Accelerated increase in plant species richness on mountain summits is linked to warming.

Globally accelerating trends in societal development and human environmental impacts since the mid-twentieth century 1-7 are known as the Great Acceleration and have been discussed as a key indicator of the onset of the Anthropocene epoch 6 . While reports on ecological responses (for example, changes in species range or local extinctions) to the Great Acceleration are multiplying 8, 9 , it is unknown whether such biotic responses are undergoing a similar acceleration over time. This knowledge gap stems from the limited availability of time series data on biodiversity changes across large temporal and geographical extents. Here we use a dataset of repeated plant surveys from 302 mountain summits across Europe, spanning 145 years of observation, to assess the temporal trajectory of mountain biodiversity changes as a globally coherent imprint of the Anthropocene. We find a continent-wide acceleration in the rate of increase in plant species richness, with five times as much species enrichment between 2007 and 2016 as fifty years ago, between 1957 and 1966. This acceleration is strikingly synchronized with accelerated global warming and is not linked to alternative global change drivers. The accelerating increases in species richness on mountain summits across this broad spatial extent demonstrate that acceleration in climate-induced biotic change is occurring even in remote places on Earth, with potentially far-ranging consequences not only for biodiversity, but also for ecosystem functioning and services.

Consistent predictors of microbial community composition across spatial scales in grasslands reveal low context-dependency.

Environmental circumstances shaping soil microbial communities have been studied extensively. However, due to disparate study designs, it has been difficult to resolve whether a globally consistent set of predictors exists, or context-dependency prevails. Here, we used a network of 18 grassland sites (11 of those containing regional plant productivity gradients) to examine (i) if similar abiotic or biotic factors predict both large-scale (across sites) and regional-scale (within sites) patterns in bacterial and fungal community composition, and (ii) if microbial community composition differs consistently at two levels of regional plant productivity (low vs. high). Our results revealed that bacteria were associated with particular soil properties (such as base saturation) and both bacteria and fungi were associated with plant community composition across sites and within the majority of sites. Moreover, a discernible microbial community signal emerged, clearly distinguishing high and low-productivity soils across different grasslands independent of their location in the world. Hence, regional productivity differences may be typified by characteristic soil microbial communities across the grassland biome. These results could encourage future research aiming to predict the general effects of global changes on soil microbial community composition in grasslands and to discriminate fertile from infertile systems using generally applicable microbial indicators.

Linking Transcriptional Dynamics of Peat Microbiomes to Methane Fluxes during a Summer Drought in Two Rewetted Fens.

Rewetted peatlands are reestablished hot spots for CH4 emissions, which are subject to increased drought events in the course of climate change. However, the dynamics of soil methane-cycling microbiomes in rewetted peatlands during summer drought are still poorly characterized. Using a quantitative metatranscriptomic approach, we investigated the changes in the transcript abundances of methanogen and methanotroph rRNA, as well as mcrA and pmoA mRNA before, during, and after the 2018 summer drought in a coastal and a percolation fen in northern Germany. Drought changed the community structure of methane-cycling microbiomes and decreased the CH4 fluxes as well as the rRNA and mRNA transcript abundances of methanogens and methanotrophs, but they showed no recovery or increase after the drought ended. The rRNA transcript abundance of methanogens was not correlated with CH4 fluxes in both fens. In the percolation fen, however, the mcrA transcript abundance showed a positive and significant correlation with CH4 fluxes. Importantly, when integrating pmoA abundance, a stronger correlation was observed between CH4 fluxes and mcrA/pmoA, suggesting that relationships between methanogens and methanotrophs are the key determinant of CH4 turnover. Our study provides a comprehensive understanding of the methane-cycling microbiome feedbacks to drought events in rewetted peatlands.

Identifying dominant environmental predictors of freshwater wetland methane fluxes across diurnal to seasonal time scales.

