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Peatlands are found on all continents, covering 3% of the global land area. However, the spatial extent and causes of metal enrichment in peatlands is understudied and no attempt has been made to evaluate global patterns of metal enrichment in bog and fen peatlands, despite that certain metals and rare earth elements (REE) arise from anthropogenic sources. We analyzed 368 peat cores sampled in 16 countries across five continents and measured metal and other element concentrations at three depths down to 70 cm as well as estimated cumulative atmospheric S deposition (1850–2009) for each site. Sites were assigned to one of three distinct broadly recognized peatland categories (bog, poor fen, and intermediate-to-moderately rich fen) that varied primarily along a pH gradient. Metal concentrations differed among peatland types, with intermediate-to-moderately rich fens demonstrating the highest concentrations of most metals. Median enrichment factors (EFs; a metric comparing natural and anthropogenic metal deposition) for individual metals were similar among bogs and fens (all groups), with metals likely to be influenced by anthropogenic sources (As, Cd, Co, Cu, Hg, Pb, and Sb) demonstrating median enrichment factors (EFs) > 1.5. Additionally, mean EFs were substantially higher than median values, and the positive correlation (< 0.40) with estimated cumulative atmospheric S deposition, confirmed some level of anthropogenic influence of all pollutant metals except for Hg that was unrelated to S deposition. Contrary to expectations, high EFs were not restricted to pollutant metals, with Mn, K and Rb all exhibiting elevated median EFs that were in the same range as pollutant metals likely due to peatland biogeochemical processes leading to enrichment of these nutrients in surface soil horizons. The global patterns of metal enrichment in bogs and fens identified in this study underscore the importance of these peatlands as environmental archives of metal deposition, but also illustrates that biogeochemical processes can enrich metals in surface peat and EFs alone do not necessarily indicate atmospheric contamination. [ABSTRACT FROM AUTHOR]

In northern peatlands, reduction of Sphagnum dominance in favour of vascular vegetation is likely to influence biogeochemical processes. Such vegetation changes occur as the water table lowers and temperatures rise. To test which of these factors has a significant influence on peatland vegetation, we conducted a 3‐year manipulative field experiment in Linje mire (northern Poland). We manipulated the peatland water table level (wet, intermediate and dry; on average the depth of the water table was 17.4, 21.2 and 25.3 cm respectively), and we used open‐top chambers (OTCs) to create warmer conditions (on average increase of 1.2°C in OTC plots compared to control plots). Peat drying through water table lowering at this local scale had a larger effect than OTC warming treatment per see on Sphagnum mosses and vascular plants. In particular, ericoid shrubs increased with a lower water table level, while Sphagnum decreased. Microclimatic measurements at the plot scale indicated that both water‐level and temperature, represented by heating degree days (HDDs), can have significant effects on the vegetation. In a large‐scale complementary vegetation gradient survey replicated in three peatlands positioned along a transitional oceanic–continental and temperate–boreal (subarctic) gradient (France–Poland–Western Siberia), an increase in ericoid shrubs was marked by an increase in phenols in peat pore water, resulting from higher phenol concentrations in vascular plant biomass. Our results suggest a shift in functioning from a mineral‐N‐driven to a fungi‐mediated organic‐N nutrient acquisition with shrub encroachment. Both ericoid shrub encroachment and higher mean annual temperature in the three sites triggered greater vascular plant biomass and consequently the dominance of decomposers (especially fungi), which led to a feeding community dominated by nematodes. This contributed to lower enzymatic multifunctionality. Our findings illustrate mechanisms by which plants influence ecosystem responses to climate change, through their effect on microbial trophic interactions. [ABSTRACT FROM AUTHOR]

Peatlands account for 15 to 30% of the world's soil carbon (C) stock and are important controls over global nitrogen (N) cycles. However, C and N concentrations are known to vary among peatlands contributing to the uncertainty of global C inventories, but there are few global studies that relate peatland classification to peat chemistry. We analyzed 436 peat cores sampled in 24 countries across six continents and measured C, N, and organic matter (OM) content at three depths down to 70 cm. Sites were distinguished between northern (387) and tropical (49) peatlands and assigned to one of six distinct broadly recognized peatland categories that vary primarily along a pH gradient. Peat C and N concentrations, OM content, and C:N ratios differed significantly among peatland categories, but few differences in chemistry with depth were found within each category. Across all peatlands C and N concentrations in the 10–20 cm layer, were 440 ± 85.1 g kg-1 and 13.9 ± 7.4 g kg-1, with an average C:N ratio of 30.1 ± 20.8. Among peatland categories, median C concentrations were highest in bogs, poor fens and tropical swamps (446–532 g kg-1) and lowest in intermediate and extremely rich fens (375–414 g kg-1). The C:OM ratio in peat was similar across most peatland categories, except in deeper samples from ombrotrophic tropical peat swamps that were higher than other peatlands categories. Peat N concentrations and C:N ratios varied approximately two-fold among peatland categories and N concentrations tended to be higher (and C:N lower) in intermediate fens compared with other peatland types. This study reports on a unique data set and demonstrates that differences in peat C and OM concentrations among broadly classified peatland categories are predictable, which can aid future studies that use land cover assessments to refine global peatland C and N stocks. [ABSTRACT FROM AUTHOR]

