Cross continental increase in methane ebullition under climate change.

Methane (CH4) strongly contributes to observed global warming. As natural CH4 emissions mainly originate from wet ecosystems, it is important to unravel how climate change may affect these emissions. This is especially true for ebullition (bubble flux from sediments), a pathway that has long been underestimated but generally dominates emissions. Here we show a remarkably strong relationship between CH4 ebullition and temperature across a wide range of freshwater ecosystems on different continents using multi-seasonal CH4 ebullition data from the literature. As these temperature-ebullition relationships may have been affected by seasonal variation in organic matter availability, we also conducted a controlled year-round mesocosm experiment. Here 4 °C warming led to 51% higher total annual CH4 ebullition, while diffusion was not affected. Our combined findings suggest that global warming will strongly enhance freshwater CH4 emissions through a disproportional increase in ebullition (6-20% per 1 °C increase), contributing to global warming.

To Float or Not to Float: How Interactions between Light and Dissolved Inorganic Carbon Species Determine the Buoyancy of Stratiotes aloides.

Structural diversity formed by dense, floating Stratiotes aloides stands, generates hotspots of biodiversity of flora and fauna in wetlands. However, only part of the populations become emergent and provide this important facilitation. Since it has been hypothesised that its buoyancy depends on the rates of underwater photosynthesis, we investigated the role of dissolved CO2 availability and PAR on photosynthesis, biomass production and buoyancy in a controlled greenhouse experiment. Photosynthesis and growth were strongly influenced by both PAR and CO2 availability. At low PAR, plants formed less biomass and produced no emergent leaves, even when CO2 was abundant. At low CO2 levels, S. aloides switched to HCO3- use, resulting in a lower photosynthetic O2 production, decreased emergent leaf formation and increased CaCO3 precipitation on its leaves, all of which impaired buoyancy. At high PAR, low CO2 availability resulted in slower colonisation of the water layer, whereas CO2 availability did not influence PAR-limited plants. Our study shows that site conditions, rather than the sole abundance of potentially facilitating species, may strongly determine whether or not they form the structure necessary to act as a facilitator for biodiversity in aquatic environments.

Ecological restoration of rich fens in Europe and North America: from trial and error to an evidence-based approach.

Fens represent a large array of ecosystem services, including the highest biodiversity found among wetlands, hydrological services, water purification and carbon sequestration. Land-use change and drainage has severely damaged or annihilated these services in many parts of North America and Europe; restoration plans are urgently needed at the landscape level. We review the major constraints on the restoration of rich fens and fen water bodies in agricultural areas in Europe and disturbed landscapes in North America: (i) habitat quality problems: drought, eutrophication, acidification, and toxicity, and (ii) recolonization problems: species pools, ecosystem fragmentation and connectivity, genetic variability, and invasive species; and here provide possible solutions. We discuss both positive and negative consequences of restoration measures, and their causes. The restoration of wetland ecosystem functioning and services has, for a long time, been based on a trial-and-error approach. By presenting research and practice on the restoration of rich fen ecosystems within agricultural areas, we demonstrate the importance of biogeochemical and ecological knowledge at different spatial scales for the management and restoration of biodiversity, water quality, carbon sequestration and other ecosystem services, especially in a changing climate. We define target processes that enable scientists, nature managers, water managers and policy makers to choose between different measures and to predict restoration prospects for different types of deteriorated fens and their starting conditions.

New insights into phosphorus mobilisation from sulphur-rich sediments: time-dependent effects of salinisation.