While wetlands are the largest natural source of methane (CH4 ) to the atmosphere, they represent a large source of uncertainty in the global CH4 budget due to the complex biogeochemical controls on CH4 dynamics. Here we present, to our knowledge, the first multi-site synthesis of how predictors of CH4 fluxes (FCH4) in freshwater wetlands vary across wetland types at diel, multiday (synoptic), and seasonal time scales. We used several statistical approaches (correlation analysis, generalized additive modeling, mutual information, and random forests) in a wavelet-based multi-resolution framework to assess the importance of environmental predictors, nonlinearities and lags on FCH4 across 23 eddy covariance sites. Seasonally, soil and air temperature were dominant predictors of FCH4 at sites with smaller seasonal variation in water table depth (WTD). In contrast, WTD was the dominant predictor for wetlands with smaller variations in temperature (e.g., seasonal tropical/subtropical wetlands). Changes in seasonal FCH4 lagged fluctuations in WTD by ~17 ± 11 days, and lagged air and soil temperature by median values of 8 ± 16 and 5 ± 15 days, respectively. Temperature and WTD were also dominant predictors at the multiday scale. Atmospheric pressure (PA) was another important multiday scale predictor for peat-dominated sites, with drops in PA coinciding with synchronous releases of CH4 . At the diel scale, synchronous relationships with latent heat flux and vapor pressure deficit suggest that physical processes controlling evaporation and boundary layer mixing exert similar controls on CH4 volatilization, and suggest the influence of pressurized ventilation in aerenchymatous vegetation. In addition, 1- to 4-h lagged relationships with ecosystem photosynthesis indicate recent carbon substrates, such as root exudates, may also control FCH4. By addressing issues of scale, asynchrony, and nonlinearity, this work improves understanding of the predictors and timing of wetland FCH4 that can inform future studies and models, and help constrain wetland CH4 emissions.

High emissions of greenhouse gases from grasslands on peat and other organic soils.

Drainage has turned peatlands from a carbon sink into one of the world's largest greenhouse gas (GHG) sources from cultivated soils. We analyzed a unique data set (12 peatlands, 48 sites and 122 annual budgets) of mainly unpublished GHG emissions from grasslands on bog and fen peat as well as other soils rich in soil organic carbon (SOC) in Germany. Emissions and environmental variables were measured with identical methods. Site-averaged GHG budgets were surprisingly variable (29.2 ± 17.4 t CO2 -eq. ha-1  yr-1 ) and partially higher than all published data and the IPCC default emission factors for GHG inventories. Generally, CO2 (27.7 ± 17.3 t CO2  ha-1  yr-1 ) dominated the GHG budget. Nitrous oxide (2.3 ± 2.4 kg N2 O-N ha-1  yr-1 ) and methane emissions (30.8 ± 69.8 kg CH4 -C ha-1  yr-1 ) were lower than expected except for CH4 emissions from nutrient-poor acidic sites. At single peatlands, CO2 emissions clearly increased with deeper mean water table depth (WTD), but there was no general dependency of CO2 on WTD for the complete data set. Thus, regionalization of CO2 emissions by WTD only will remain uncertain. WTD dynamics explained some of the differences between peatlands as sites which became very dry during summer showed lower emissions. We introduced the aerated nitrogen stock (Nair ) as a variable combining soil nitrogen stocks with WTD. CO2 increased with Nair across peatlands. Soils with comparatively low SOC concentrations showed as high CO2 emissions as true peat soils because Nair was similar. N2 O emissions were controlled by the WTD dynamics and the nitrogen content of the topsoil. CH4 emissions can be well described by WTD and ponding duration during summer. Our results can help both to improve GHG emission reporting and to prioritize and plan emission reduction measures for peat and similar soils at different scales.

Resurveying historical vegetation data - opportunities and challenges.