Background: Northern peatlands have accumulated vast amounts of carbon (C) as peat. Warming temperatures may affect peatland C stores by increasing microbial decomposition of ancient peat through enhanced input of labile root exudates by expansion of vascular plants, thereby accelerating atmospheric warming. Aims: We set out to explore how much freshly assimilated C is allocated belowground by vascular plants, and if the above‐ to belowground allocation is affected by temperature and plant functional types. Methods: We traced the C allocation pathways of two dominant plant functional types (i.e., sedges and shrubs) in two peatlands under different temperature regimes by combining selective plant removal in mixed sedge‐shrub vegetation and in situ 13C pulse‐labelling. Aboveground to belowground C allocation as well as the C turnover were assessed by quantifying 13C in plant leaves and soil respiration and by measuring δ13C in dissolved organic C. A depth‐resolved quantification of 13C in the peat soil gave additional insight into belowground C allocation patterns. Results: Temperature did not affect the rate at which 13C was assimilated into shoots, but higher temperature decreased the fraction of assimilated C that was allocated belowground by vascular plants. Sedges assimilated CO2 faster into their shoot biomass (faster depletion in 13C in shoots) and allocated more of the assimilated 13C belowground than shrubs. Conversely, sedges retained this belowground allocated C better than shrubs, leading to lower 13C in soil respiration measured under sedges. Conclusions: Climate induced vascular plant expansion will increase input of fresh assimilates into the peat substantially, even though part of this effect will be offset by reduced above‐ to belowground allocation rates. If shrub density increases relative to sedges, fresh assimilates are more likely to be respired than translocated to roots where they could reach and, potentially mobilize, ancient C stored in deeper peat layers. [ABSTRACT FROM AUTHOR]

Peatlands contain a significant fraction of global soil carbon, but how these reservoirs will respond to the changing climate is still relatively unknown. A global picture of the variations in peat organic matter chemistry will aid our ability to gauge peatland soil response to climate. The goal of this research is to test the hypotheses that (a) peat carbohydrate content, an indicator of soil organic matter reactivity, will increase with latitude and decrease with mean annual temperatures, (b) while peat aromatic content, an indicator of recalcitrance, will vary inversely, and (c) elevation will have a similar effect to latitude. We used Fourier Transform Infrared Spectroscopy to examine variations in the organic matter functional groups of 1034 peat samples collected from 10 to 20, 30–40, and 60–70 cm depths at 165 individual sites across a latitudinal gradient of 79°N–65°S and from elevations of 0–4,773 m. Carbohydrate contents of high latitude peat were significantly greater than peat originating near the equator, while aromatic content showed the opposite trend. For peat from similar latitudes but different elevations, the carbohydrate content was greater and aromatic content was lower at higher elevations. Higher carbohydrate content at higher latitudes indicates a greater potential for mineralization, whereas the chemical composition of low latitude peat is consistent with their apparent relative stability in the face of warmer temperatures. The combination of low carbohydrates and high aromatics at warmer locations near the equator suggests the mineralization of high latitude peat until reaching recalcitrance under a new temperature regime. Plain Language Summary: Peatlands are a large global soil carbon reservoir, containing a quantity of carbon that is equivalent to about half or more of the carbon dioxide in the atmosphere. What will be their fate on a warming planet? Across a latitudinal gradient from 79°N to 65°S, we measured the fraction of the peat made up of carbohydrates, which are easily decomposed, and of aromatics, which are less easily decomposed. We found that peat from high latitudes and high elevations had greater carbohydrate content, while aromatic content showed the opposite trend. Larger carbohydrate content in organic matter indicates greater decomposability, while greater aromatic content indicates lower decomposability. We suggest that this latitude/elevation difference that we observed predicts how high‐latitude and high‐elevation peats may change under warmer conditions. Our work indicates that while a large portion of the carbohydrate fraction in these peats could be lost upon warming, releasing greenhouse gases, a residual fraction will survive and become more aromatic‐rich, making the remainder more resistant to rapid decomposition. Key Points: Peatland soil organic matter stability can be evaluated through the relative abundances of carbohydrate and aromatic content in peatPeat at higher latitudes and elevations has larger carbohydrate and smaller aromatic content relative lower latitudes and elevationsOur results foreshadow a transition of organic matter from higher latitude peatlands to a more recalcitrant form in response to warming [ABSTRACT FROM AUTHOR]