Internal phosphorus (P) mobilisation from aquatic sediments is an important process adding to eutrophication problems in wetlands. Salinisation, a fast growing global problem, is thought to affect P behaviour. Although several studies have addressed the effects of salinisation, interactions between salinity changes and nutrient cycling in freshwater systems are not fully understood. To tackle eutrophication, a clear understanding of the interacting effects of sediment characteristics and surface water quality is vital. In the present study, P release from two eutrophic sediments, both characterized by high pore water P and very low pore water iron (Fe(2+)) concentrations, was studied in a long-term aquarium experiment, using three salinity levels. Sediment P release was expected to be mainly driven by diffusion, due to the eutrophic conditions and low iron availability. Unexpectedly, this only seemed to be the driving mechanism in the short term (0-10 weeks). In the long term (>80 weeks), P mobilisation was absent in most treatments. This can most likely be explained by the oxidation of the sediment-water interface where Fe(2+) immobilises P, even though it is commonly assumed that free Fe(2+) concentrations need to be higher for this. Therefore, a controlling mechanism is suggested in which the partial oxidation of iron-sulphides in the sediment plays a key role, releasing extra Fe(2+) at the sediment-water interface. Although salinisation was shown to lower short-term P mobilisation as a result of increased calcium concentrations, it may increase long-term P mobilisation by the interactions between sulphate reduction and oxygen availability. Our study showed time-dependent responses of sediment P mobilisation in relation to salinity, suggesting that sulphur plays an important role in the release of P from FeSx-rich sediments, its biogeochemical effect depending on the availability of Fe(2+) and O2.

Sulfide as a soil phytotoxin-a review.

In wetland soils and underwater sediments of marine, brackish and freshwater systems, the strong phytotoxin sulfide may accumulate as a result of microbial reduction of sulfate during anaerobiosis, its level depending on prevailing edaphic conditions. In this review, we compare an extensive body of literature on phytotoxic effects of this reduced sulfur compound in different ecosystem types, and review the effects of sulfide at multiple ecosystem levels: the ecophysiological functioning of individual plants, plant-microbe associations, and community effects including competition and facilitation interactions. Recent publications on multi-species interactions in the rhizosphere show even more complex mechanisms explaining sulfide resistance. It is concluded that sulfide is a potent phytotoxin, profoundly affecting plant fitness and ecosystem functioning in the full range of wetland types including coastal systems, and at several levels. Traditional toxicity testing including hydroponic approaches generally neglect rhizospheric effects, which makes it difficult to extrapolate results to real ecosystem processes. To explain the differential effects of sulfide at the different organizational levels, profound knowledge about the biogeochemical, plant physiological and ecological rhizosphere processes is vital. This information is even more important, as anthropogenic inputs of sulfur into freshwater ecosystems and organic loads into freshwater and marine systems are still much higher than natural levels, and are steeply increasing in Asia. In addition, higher temperatures as a result of global climate change may lead to higher sulfide production rates in shallow waters.

Micro-halocline enabled nutrient recycling may explain extreme Azolla event in the Eocene Arctic Ocean.

In order to understand the physicochemical mechanisms that could explain the massive growth of Azolla arctica in the Eocene Arctic Ocean, we carried out a laboratory experiment in which we studied the interacting effects of rain and wind on the development of salinity stratification, both in the presence and in the absence of a dense Azolla cover. Additionally, we carried out a mesocosm experiment to get a better understanding of the nutrient cycling within and beneath a dense Azolla cover in both freshwater and brackish water environments. Here we show that Azolla is able to create a windproof, small-scale salinity gradient in brackish waters, which allows for efficient recycling of nutrients. We suggest that this mechanism ensures the maintenance of a large standing biomass in which additional input of nutrients ultimately result in a further expansion of an Azolla cover. As such, it may not only explain the extent of the Azolla event during the Eocene, but also the absence of intact vegetative Azolla remains and the relatively low burial efficiency of organic carbon during this interval.

Microbial transformations of nitrogen, sulfur, and iron dictate vegetation composition in wetlands: a review.

The majority of studies on rhizospheric interactions focus on pathogens, mycorrhizal symbiosis, or carbon transformations. Although the biogeochemical transformations of N, S, and Fe have profound effects on vegetation, these effects have received far less attention. This review, meant for microbiologists, biogeochemists, and plant scientists includes a call for interdisciplinary research by providing a number of challenging topics for future ecosystem research. Firstly, all three elements are plant nutrients, and microbial activity significantly changes their availability. Secondly, microbial oxidation with oxygen supplied by radial oxygen loss from roots in wetlands causes acidification, while reduction using alternative electron acceptors leads to generation of alkalinity, affecting pH in the rhizosphere, and hence plant composition. Thirdly, reduced species of all three elements may become phytotoxic. In addition, Fe cycling is tightly linked to that of S and P. As water level fluctuations are very common in wetlands, rapid changes in the availability of oxygen and alternative terminal electron acceptors will result in strong changes in the prevalent microbial redox reactions, with significant effects on plant growth. Depending on geological and hydrological settings, these interacting microbial transformations change the conditions and resource availability for plants, which are both strong drivers of vegetation development and composition by changing relative competitive strengths. Conversely, microbial composition is strongly driven by vegetation composition. Therefore, the combination of microbiological and plant ecological knowledge is essential to understand the biogeochemical and biological key factors driving heterogeneity and total (i.e., microorganisms and vegetation) community composition at different spatial and temporal scales.