BACKGROUND: Resurveying historical vegetation plots has become more and more popular in recent years as it provides a unique opportunity to estimate vegetation and environmental changes over the past decades. Most historical plots, however, are not permanently marked and uncertainty in plot location, in addition to observer bias and seasonal bias, may add significant error to temporal change. These errors may have major implications for the reliability of studies on long-term environmental change and deserve closer attention of vegetation ecologists. MATERIAL & METHODS: Vegetation data obtained from the resurveying of non-permanently marked plots are assessed for their potential to study environmental-change effects on plant communities and the challenges the use of such data have to meet. We describe the properties of vegetation resurveys distinguishing basic types of plots according to relocation error, and we highlight the potential of such data types for studying vegetation dynamics and their drivers. Finally, we summarise the challenges and limitations of resurveying non-permanently marked vegetation plots for different purposes in environmental change research. RESULTS AND CONCLUSIONS: Resampling error is caused by three main independent sources of error: error caused by plot relocation, observer bias, and seasonality bias. For relocation error, vegetation plots can be divided into permanent and non-permanent plots, while the latter are further divided into quasi-permanent (with approximate relocation) and non-traceable (with random relocation within a sampled area) plots. To reduce the inherent sources of error in resurvey data, the following precautions should be followed: (i) resurvey historical vegetation plots whose approximate plot location within a study area is known; (ii) consider all information available from historical studies in order to keep plot relocation errors low; (iii) resurvey at times of the year when vegetation development is comparable to the historical survey to control for seasonal variability in vegetation; (iv) keep a high level of experience of the observers to keep observer bias low; and (v) edit and standardise datasets before analyses.

Opaque closed chambers underestimate methane fluxes of Phragmites australis (Cav.) Trin. ex Steud.

Closed chamber measurements for methane emission estimation are often carried out with opaque chambers to avoid heating of the headspace. However, mainly in wetlands, some plants possess an internal convective gas transport which quickly responds to changes in irradiation. These plants have also been found to often channel a large part of the released methane in temperate fens. We compare methane fluxes derived from transparent versus opaque chambers on Carex-, Phragmites-, and Typha-dominated stands of a temperate fen. Transparent chamber fluxes almost doubled opaque chamber fluxes in the convective transporting Phragmites stand. In Typha, a trend of higher fluxes determined with the transparent chambers was detectable, whereas in Carex, transparent and opaque chamber fluxes did not differ significantly. Thus, opaque chambers bias the outcome of methane measurements, depending on dominant vegetation. We recommend the use of transparent chambers when determining emissions of convective plants or extrapolating fluxes to larger scales.

Plant roots but not hydrology control microbiome composition and methane flux in temperate fen mesocosms.

The rewetting of formerly drained peatlands can help to counteract climate change through the reduction of CO2 emissions. However, this can lead to resuming CH4 emissions due to changes in the microbiome, favoring CH4-producing archaea. How plants, hydrology and microbiomes interact as ultimate determinants of CH4 dynamics is still poorly understood. Using a mesocosm approach, we studied peat microbiomes, below-ground root biomass and CH4 fluxes with three different water level regimes (stable high, stable low and fluctuating) and four different plant communities (bare peat, Carex rostrata, Juncus inflexus and their mixture) over the course of one growing season. A significant difference in microbiome composition was found between mesocosms with and without plants, while the difference between plant species identity or water regimes was rather weak. A significant difference was also found between the upper and lower peat, with the difference increasing as plants grew. By the end of the growing season, the methanogen relative abundance was higher in the sub-soil layer, as well as in the bare peat and C. rostrata pots, as compared to J. inflexus or mixture pots. This was inversely linked to the larger root area of J. inflexus. The root area also negatively correlated with CH4 fluxes which positively correlated with the relative abundance of methanogens. Despite the absence or low abundance of methanotrophs in many samples, the integration of methanotroph abundance improved the quality of the correlation with CH4 fluxes, and methanogens and methanotrophs together determined CH4 fluxes in a structural equation model. However, water regime showed no significant impact on plant roots and methanogens, and consequently, on CH4 fluxes. This study showed that plant roots determined the microbiome composition and, in particular, the relative abundance of methanogens and methanotrophs, which, in interaction, drove the CH4 fluxes.