The relative importance of global versus local environmental factors for growth and thus carbon uptake of the bryophyte genus Sphagnum—the main peat‐former and ecosystem engineer in northern peatlands—remains unclear.We measured length growth and net primary production (NPP) of two abundant Sphagnum species across 99 Holarctic peatlands. We tested the importance of previously proposed abiotic and biotic drivers for peatland carbon uptake (climate, N deposition, water table depth and vascular plant cover) on these two responses. Employing structural equation models (SEMs), we explored both indirect and direct effects of drivers on Sphagnum growth.Variation in growth was large, but similar within and between peatlands. Length growth showed a stronger response to predictors than NPP. Moreover, the smaller and denser Sphagnum fuscum growing on hummocks had weaker responses to climatic variation than the larger and looser Sphagnum magellanicum growing in the wetter conditions. Growth decreased with increasing vascular plant cover within a site. Between sites, precipitation and temperature increased growth for S. magellanicum. The SEMs indicate that indirect effects are important. For example, vascular plant cover increased with a deeper water table, increased nitrogen deposition, precipitation and temperature. These factors also influenced Sphagnum growth indirectly by affecting moss shoot density.Synthesis. Our results imply that in a warmer climate, S. magellanicum will increase length growth as long as precipitation is not reduced, while S. fuscum is more resistant to decreased precipitation, but also less able to take advantage of increased precipitation and temperature. Such species‐specific sensitivity to climate may affect competitive outcomes in a changing environment, and potentially the future carbon sink function of peatlands. [ABSTRACT FROM AUTHOR]

Northern peatlands sequester carbon (C) and nitrogen (N) over millennia, at variable rates that depend on climate, environmental variables and anthropogenic activity. The ombrotrophic peatlands of central and northern Alberta (Canada) have developed under variable climate conditions during the last hundreds to thousands of years, while in the course of the twentieth century, some regions were also likely subjected to anthropogenic disturbance. We aimed to quantify peat C and N accumulation rates for the last millennium from seven peatlands to estimate the relative influence of climate and anthropogenic disturbance on C accumulation dynamics. Peatlands have accumulated C at an average rate of 25.3 g C m−2 year−1 over the last millennium. Overall, climate was likely a major factor as, on average, highest apparent rates of C accumulation were found around 1100 CE, during the warmer Medieval Climate Anomaly, with lowest rates during the Little Ice Age, around 1750 CE. Local factors, such as disturbance, played a role in C sequestration at the site scale. The average N accumulation rate was 0.55 g N m−2 year−1, with high inter- and intra-site variability. In general, N accumulation mirrored patterns in C sequestration for peat deposited pre-1850 CE. However, higher N accumulation rates observed after 1850 CE, averaging 0.94 g N m−2 year−1, were not correlated with C accumulation. Moreover, some of the historically strongly accumulating sites may have become less efficient in sequestering C, and vice versa. All seven sites showed a marked decrease in δ15N when comparing pre- and post-1850 timeframes, consistent with increasing post-1850 N additions from an atmospheric source, likely biological N fixation. Overall, N was not a driving factor for C accumulation. [ABSTRACT FROM AUTHOR]

