditions. In contrast, porewater ANC did not increase in Sphagnum-plots as a result of the ability of Sphagnum spp. to acidify their environment. In both rich and poor fens, flooding-induced P-mobilization remained sufficiently low to safeguard P-limited vegetation. NO3(-) and NH4(+) dynamics showed no considerable changes either. In conclusion, short-term summer inundation with base-rich and nutrient-poor surface water is considered beneficial in the management of non-floating rich fens, and much more effective than winter inundation.

Aim To further unravel P availability in mineral-rich fens, and test whether high Fe in the soil would lead to low P availability to the vegetation. Methods Mesotrophic fens were selected over gradients in Ca and Fe in central Sweden and the Netherlands, to study characteristics of vegetation, pore water and peat soil, including inorganic and organic forms of P, Fe and Al. Results Soil Fe was more important than region or soil Ca, and P availability to the vegetation increased from Fe-poor to Fe-rich fens. Contrary to expectations, precipitation of iron phosphates played a minor role in Fe-rich fens. Fe-rich fens were P-rich for three reasons: (1) high P sorption capacity, (2) relatively weak sorption to Fe-OM complexes and (3) high amounts of sorbed organic P, which probably consists of labile P. Also, nonmycorrhizal wetland plants probably especially take up weakly sorbed (organic) P. However, high P did not lead to high biomass or low plant diversity. Fe-rich fens were limited by other nutrients, and high P may help protect the vegetation against Fe-toxicity. Conclusions Fe-poor fens are P-poor, irrespective of Ca, and Fe-rich fens P-rich even under mesotrophic conditions. However, high P itself does not endanger Fe-rich fens.

BACKGROUND AND AIMS: Many rich fens are threatened by high nutrient inputs, but the literature is inconsistent with respect to the type of nutrient limitation and the influence of edaphic characteristics. METHODS: We performed experiments with N- and P-fertilization in three endangered rich fen types: floating fen with Scorpidium scorpioides, non-floating fen with Scorpidium cossonii, floodplain fen with Hamatocaulis vernicosus. In addition, K-fertilization was carried out in the floodplain fen. RESULTS: The floodplain fen showed no response to P-addition, but N- and K-addition led to grass encroachment and decline of moss cover and species richness. In contrast, in the P-limited floating fen with S. scorpioides, P-addition led to increased vascular plant production at the expense of moss cover. Scorpidium scorpioides, however, also declined after N-addition, presumably due to ammonium toxicity. The fen with S. cossonii took an intermediate position, with NP co-limitation. These striking contrasts corresponded with edaphic differences. The N-limited fen showed low Ca:Fe ratios and labile N-concentrations, and high concentrations of plant-available P and Fe-bound P. The P-limited fen showed an opposite pattern with high Ca:Fe ratios and labile N-concentrations, and low P-concentrations. CONCLUSIONS: This implies that edaphic characteristics dictate the nature of nutrient limitation, and explain contrasting effects of N- and P-eutrophication in different fens.

The moss Calliergon megalophyllum is rediscovered in the Netherlands after approximately 50 years of absence, in a location different from before: National Park Weerribben-Wieden. This is a Natura 2000 wetland area, and a Dutch hotspot for rich-fen bryophytes. The species was growing in a fen pool. Plant species composition and water chemistry were compared with Swedish samples collected throughout the country. Water chemistry of C. megalophyllum in Sweden was also compared with four other (semi-)aquatic species: C. giganteum, Scorpidium scorpioides,Sarmentypnum trichophyllum and S. exannulatum. The species is characteristic for poorly buffered habitats, but has nevertheless relatively high pH, which makes it sensitive to acidification, especially when atmospheric deposition is high. In the Dutch locality, buffer capacity is maintained by input of base-rich ditch water through small channels in the fen. The data further suggest that, like other Calliergon species, C. megalophyllum is growing in relatively nutrient-rich habitats, especially with respect to P and K. In the Netherlands, plant nutrient concentrations suggest that P is indeed not limiting, which may enhance survival of the species, as P-poor habitats in this country have become very rare.