Unraveling microbial processes involved in carbon and nitrogen cycling and greenhouse gas emissions in rewetted peatlands by molecular biology.

Restoration of drained peatlands through rewetting has recently emerged as a prevailing strategy to mitigate excessive greenhouse gas emissions and re-establish the vital carbon sequestration capacity of peatlands. Rewetting can help to restore vegetation communities and biodiversity, while still allowing for extensive agricultural management such as paludiculture. Belowground processes governing carbon fluxes and greenhouse gas dynamics are mediated by a complex network of microbial communities and processes. Our understanding of this complexity and its multi-factorial controls in rewetted peatlands is limited. Here, we summarize the research regarding the role of soil microbial communities and functions in driving carbon and nutrient cycling in rewetted peatlands including the use of molecular biology techniques in understanding biogeochemical processes linked to greenhouse gas fluxes. We emphasize that rapidly advancing molecular biology approaches, such as high-throughput sequencing, are powerful tools helping to elucidate the dynamics of key biogeochemical processes when combined with isotope tracing and greenhouse gas measuring techniques. Insights gained from the gathered studies can help inform efficient monitoring practices for rewetted peatlands and the development of climate-smart restoration and management strategies. Supplementary Information: The online version contains supplementary material available at 10.1007/s10533-024-01122-6.

Active afforestation of drained peatlands is not a viable option under the EU Nature Restoration Law.

The EU Nature Restoration Law (NRL) is critical for the restoration of degraded ecosystems and active afforestation of degraded peatlands has been suggested as a restoration measure under the NRL. Here, we discuss the current state of scientific evidence on the climate mitigation effects of peatlands under forestry. Afforestation of drained peatlands without restoring their hydrology does not fully restore ecosystem functions. Evidence on long-term climate benefits is lacking and it is unclear whether CO2 sequestration of forest on drained peatland can offset the carbon loss from the peat over the long-term. While afforestation may offer short-term gains in certain cases, it compromises the sustainability of peatland carbon storage. Thus, active afforestation of drained peatlands is not a viable option for climate mitigation under the EU Nature Restoration Law and might even impede future rewetting/restoration efforts. Instead, restoring hydrological conditions through rewetting is crucial for effective peatland restoration.

Influence of rewetting on N2O emissions in three different fen types.

In recent years, many peatlands in Europe have been rewetted for nature conservation and global warming mitigation. However, the effects on emissions of the greenhouse gas nitrous oxide (N2O) have been found to be highly variable and driving factors are poorly understood. Therefore, we measured N2O fluxes every two weeks over three years on pairs of sites (one drained, one rewetted) of three important peatland types in North-Eastern Germany, namely, percolation fen, alder forest and coastal fen. Additionally, every three months, sources of N2O were determined using a stable isotope mapping approach. Overall, fluxes were under the very dry conditions of the study years usually small with large temporal and spatial variations. Ammonium concentrations consistently and significantly correlated positively with N2O fluxes for all sites. Cumulative fluxes were often not significantly different from zero and apart from the rewetted alder forest, which was always a source of N2O, sites showed varying cumulative emission behavior (insignificant, source, potentially sink in one case) among years. Precipitation was positively correlated with cumulative fluxes on all drained sites and the rewetted alder forest. Isotope mapping indicated that N2O was always produced by more than one process simultaneously, with the estimated contribution of denitrification varying between 20 and 80%. N2O reduction played a potentially large role, with 5 to 50% of total emissions, showing large variations among sites and over time. Overall, neither the effect of rewetting, water level nor seasonality was clearly reflected in the fluxes or sources. Emissions were concentrated in hotspots and hot moments. A better understanding of the driving factors of N2O production and reduction in (rewetted) fens is essential and stable isotope methods including measurements of 15N and 18O as well as site preferences can help foster the necessary comprehension of the underlying mechanisms. Supplementary Information: The online version contains supplementary material available at 10.1007/s10705-022-10244-y.

