Of all terrestrial ecosystems, peatlands store carbon most effectively in long-term scales of millennia. However, many peatlands have been drained for peat extraction or agricultural use. This converts peatlands from sinks to sources of carbon, causing approx. 5% of the anthropogenic greenhouse effect and additional negative effects on other ecosystem services. Rewetting peatlands can mitigate climate change and may be combined with management in the form of paludiculture. Rewetted peatlands, however, do not equal their pristine ancestors and their ecological functioning is not understood. This holds true especially for groundwater-fed fens. Their functioning results from manifold interactions and can only be understood following an integrative approach of many relevant fields of science, which we merge in the interdisciplinary project WETSCAPES. Here, we address interactions among water transport and chemistry, primary production, peat formation, matter transformation and transport, microbial community, and greenhouse gas exchange using state of the art methods. We record data on six study sites spread across three common fen types (Alder forest, percolation fen, and coastal fen), each in drained and rewetted states. First results revealed that indicators reflecting more long-term effects like vegetation and soil chemistry showed a stronger differentiation between drained and rewetted states than variables with a more immediate reaction to environmental change, like greenhouse gas (GHG) emissions. Variations in microbial community composition explained differences in soil chemical data as well as vegetation composition and GHG exchange. We show the importance of developing an integrative understanding of managed fen peatlands and their ecosystem functioning.

Drained peatlands are significant sources of the greenhouse gas (GHG) carbon dioxide. Rewetting is a proven strategy used to protect carbon stocks; however, it can lead to increased emissions of the potent GHG methane. The response to rewetting of soil microbiomes as drivers of these processes is poorly understood, as are the biotic and abiotic factors that control community composition. We analyzed the pro- and eukaryotic microbiomes of three contrasting pairs of minerotrophic fens subject to decade-long drainage and subsequent long-term rewetting. Abiotic soil properties including moisture, dissolved organic matter, methane fluxes, and ecosystem respiration rates were also determined. The composition of the microbiomes was fen-type-specific, but all rewetted sites showed higher abundances of anaerobic taxa compared to drained sites. Based on multi-variate statistics and network analyses, we identified soil moisture as a major driver of community composition. Furthermore, salinity drove the separation between coastal and freshwater fen communities. Methanogens were more than 10-fold more abundant in rewetted than in drained sites, while their abundance was lowest in the coastal fen, likely due to competition with sulfate reducers. The microbiome compositions were reflected in methane fluxes from the sites. Our results shed light on the factors that structure fen microbiomes via environmental filtering.

Tree stems can be a source of the greenhouse gas methane (CH₄). However, assessments of the global importance of stem CH₄ emissions are complicated by a lack of research and high variability between individual ecosystems. Here, we determined the contribution of emissions from stems of mature black alder (Alnus glutinosa (L.) Gaertn.) to overall CH₄ exchange in two temperate peatlands. We measured emissions from stems and soils using closed chambers in a drained and an undrained alder forest over 2 years. Furthermore, we studied the importance of alder leaves as substrate for methanogenesis in an incubation experiment. Stem CH₄ emissions were short‐lived and occurred only during times of inundation at the undrained site. The drained site did not show stem emissions and the soil acted as a small CH₄ sink. The contribution of stem emissions to the overall CH₄ budget was below 0.3% in both sites. Our results show that mature black alder can be an intermittent source of CH₄ to the atmosphere. However, the low share of stem CH₄ emissions in both investigated stands indicates that this pathway may be of minor relative importance in temperate peatlands, yet strongly depend on the hydrologic regime.

Root phenology influences the timing of plant resource acquisition and carbon fluxes into the soil. This is particularly important in fen peatlands, in which peat is primarily formed by roots and rhizomes of vascular plants. However, most fens in Central Europe are drained for agriculture, leading to large carbon losses, and further threatened by increasing frequency and intensity of droughts. Rewetting fens aims to restore the original carbon sink, but how root phenology is affected by drainage and rewetting is largely unknown. We monitored root phenology with minirhizotrons in drained and rewetted fens (alder forest, percolation fen and coastal fen) as well as its soil temperature and water table depth during the 2018 drought. For each fen type, we studied a drained site and a site that was rewetted ~25 years ago, while all the sites studied had been drained for almost a century. Overall, the growing season was longer with rewetting, allowing roots to grow over a longer period in the year and have a higher root production than under drainage. With increasing depth, the growing season shifted to later in time but remained a similar length, and the relative importance of soil temperature for root length changes increased with soil depth. Synthesis and applications. Rewetting extended the growing season of roots, highlighting the importance of phenology in explaining root productivity in peatlands. A longer growing season allows a longer period of carbon sequestration in form of root biomass and promotes the peatlands' carbon sink function, especially through longer growth in deep soil layers. Thus, management practices that focus on rewetting peatland ecosystems are necessary to maintain their function as carbon sinks, particularly under drought conditions, and are a top priority to reduce carbon emissions and address climate change.

Tree stems in both wetland and upland forests are a potentially significant source of the greenhouse gas methane (CH₄). Stem CH₄ emissions are assumed to play an important role on a global scale. Yet, uncertainties regarding temporal and spatial variability as well as transport processes of stem emissions remain high. Here, we assess the spatial variability and range of CH₄ egress from 28 black alder (Alnus glutinosa (L.) Gaertn.) trees based on data we recorded during three measurement campaigns in a mature alder carr in spring 2019. Fluxes varied significantly in time and space. The overall temporal variability during the three campaigns was larger than the spatial variability. The fluxes ranged from less than 1 µg m⁻² h⁻¹ to 450 µg m⁻² h⁻¹ with an average overall flux of 25 ± 59 µg m⁻² h⁻¹ (± SD). Stem CH₄ fluxes were highest (61.6 ± 104.5 µg m⁻² h⁻¹) during higher soil temperatures in spring, with increasing sap flux density and an intermediate water level just above the surface (+ 2.7 ± 0.3 cm). Morphological properties of the trees, including diameter at breast height, their position on hummocks or hollows and height of the measurement on the stem, largely failed to explain variability in stem CH₄ fluxes. Large-scale topography (overall elevation gradient) within the study site and CO₂ flux were the best predictors of stem CH₄ emissions, but inundation and leaf out also significantly influenced stem CH₄ fluxes. Thus, large-scale factors such as topography as an indirect indicator for water level can potentially facilitate upscaling of tree stem CH₄ emissions.