Aims: Northern peatlands store large amounts of soil organic carbon (C) that can be very sensitive to ongoing global warming. Recently it has been shown that temperature-enhanced growth of vascular plants in these typically moss-dominated ecosystems may promote microbial peat decomposition by increased C input via root exudates. To what extent different plant functional types (PFT) and soil temperature interact in controlling root C input is still unclear. In this study we explored how root C input is related to the presence of ericoid shrubs (shrubs) and graminoid sedges (sedges) by means of a factorial plant clipping experiment (= PFT effect) in two peatlands located at different altitude (= temperature effect). Methods: By selective clipping of shrub and sedge shoots in mixed vegetation at two Alpine peatland sites we interrupted the above- to belowground translocation of C, thus temporarily inhibiting root C release. Subsequent measurements of soil respiration, dissolved organic carbon (DOC) concentration and stable isotope composition (13C) of DOC in pore water were used as proxies to estimate the above- to belowground transfer of C by different PFT. Results: We found that soil respiration rates and DOC concentrations temporarily decreased within 24 h after clipping, with the decrease in soil respiration being most pronounced at the 1.4 °C warmer peatland after clipping shrubs. The transient drop in DOC concentration coincided with a shift towards a heavier C isotope signature, indicating that the decrease was associated with inhibition of a light C source that we attribute to root exudates. Together these results imply that shrubs translocated more C into the peat than sedges, particularly at higher temperature. Conclusions: We showed that plant functional type and temperature interact in controlling root C input under field conditions in peatlands. Our results provide a mechanistic evidence that shrubs may potentially promote the release of stored soil C through root-derived C input. [ABSTRACT FROM AUTHOR]

Abstract: Climate change can alter peatland plant community composition by promoting the growth of vascular plants. How such vegetation change affects peatland carbon dynamics remains, however, unclear. In order to assess the effect of vegetation change on carbon uptake and release, we performed a vascular plant‐removal experiment in two Sphagnum‐dominated peatlands that represent contrasting stages of natural vegetation succession along a climatic gradient. Periodic measurements of net ecosystem CO2 exchange revealed that vascular plants play a crucial role in assuring the potential for net carbon uptake, particularly with a warmer climate. The presence of vascular plants, however, also increased ecosystem respiration, and by using the seasonal variation of respired CO2 radiocarbon (bomb‐14C) signature we demonstrate an enhanced heterotrophic decomposition of peat carbon due to rhizosphere priming. The observed rhizosphere priming of peat carbon decomposition was matched by more advanced humification of dissolved organic matter, which remained apparent beyond the plant growing season. Our results underline the relevance of rhizosphere priming in peatlands, especially when assessing the future carbon sink function of peatlands undergoing a shift in vegetation community composition in association with climate change. [ABSTRACT FROM AUTHOR]

Rain-fed peatlands are dominated by peat mosses (Sphagnum sp.), which for their growth depend on nutrients, water and CO2 uptake from the atmosphere. As the isotopic composition of carbon (12;13C) and oxygen (16;18O) of these Sphagnum mosses are affected by environmental conditions, Sphagnum tissue accumulated in peat constitutes a potential long-term archive that can be used for climate reconstruction. However, there is inadequate understanding of how isotope values are influenced by environmental conditions, which restricts their current use as environmental and palaeoenvironmental indicators. Here we tested (i) to what extent C and O isotopic variation in living tissue of Sphagnum is speciesspecific and associated with local hydrological gradients, climatic gradients (evapotranspiration, temperature, precipitation) and elevation; (ii) whether the C isotopic signature can be a proxy for net primary productivity (NPP) of Sphagnum; and (iii) to what extent Sphagnum tissue δ18O tracks the δ18O isotope signature of precipitation. In total, we analysed 337 samples from 93 sites across North America and Eurasia using two important peat-forming Sphagnum species (S. magellanicum, S. fuscum) common to the Holarctic realm. There were differences in δ13C values between species. For S. magellanicum δ13C decreased with increasing height above the water table (HWT, R2 D 17 %) and was positively correlated to productivity (R2 D 7 %). Together these two variables explained 46% of the between-site variation in δ13C values. For S. fuscum, productivity was the only significant predictor of δ13C but had low explanatory power (total R2 D 6 %). For δ18O values, approximately 90% of the variation was found between sites. Globally modelled annual δ18O values in precipitation explained 69% of the between-site variation in tissue δ18O. S. magellanicum showed lower δ18O enrichment than S. fuscum (-0.83%lower). Elevation and climatic variables were weak predictors of tissue δ18O values after controlling for δ18O values of the precipitation. To summarize, our study provides evidence for (a) good predictability of tissue δ18O values from modelled annual δ18O values in precipitation, and (b) the possibility of relating tissue δ13C values to HWT and NPP, but this appears to be species-dependent. These results suggest that isotope composition can be used on a large scale for climatic reconstructions but that such models should be species-specific. [ABSTRACT FROM AUTHOR]