Full-cycle greenhouse gas balance of a Sphagnum paludiculture site on former bog grassland in Germany.

Growing Sphagnum on rewetted bogs (=Sphagnum paludiculture) is an alternative to drainage-based land use because it retains its value as productive land while mitigating greenhouse gas (GHG) emissions. However, studies on GHG exchange covering the full production system and cycle are missing. Here, we combined data of the establishment phase with newly recorded data of a 7-year old Sphagnum paludiculture site in Germany including partial Sphagnum harvest. GHGs were measured with closed chambers at all elements of the system (production fields, ditches, causeways). Over the full production cycle, the production fields were GHG sinks with -3.2 ± 4.2 t ha-1 a-1 (in CO2-eq), while ditches represented sources emitting 13.8 ± 11.5 t ha-1 a-1. New measurements on the causeway indicated that it was a stronger GHG source with 29.3 ± 9.8 t ha-1 a-1 than previously assumed from literature values. Corrected for the area share of its elements and including the partial Sphagnum harvest (in dry mass) of ~13.8 ± 0.6 t ha-1 (=average 7-year CO2 emissions of 3.3 ± 0.1 t ha-1 a-1), the site was a GHG source of 10.7 ± 4.6 t ha-1 a-1, thus reducing emissions by ~20 t ha-1 a-1 compared to the German emission factor for grassland on drained organic soils. Per ton harvested dry biomass, the paludiculture site emitted 9.9 ± 4.6 t of CO2-eq. The causeways were the major contributor to the warming, calling for reducing causeway area in Sphagnum paludicultures. Future 'best-practice' could realistically comprise areal shares of 80 % production fields, 5 % ditches, 15 % causeways and a full Sphagnum harvest with the uppermost 5 cm remaining on site for recovery. In this scenario the site would emit CO2-eq emissions of 4.3 ± 1.9 t ha-1 a-1 or 0.9 ± 2.1 t per ton harvested dry mass.

Commentary: do we have a consistent terminology for species diversity? We are on the way.

A consistent terminology for species diversity is subject of an ongoing debate. Recently Tuomisto (Oecologia 164:853-860, 2010) stated that a consistent terminology for diversity already exists. The paper comments on recent papers by ourselves (Jurasinski et al. Oecologia 159:15-26, 2009) and by Moreno and Rodriguez (Oecologia 163:279-282, 2010). Both started from Whittaker's diversity concept to discuss the ambiguities of the terminology and propose a new, more consistent terminology that is based on the different approaches to diversity analysis. In contrast, Tuomisto adheres to a strict school of thinking and derives a diversity framework in the sense of Whittaker (alpha, beta, gamma) from the conceptual definition of diversity itself. A third group of papers discusses appropriate methods for the analysis of the variation in species composition. Here, we support the idea that alpha, beta and gamma diversity should be used in a strict sense that is based only on the conceptual definition of diversity. We accordingly extend and modify our terminological concept for species diversity. All approaches to the analysis and quantification of species composition and diversity can be assigned to three abstraction levels (species composition, variation in species composition,and variation in variation in species composition) and two scale levels (sample scale, aggregation scale). All methods that investigate the variation in species composition across scale levels evaluate beta relation with beta diversity being just one form of beta relation, which is calculated by dividing gamma diversity of order q by the appropriate alpha diversity of the same order. In contrast, differentiation refers to a pairwise calculation of resemblance in species composition. It is restricted to sample scale and is therefore most often only an intermediate step of analysis. Many ecological questions can be addressed either by direct analysis of the variation in species composition using raw data approaches or by further analysis of differentiation datasets on aggregation scale with or without respect to an external gradient.

