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

Salinity-induced increase of the hydraulic conductivity in the hyporheic zone of coastal wetlands

In coastal zones globally, salinization is rapidly taking place due to the combined effects of sea level rise, land subsidence, altered hydrology, and climate change. Although increased salinity levels are known to have a great impact on both biogeochemical and hydrological processes in aquatic sediments, only few studies have included both types of processes and their potential interactions. In the present paper, we used a controlled 3-year experimental mesocosm approach to test salinity induced interactions and discuss mechanisms explaining the observed hydrological changes. Surface water salinity was experimentally increased from 14 to 140 mmol Cl per L (0.9 and 9 PSU) by adding sea salt which increased pore water salinity but also increased sulfate reduction rates, leading to higher sulfide, and lower methane concentrations. By analyzing slug test data with different slug test analysis methods, we were able to show that hydraulic conductivity of the hyporheic zone increased 2.8 times by salinization. Based on our hydrological and biogeochemical measurements, we conclude that the combination of pore dilation and decreased methane production rates were major controls on the observed increase in hydraulic conductivity. The slug test analysis method comparison allowed to conclude that the adjusted Bouwer and Rice method results in the most reliable estimate of the hydraulic conductivity for hyporheic zones. Our work shows that both physical and biogeochemical processes are vital to explain and predict hydrological changes related to the salinization of hyporheic zones in coastal wetlands and provides a robust methodological approach for doing so

Hogere zoutconcentratie leidt tot een verhoogde waterdoorlatendheid van de waterbodem

Door de combinatie van klimaatverandering (zeespiegelstijging en langere droogteperiodes) en ontwatering voor landbouw neemt de kans op stijgende zoutconcentratiesin het oppervlaktewater in laag Nederland toe. Hoewel bekend is dat dit chemische enfysische effecten kan hebben, is de interactie tussen fysische en biogeochemische processen onderbelicht gebleven. In dit artikel wordt een veldexperiment gepresenteerdwaarin de effecten van verhoogde zoutconcentraties op de combinatie van chemischeen fysische processen in een voormalig brak laagveen zijn bestudeerd. Met behulp vanbiogeochemische analyses en de omgekeerde boorgatmethode in de waterbodem wordtaangetoond dat een verhoogde zoutconcentratie in het oppervlaktewater kan leiden toteen verhoogde waterdoorlatendheid van de waterbodem

Growth forms and life-history strategies predict the occurrence of aquatic macrophytes in relation to environmental factors in a shallow peat lake complex

Aquatic ecosystems provide vital services, and macrophytes play a critical role in their functioning. Conceptual models indicate that in shallow lakes, plants with different growth strategies are expected to inhabit contrasting habitats. For shallow peat lakes, characterized by incohesive sediments, roles of growth forms, life-history strategies and environmental factors in determining the occurrence of aquatic vegetation remain unknown. In a field survey, we sampled 64 points in a peat lake complex and related macrophyte occurrence to growth forms (floating-leaved rooted and submerged), life-history strategies for overwintering (turions, seeds, rhizomes) and environmental factors (water depth, fetch, and porewater nutrients). Our survey showed that macrophyte occurrence relates to water depth, wind-fetch, and nutrients, and depends on growth form and life-history strategies. Specifically, rooted floating-leaved macrophytes occur at lower wind-fetch/shallower waters. Submerged macrophytes occur from low to greater wind-fetch/water depth, depending on life-history strategies; macrophytes with rhizomes occur at greater wind-fetch/depth relative to species that overwinter with seeds or turions. We conclude that growth form and life-history strategies for overwintering predict macrophytes occurrence regarding environmental factors in peat lakes. Therefore, we propose an adapted model for macrophyte occurrence for such lakes. Altogether, these results may aid in species-selection to revegetate peat lakes depending on its environment

Factors influencing submerged macrophyte presence in fresh and brackish eutrophic waters and their impact on carbon emissions

In agricultural landscapes of North-Western Europe, the majority of water bodies do not meet the targets set by the European Water Framework Directive due to a lack of submerged macrophytes and associated biodiversity. These eutrophic waters can also be a substantial source of methane (CH4) and carbon dioxide (CO2) to the atmosphere. Here we present a two-year field experiment on the island of Goeree-Overflakkee (southwest Netherlands), conducted in six drainage ditches varying in salinity, where we monitored four permanent plots per ditch and varied the presence of both fish and macrophytes. We aimed to: 1) investigate factors limiting submerged macrophyte growth, focussing on exclusion of grazing pressure and bioturbation by fish; and 2) quantify the CO2 and CH4 emission under these conditions. Even in highly eutrophic, semi turbid ditches with fluctuating salinity levels and sulphide presence in the root zone, submerged macrophytes established successfully after introduction when the influence of grazing and bioturbation by fish was excluded. In the exclosures, diffusive CH4 and CO2 emissions, but not ebullitive CH4 emissions were significantly reduced. The spontaneous development of submerged macrophytes in the exclosures without macrophyte introduction underlined the effect of grazing and bioturbation by fish and suggest that abiotic conditions did not hamper submerged macrophyte development. Our results provide important insights into the influential factors for submerged macrophyte development and potential for future management practices. Large-scale fish removal may stimulate submerged macrophyte growth and reduce methane emissions, albeit that the macrophyte diversity will likely stay low in our study region due to fluctuating salinity and eutrophic conditions.</p