Peatlands are important sinks of atmospheric carbon (C) that, in response to climate warming, are undergoing dynamic vegetation succession. Here we examined the hypothesis that the uptake of nutrients by different plant growth forms (PGFs) is one key mechanism driving changes in species abundance in peatlands. Along an altitude gradient representing a natural climate experiment, we compared the variability of the stable C isotope composition (δC) and stable nitrogen (N) isotope composition (δN) in current-year leaves of two major PGFs, i.e. ericoids and graminoids. The climate gradient was associated with a gradient of vascular plant cover, which was parallelled by different concentrations of organic and inorganic N as well as the fungal/bacterial ratio in peat. In both PGFs the C natural abundance showed a marginal spatial decrease with altitude and a temporal decrease with progression of the growing season. Our data highlight a primary physical control of foliar δC signature, which is independent from the PGFs. Natural abundance of foliar N did not show any seasonal pattern and only in the ericoids showed depletion at lower elevation. This decreasing δN pattern was primarily controlled by the higher relative availability of organic versus inorganic N and, only for the ericoids, by an increased proportion of fungi to bacteria in soil. Our space-for-time approach demonstrates that a change in abundance of PGFs is associated with a different strategy of nutrient acquisition (i.e. transfer via mycorrhizal symbiosis versus direct fine-root uptake), which could likely promote observed and predicted dwarf shrub expansion under climate change. [ABSTRACT FROM AUTHOR]

Peatlands are important sinks for atmospheric carbon (C), yet the role of plant functional types (PFTs) for C sequestration under climatic perturbations is still unclear. A plant-removal experiment was used to study the importance of vascular PFTs for the net ecosystem C02 exchange (NEE) during (i.e., resistance) and after (i.e., recovery) an experimental drought. The removal of PFTs caused a decrease of NEE, but the rate differed between microhabitats (i.e., hummocks and lawns) and the type of PFTs. Ericoid removal had a large effect on NEE in hummocks, while the graminoids played a major role in the lawns. The removal of PFTs did not affect the resistance or the recovery after the experimental drought. We argue that the response of Sphagnum mosses (the only PFT present in all treatments) to drought is dominant over that of coexisting PFTs. However, we observed that the moment in time when the system switched from C sink to C source during the drought was controlled by the vascular PFTs. In the light of climate change, the shifts in species composition or even the loss of certain PFTs are expected to strongly affect the future C dynamics in response to environmental stress. [ABSTRACT FROM AUTHOR]

In this study, we describe the seasonal variation in N abundance in the litter of two Sphagnum species and four vascular plant species during 3 years of field decomposition in an Italian Alpine bog. Litter bags were periodically retrieved at the end of summer and winter periods, and the δN in residual litter was related to mass loss, litter chemistry, and climatic conditions. In Sphagnum litter, higher rates of decomposition during summer months were associated with an increase of δN probably due to the incorporation of microbial organic compounds rich in N. The litter of Eriophorum vaginatum and Carex rostrata was characterized by a decrease of δN, so that the final signature was significantly lower than in initial litter. On the other hand, the residual litter of Potentilla erecta and Calluna vulgaris was characterized by a final δN higher than in initial litter. Our data reported a seasonality of N abundance in the residual litter of Sphagnum species, but not in that of vascular plant species, thus highlighting the role of differences in litter chemistry. [ABSTRACT FROM AUTHOR]

In the European Union, the Directive 92/43/EEC defines a number of species and habitats of community interest that are worthy to be preserved because in danger to disappear or because they are representative of the different European bio-geographical regions. In the light of the limited economic resources generally allocated to conservation efforts, there is the necessity to prioritise conservation actions in order to avoid deterioration of protected areas. To this aim, in the present study the most representative habitats of the Italian Alps are compared on the basis of vascular plant biodiversity and a conservation priority index is proposed for each habitat taking into account the potential distribution of 252 threatened vascular plant species. Rocky slopes, screes and alpine grasslands resulted to have the greatest percentage of endemic plant species so reflecting the general distributional pattern of endemic plant species at high altitudes in Eurasian mountains. The relationship between the conservation priority index and the corresponding habitat extent within the Natura 2000 network suggests that peatlands, arid grasslands, wet meadows and freshwater habitats deserve a higher priority in conservation actions. Although vascular plant biodiversity is not necessarily a surrogate of overall biodiversity of Alpine habitats, the results here reported can be used as an initial reference framework for prioritising conservation actions, so as to accomplish the provisions of Article 6 of Habitats Directive. [ABSTRACT FROM AUTHOR]