Short-lived peaks of stem methane emissions from mature black alder (Alnus glutinosa (L.) Gaertn.) - Irrelevant for ecosystem methane budgets?

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.

Long-term vegetation change in the Western Tien-Shan Mountain pastures, Central Asia, driven by a combination of changing precipitation patterns and grazing pressure.

In mountain pastures worldwide, studies investigating vegetation changes due to long-term grazing and environmental changes are sparse, especially regarding the effects of changes in snowmelt patterns. The outstanding availability of historical vegetation data from Kyrgyz mountain pastures creates unique opportunities to study past and forecast future changes, making them ideal model ecosystems. Using a resurvey approach, we explored the response of mountain vegetation to management and environmental changes in the Western Tien-Shan to investigate whether plant communities of six vegetation types (ecozones) had changed over 42 years, whether changes were related to management or ecological causes and whether species' mean elevational ranges had changed. We assembled historic vegetation data (1973-1987) in six ecozones that were resurveyed annually from 2008 to 2015 and connected them with species' management-related traits and ecological indicator values. Overall, a homogenization of vegetation within and among ecozones was observed. Mountain steppe, meadow-steppe, and subalpine meadows showed the strongest convergence towards a dominance of mesic shrubs, related to increasing precipitation changing soil moisture and soil-salt regimes. In the high mountain steppe and the alpine ecozone, cushion dwarf shrubs increased, driven by increased soil moisture following faster snowmelt. Changes in the semidesert were related to highly variable spring soil moisture. Compositional changes accelerated over time. Mostly palatable species declined in abundance. More competitive unpalatable species replaced abundant (1973) unpalatable species. Mean elevation shifted significantly for 35 species (out of 136), with 60% shifting >100 m, more often upward (low and high elevations) than downward (mid-elevations). These mountain ecosystems seem more sensitive to changing precipitation than temperature- or grazing-induced changes, making climatic change a more important driver than management. Further adaptive management should consider the response of the vegetation to environmental changes and promote alternative land-use options to maintain ecosystem functioning. In mountain ecosystems worldwide, the observed acceleration of changes might go unnoticed, calling for long-term studies and global climate-vegetation-management interaction models.

Eukaryotic rather than prokaryotic microbiomes change over seasons in rewetted fen peatlands.

In the last decades, rewetting of drained peatlands is on the rise worldwide, to restore their significant carbon sink function. Despite the increasing understanding of peat microbiomes, little is known about the seasonal dynamics and network interactions of the microbial communities in these ecosystems, especially in rewetted fens (groundwater-fed peatlands). Here, we investigated the seasonal dynamics in both prokaryotic and eukaryotic microbiomes in three common fen types in Northern Germany. The eukaryotic microbiomes, including fungi, protists and microbial metazoa, showed significant changes in their community structures across the seasons in contrast to largely unaffected prokaryotic microbiomes. Furthermore, our results proved that the dynamics in eukaryotic microbiomes in the rewetted sites differed between fen types, specifically in terms of saprotrophs, arbuscular mycorrhiza and grazers of bacteria. The co-occurrence networks also exhibited strong seasonal dynamics that differed between rewetted and drained sites, and the correlations involving protists and prokaryotes were the major contributors to these dynamics. Our study provides the insight that microbial eukaryotes mainly define the seasonal dynamics of microbiomes in rewetted fen peatlands. Accordingly, future research should unravel the importance of eukaryotes for biogeochemical processes, especially the under-characterized protists and metazoa, in these poorly understood ecosystems.

Prompt rewetting of drained peatlands reduces climate warming despite methane emissions.