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 strong drainage has severely damaged or annihilated these services in many parts of North America and Europe, which urges the need of restoration plans at the landscape level. We review the major constraints for the restoration of rich fens and fen water bodies in agricultural areas in Europe and disturbed landscapes in North America: 1) habitat quality problems: drought, eutrophication, acidification, and toxicity, 2) recolonization problems: species pools, ecosystem fragmentation and connectivity, genetic variability, invasive species, and 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

Rewetting drained peatlands through subsoil infiltration stabilises redox-dependent soil carbon and nutrient dynamics

Centuries of drainage have stimulated peat decomposition. To counteract the resulting increase in greenhouse gas emission and land subsidence in Dutch agricultural peatlands, passive and active subsurface infiltration (SSI) systems have been developed for peatland rewetting. Here, we studied the effects of SSI systems on groundwater levels, porewater composition and redox potential in four drained peatlands in the Netherlands to determine how soil processes are affected, especially carbon and nutrient dynamics. For three years, groundwater levels were measured continuously, and porewater samples were collected 8–10 times per year in paired SSI (active and passive) and control plots. SSI plots had higher summer groundwater levels and less seasonal fluctuation in groundwater levels than control plots. Redox potential and porewater composition in control plots reflected dominance of oxidation processes during dry periods in the upper soil layers, whereas SSI plots showed dominance of reduction processes in these layers throughout the year. These differences between control and SSI plots were strongest at locations with active SSI systems. Our results show that SSI systems can be effective measures to raise and stabilise groundwater levels in drained peatlands, which results in more stable redox zonation and dominance of anaerobic soil processes, especially during dry periods and when active SSI systems are applied. On the short term, a switch from oxic to anoxic conditions can cause mobilisation of phosphorus and ammonium, while on the longer term the application of SSI can lead to substantial change in carbon and nutrient dynamics. Understanding the full effects of implementing SSI systems and other mitigation measures in drained peatlands is important before they are applied on a large scale

The impact of warming and nutrients on algae production and microcystins in seston from the iconic lake lesser Prespa, Greece

Lake Lesser Prespa and its adjacent pond, Vromolimni in Greece, is a shallow freshwater system and a highly protected area hosting an exceptional biodiversity. The occurrence of microcystins (MCs) producing cyanobacterial blooms in these waters during recent years can be harmful to the wildlife. We tested the hypothesis that both cyanobacterial biomass and MCs are strongly influenced by nutrients (eutrophication) and warming (climate change). Lake and pond water was collected from two sites in each water body in 2013 and incubated at three temperatures (20°C, 25°C, 30°C) with or without additional nutrients (nitrogen +N, phosphorus +P and both +N and +P). Based on both biovolume and chlorophyll-a concentrations, cyanobacteria in water from Lesser Prespa were promoted primarily by combined N and P additions and to a lesser extent by N alone. Warming seemed to yield more cyanobacteria biomass in these treatments. In water from Vromolimni, both N alone and N+P additions increased cyanobacteria and a warming effect was hardly discernible. MC concentrations were strongly increased by N and N+P additions in water from all four sites, which also promoted the more toxic variant MC-LR. Hence, both water bodies seem particularly vulnerable to further N-loading enhancing MC related risks

Colonial nesting waterbirds as vectors of nutrients to Lake Lesser Prespa (Greece)

The nutrients imported by breeding waterbirds should be considered when identifying the main sources of nutrient input to lakes. Lake Lesser Prespa (Greece), including the adjacent Vromolimni pond, hosts numerous protected waterbirds that nest in densely populated colonies across the reedbeds. The accelerated eutrophication of the lake in recent years has been of increasing concern. In addition to likely large sources of nutrients (i.e., anthropogenic activities, especially agriculture), nutrient input via waterbird excrement may further trigger eutrophication. We estimated the annual phosphorus (P) and nitrogen (N) input by the most abundant colonial-nesting waterbirds (great white pelican, Dalmatian pelican, great cormorant, and pygmy cormorant) into the lake and investigated their influence on water and sediment quality. Near the waterbird colonies, soluble nutrient concentrations in the lake sediments were higher, and chlorophyll measurements indicated higher algal growth near these sites in summer. Stable isotope analysis suggests that near the colonies, waterbirds are responsible for nutrient loadings that affect the lake sediment. The estimated N and P nutrient input into the lake by both pelican and cormorant species is at least 1243 and 1649 kg/yr, respectively. On a landscape scale, this level of loading could be of minor importance for the lake because N and P can reach 32.8 (SD 9.3) and 38.9 (5.8) mg/m2 per year, respectively. Locally, however, this level of loading might induce cyanobacterial blooms, illustrated by the analysis of isolated Vromolimni pond near the lake. Our findings emphasize the likely importance of nutrient loading by waterbirds for the lake system.</p

Hogere zoutconcentratie leidt tot een verhoogde waterdoorlatendheid van de waterbodem

Door de combinatie van klimaatverandering (zeespiegelstijging en langere droogteperiodes) en ontwatering voor landbouw neemt de kans op stijgende zoutconcentratiesin het oppervlaktewater in laag Nederland toe. Hoewel bekend is dat dit chemische enfysische effecten kan hebben, is de interactie tussen fysische en biogeochemische processen onderbelicht gebleven. In dit artikel wordt een veldexperiment gepresenteerdwaarin de effecten van verhoogde zoutconcentraties op de combinatie van chemischeen fysische processen in een voormalig brak laagveen zijn bestudeerd. Met behulp vanbiogeochemische analyses en de omgekeerde boorgatmethode in de waterbodem wordtaangetoond dat een verhoogde zoutconcentratie in het oppervlaktewater kan leiden toteen verhoogde waterdoorlatendheid van de waterbodem