Zero methane emission bogs: extreme rhizosphere oxygenation by cushion plants in Patagonia.

• Vascular wetland plants may substantially increase methane emissions by producing root exudates and easily degradable litter, and by providing a low-resistance diffusion pathway via their aerenchyma. However, model studies have indicated that vascular plants can reduce methane emission when soil oxygen demand is exceeded by oxygen released from roots. Here, we tested whether these conditions occur in bogs dominated by cushion plants. • Root-methane interactions were studied by comparing methane emissions, stock and oxygen availability in depth profiles below lawns of either cushion plants or Sphagnum mosses in Patagonia. • Cushion plants, Astelia pumila and Donatia fascicularis, formed extensive root systems up to 120 cm in depth. The cold soil (< 10°C) and highly decomposed peat resulted in low microbial activity and oxygen consumption. In cushion plant lawns, high soil oxygen coincided with high root densities, but methane emissions were absent. In Sphagnum lawns, methane emissions were substantial. High methane concentrations were only found in soils without cushion plant roots. • This first methane study in Patagonian bog vegetation reveals lower emissions than expected. We conclude that cushion plants are capable of reducing methane emission on an ecosystem scale by thorough soil and methane oxidation.

The interaction between decomposition, net N and P mineralization and their mobilization to the surface water in fens.

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:PO(4) ratios below 1molmol(-1), PO(4) mobilization from the sediment to the water layer was considerable and linearly related to the pore water PO(4) concentration. At higher ratios, there was a strong linear correlation between the Fe:PO(4) ratio and PO(4) mobilization. Hence, measuring Fe and PO(4) in anaerobic sediment pore water provides a powerful tool for a quick assessment of internal PO(4) 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 PO(4). Pore water Fe concentrations decreased at ratios <1molmol(-1), whereas pore water PO(4) concentrations and PO(4) mobilization to the water layer increased. As PO(4) 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.

Spatial self-organized patterning in seagrasses along a depth gradient of an intertidal ecosystem.

The spatial structure of seagrass landscapes is typically ascribed to the direct influence of physical factors such as hydrodynamics, light, and sediment transport. We studied regularly interspaced banded patterns, formed by elongated patches of seagrass, in a small-scale intertidal ecosystem. We investigated (1) whether the observed spatial patterns may arise from feedback interactions between seagrass and its abiotic environment and (2) whether changes in abiotic conditions may lead to predictable changes in these spatial patterns. Field measurements, experiments, and a spatially explicit computer model identified a "scale-dependent feedback" (a mechanism for spatial self-organization) as a possible cause for the banded patterns. Increased protection from uprooting by improved anchoring with increasing seagrass density caused a local positive feedback. Sediment erosion around seagrass shoots increased with distance through the seagrass bands, hence causing a long-range negative feedback. Measurements across the depth gradient of the intertidal, together with model simulations, demonstrated that seagrass cover and mean patch size were predictably influenced by additional external stress caused by light limitation and desiccation. Thus, our study provides direct empirical evidence for a consistent response of spatial self-organized patterns to changing abiotic conditions, suggesting a potential use for self-organized spatial patterns as stress indicators in ecosystems.

Interacting effects of sulphate pollution, sulphide toxicity and eutrophication on vegetation development in fens: a mesocosm experiment.

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.

Even low to medium nitrogen deposition impacts vegetation of dry, coastal dunes around the Baltic Sea.