Peatlands are strategic areas for climate change mitigation because of their matchless carbon stocks. Drained peatlands release this carbon to the atmosphere as carbon dioxide (CO2). Peatland rewetting effectively stops these CO2 emissions, but also re-establishes the emission of methane (CH4). Essentially, management must choose between CO2 emissions from drained, or CH4 emissions from rewetted, peatland. This choice must consider radiative effects and atmospheric lifetimes of both gases, with CO2 being a weak but persistent, and CH4 a strong but short-lived, greenhouse gas. The resulting climatic effects are, thus, strongly time-dependent. We used a radiative forcing model to compare forcing dynamics of global scenarios for future peatland management using areal data from the Global Peatland Database. Our results show that CH4 radiative forcing does not undermine the climate change mitigation potential of peatland rewetting. Instead, postponing rewetting increases the long-term warming effect through continued CO2 emissions.

The impact of occasional drought periods on vegetation spread and greenhouse gas exchange in rewetted fens.

Peatland rewetting aims at stopping the emissions of carbon dioxide (CO2) and establishing net carbon sinks. However, in times of global warming, restoration projects must increasingly deal with extreme events such as drought periods. Here, we evaluate the effect of the European summer drought 2018 on vegetation development and the exchange of methane (CH4) and CO2 in two rewetted minerotrophic fens (Hütelmoor-Hte and Zarnekow-Zrk) including potential carry-over effects in the post-drought year. Drought was a major stress factor for the established vegetation but also promoted the rapid spread of new vegetation, which will likely gain a lasting foothold in Zrk. Accordingly, drought increased not only respiratory CO2 losses but also photosynthetic CO2 uptake. Altogether, the drought reduced the net CO2 sink in Hte, while it stopped the persistent net CO2 emissions of Zrk. In addition, the drought reduced CH4 emissions in both fens, though this became most apparent in the post-drought year and suggests a lasting shift towards non-methanogenic organic matter decomposition. Occasional droughts can be beneficial for the restoration of the peatland carbon sink function if the newly grown vegetation increases CO2 sequestration in the long term. Nonetheless, care must be taken to prevent extensive peat decay. This article is part of the theme issue 'Impacts of the 2018 severe drought and heatwave in Europe: from site to continental scale'.

Long-Term Rewetting of Three Formerly Drained Peatlands Drives Congruent Compositional Changes in Pro- and Eukaryotic Soil Microbiomes through Environmental Filtering.

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.

Topsoil removal reduced in-situ methane emissions in a temperate rewetted bog grassland by a hundredfold.

Peatland restoration is seen as an effective contribution to help achieve the aims of the Paris Agreement because currently huge amounts of peatlands in Northern Central Europe are under unsustainable drainage-based land use. If net zero greenhouse gas emissions from peatlands shall be reached by 2050, restoration measures have to be done as soon as possible. However, rewetting drained peatlands that were under intensive grassland use frequently results in high methane (CH4) emissions, which is often seen as a counter-argument against rewetting. To find the source of high CH4 emissions after rewetting and to explore the best possible way of peatland restoration (i.e., low CH4 emissions after rewetting) under near-natural conditions, we installed a field trial in a drained bog in north-western Germany. The trial consists of seven plots (~8 × 24 m2) representing the status quo-intensive grassland use- and six restoration approaches with combinations of rewetting either on the original surface or after topsoil removal (TSR), biomass harvesting or spreading Sphagnum spp. to initiate vegetation succession. On all seven plots we measured CH4 fluxes using closed chambers. In addition, we investigated CH4 production potential by incubating soil samples and determining methanogen abundance by quantitative PCR. Compared to rewetting on the original surface, CH4 emissions were reduced on TSR plots by factor 30 to 400. Spreading of Sphagnum spp. had only little effect on CH4 emissions during the first year of establishment. TSR also reduced CH4 production potential and methanogen abundance. Further, the response of CH4 fluxes to methanogen abundance was lower after TSR. This suggests that both reduction in labile substrate and in methanogen abundance contribute to near-zero CH4 emissions after TSR. These are the first field-scale results that demonstrate the efficiency of removing degraded topsoil to avoid high CH4 emissions after rewetting.