Coastal dunes around the Baltic Sea have received small amounts of atmospheric nitrogen and are rather pristine ecosystems in this respect. In 19 investigated dune sites the atmospheric wet nitrogen deposition is 3-8kg Nha(-1)yr(-1). The nitrogen content of Cladonia portentosa appeared to be a suitable biomonitor of these low to medium deposition levels. Comparison with EMEP-deposition data showed that Cladonia reflects the deposition history of the last 3-6 years. With increasing nitrogen load, we observed a shift from lichen-rich short grass vegetation towards species-poor vegetation dominated by the tall graminoid Carex arenaria. Plant species richness per field site, however, does not decrease directly with these low to medium N deposition loads, but with change in vegetation composition. Critical loads for acidic, dry coastal dunes might be lower than previously thought, in the range of 4-6kg Nha(-1)yr(-1) wet deposition.

Multi-level effects of sulphur-iron interactions in freshwater wetlands in The Netherlands.

Although sulphur deposition rates in Europe have considerably decreased over the last decades, sulphate concentrations in freshwater wetlands are still high, as a result of drainage, nitrate pollution, and increased sulphur loads in rivers. High sulphur fluxes may cause sulphide toxicity and eutrophication, and strongly interfere with the biogeochemical cycling of iron and phosphorus. In the present study the ecotoxicological interactions between sulphur, phosphate, iron, and trace metals in freshwater wetlands are reviewed.

Long term changes in atmospheric N and S throughfall deposition and effects on soil solution chemistry in a Scots pine forest in the Netherlands.

In a Scots pine forest the throughfall deposition and the chemical composition of the soil solution was monitored since 1984. (Inter)national legislation measures led to a reduction of the deposition of nitrogen and sulphur. The deposition of sulphur has decreased by approximately 65%. The total mineral-nitrogen deposition has decreased by ca. 25%, which is mainly due to a reduction in ammonium-N deposition (-40%), since nitrate-N deposition has increased (+50%). The nitrogen concentration in the upper mineral soil solution at 10 cm depth has decreased, leading to an improved nutritional balance, which may result in improved tree vitality. In the drainage water at 90 cm depth the fluxes of NO3(-) and SO4(2-) have decreased, resulting in a reduced leeching of accompanying base cations, thus preserving nutrients in the ecosystem. It may take still several years, however, before this will meet the prerequisite of a sustainable ecosystem.

Prediction of phosphorus mobilisation in inundated floodplain soils.

After flooding, iron reduction in riverine wetlands may cause the release of large quantities of phosphorus. As phosphorus is an important nutrient causing eutrophication in aquatic systems, it is important to have a tool to predict this potential release. In this study we examined the P release to the soil pore water in soil cores from floodplains in the Netherlands and from less anthropogenically influenced floodplains from Poland. During the inundation experiment, concentrations of P in the pore water rose to 2-90 times the initial concentrations. P release was not directly related to the geographic origin of the soils. An important predictor variable of P release was found in the ratio between the concentration of iron-bound P and amorphous iron. This ratio may provide a practical tool for the selection of new areas for wetland creation, and for impact assessment of plans for riverine wetland restoration and floodwater storage.

Predicting metal uptake by wetland plants under aerobic and anaerobic conditions.

Metal pollution can be a serious threat to ecosystems at a global scale. Although the bioavailability of potentially toxic metals is determined by many biotic and abiotic factors, including pH and redox potential, total metal concentrations in the soil are used widely to assess or predict toxicity. In the present study we tested the effect of desiccation of soils differing in acidification potential and total heavy metal contamination on the growth and metal uptake of three typical, common wetland species: Caltha palustris, Juncus effusus, and Rumex hydrolapathum. We found that plant growth in wet soils mainly was determined by nutrient availability, though in dry soils the combined effects of acidification and increased metal availability prevailed. Metal uptake under anaerobic conditions was best predicted by the acidification potential (sediment S/[Ca + Mg] ratio), not by total metal concentrations. We propose that this is related to radial oxygen loss by wetland plant roots, which leads to acidification of the rhizosphere. Under aerobic conditions, plant metal uptake was best predicted by the amount of CaCl2-extractable metals. We conclude that total metal concentrations are not suitable for predicting bioavailability and that the above diagnostic parameters will provide insight into biogeochemical processes involved in toxicity assessment and soil policy.

Differential responses of the freshwater wetland species Juncus effusus L. and Caltha palustris L. to iron supply in sulfidic environments.

Sulfur pollution can lead to serious problems in freshwater wetlands, including phosphorus eutrophication and sulfide toxicity. We tested the effects of anaerobic iron-rich groundwater discharge in fens, simulated by iron injection, on two characteristic species (Juncus effusus and Caltha palustris) in a sulfidic environment. Biomass production of C. palustris roots showed an optimum response to the combined addition of iron and sulfide, with highest values at intermediate concentrations of both substances. Iron deficiency apparently occurred at low iron concentrations, while at high iron concentrations, growth was decreased. For J. effusus, in contrast, no toxic effects were found of both iron and sulfide. This could be explained by larger radial oxygen loss (ROL) of J. effusus and could not be explained by differences in phosphorous concentrations. The results of our experiments confirm that iron-rich groundwater discharge has the potential to affect vegetation composition through toxicity modification in sulfidic environments.

Methanotrophic symbionts provide carbon for photosynthesis in peat bogs.

Wetlands are the largest natural source of atmospheric methane, the second most important greenhouse gas. Methane flux to the atmosphere depends strongly on the climate; however, by far the largest part of the methane formed in wetland ecosystems is recycled and does not reach the atmosphere. The biogeochemical controls on the efficient oxidation of methane are still poorly understood. Here we show that submerged Sphagnum mosses, the dominant plants in some of these habitats, consume methane through symbiosis with partly endophytic methanotrophic bacteria, leading to highly effective in situ methane recycling. Molecular probes revealed the presence of the bacteria in the hyaline cells of the plant and on stem leaves. Incubation with (13)C-methane showed rapid in situ oxidation by these bacteria to carbon dioxide, which was subsequently fixed by Sphagnum, as shown by incorporation of (13)C-methane into plant sterols. In this way, methane acts as a significant (10-15%) carbon source for Sphagnum. The symbiosis explains both the efficient recycling of methane and the high organic carbon burial in these wetland ecosystems.

Effects of nitrogen enrichment on coastal dune grassland: a mesocosm study.

Mesocosms filled with dune sand were planted with graminoid (Calamagrostis epigejos, Carex arenaria) and herbaceous species (Carlina vulgaris, Galium verum). Strong effects of nitrogen addition on the vegetation were found within two to three years. The above-ground biomass of C. epigejos and C. arenaria increased at deposition rates between 10 and 80 kg N ha(-1) yr(-1). Both grasses were limited by N. In latter stages P limitation was suggested for C. arenaria. At high N-levels, C. epigejos dominated the vegetation within two years. C. vulgaris and G. verum declined drastically as a result of increased competition for light by the highly competitive grass C. epigejos. It is concluded that increased (ambient) N inputs are of major importance for the increased dominance of tall grasses in stable dune grasslands.

Decline of acid-sensitive plant species in heathland can be attributed to ammonium toxicity in combination with low pH.

The effects of increasing ammonium concentrations in combination with different pH levels were studied on five heathland plant species to determine whether their occurrence and decline could be attributed to ammonium toxicity and/or pH levels. Plants were grown in growth media amended with four different ammonium concentrations (10, 100, 500 and 1000 micromol l(-1)) and two pH levels resembling acidified (pH 3.5 or 4) and weakly buffered (pH 5 or 5.5) situations. Survival of Antennaria dioica and Succisa pratensis was reduced by low pH in combination with high ammonium concentrations. Biomass decreased with increased ammonium concentrations and decreasing pH levels. Internal pH of the plants decreased with increasing ammonium concentrations. Survival of Calluna vulgaris, Deschampsia flexuosa and Gentiana pneumonanthe was not affected by ammonium. Moreover, biomass increased with increasing ammonium concentrations. Biomass production of G. pneumonanthe reduced at low pH levels. A decline of acid-sensitive species in heathlands was attributed to ammonium toxicity effects in combination with a low pH.