Abstract: Peat mosses of the genus Sphagnum constitute the bulk of living and dead biomass in bogs. These plants contain peculiar polyphenols which hamper litter peat decomposition through their inhibitory activity on microbial breakdown. In the light of the increasing availability of biologically active nitrogen in natural ecosystems, litter derived from Sphagnum mosses is an ideal substrate to test the potential effects of increased atmospheric nitrogen deposition on polyphenol content in litter peat. To this aim, we measured total nitrogen and soluble polyphenol concentration in Sphagnum litter peat collected in 11 European bogs under a chronic gradient of atmospheric nitrogen deposition. Our results demonstrate that increasing nitrogen concentration in Sphagnum litter, as a consequence of increased exogenous nitrogen availability, is accompanied by a decreasing concentration of polyphenols. This inverse relationship is consistent with reports that in Sphagnum mosses, polyphenol and protein biosynthesis compete for the same precursor. Our observation of modified Sphagnum litter chemistry under chronic nitrogen eutrophication has implications in the context of the global carbon balance, because a lower content of decay-inhibiting polyphenols would accelerate litter peat decomposition. [Copyright &y& Elsevier]

Alteration of the global nitrogen (N) cycle because of human-enhanced N fixation is a major concern particularly for those ecosystems that are nutrient poor by nature. BecauseSphagnum-dominated mires are exclusively fed by wet and dry atmospheric deposition, they are assumed to be very sensitive to increased atmospheric N input. We assessed the consequences of increased atmospheric N deposition on total N concentration, N retention ability, andδ15N isotopic signature ofSphagnumplants collected in 16 ombrotrophic mires across 11 European countries. The mires spanned a gradient of atmospheric N deposition from about 0.1 up to about 2 g m−2 yr−1. Mean N concentration inSphagnumcapitula was about 6 mg g−1 in less polluted mires and about 13 mg g−1 in highly N-polluted mires. The relative difference in N concentration between capitulum and stem decreased with increasing atmospheric N deposition, suggesting a possible metabolic mechanism that reduces excessive N accumulation in the capitulum.Sphagnumplants showed lower rates of N absorption under increasing atmospheric N deposition, indicating N saturation inSphagnumtissues. The latter probably is related to a shift from N-limited conditions to limitation by other nutrients. The capacity of theSphagnumlayer to filter atmospheric N deposition decreased exponentially along the depositional gradient resulting in enrichment of the mire pore water with inorganic N forms (i.e., NO3−+NH4+).Sphagnumplants hadδ15N signatures ranging from about−8‰ to about−3‰. The isotopic signatures were rather related to the ratio of reduced to oxidized N forms in atmospheric deposition than to total amount of atmospheric N deposition, indicating thatδ15N signature ofSphagnumplants can be used as an integrated measure ofδ15N signature of atmospheric precipitation. Indeed, mires located in areas characterized by greater emissions of NH3 (i.e., mainly affected by agricultural activities) hadSphagnumplants with a lowerδ15N signature compared with mires located in areas dominated by NO x emissions (i.e., mainly affected by industrial activities). [ABSTRACT FROM AUTHOR]

Several studies on the impact of climate warming have indicated that peat decomposition/mineralization will be enhanced. Most of these studies deal with the impact of experimental warming during summer when prevalent abiotic conditions are favorable to decomposition. Here, we investigated the effect of experimental air warming by open-top chambers (OTCs) on water-extractable organic matter (WEOM), microbial biomasses and enzymatic activities in two contrasted moisture sites named Bog and Fen sites, the latter considered as the wetter ones. While no or few changes in peat temperature and water content appeared under the overall effect of OTCs, we observed that air warming smoothed water content differences and led to a decrease in mean peat temperature at the warmed Bog sites. This thermal discrepancy between the two sites led to contrasting changes in microbial structure and activities: a rise in hydrolytic activity at the warmed Bog sites and a relative enhancement of bacterial biomass at the warmed Fen sites. These features were not associated with any change in WEOM properties namely carbon and sugar contents and aromaticity, suggesting that air warming did not trigger any shift in OM decomposition. Using various tools, we show that the use of single indicators of OM decomposition can lead to fallacious conclusions. Lastly, these patterns may change seasonally as a consequence of complex interactions between groundwater level and air warming, suggesting the need to improve our knowledge using a high time-resolution approach. [ABSTRACT FROM AUTHOR]

Abstract: Peatlands represent massive global C pools and sinks. Carbon accumulation depends on the ratio between net primary production and decomposition, both of which can change under projected increases of atmospheric CO2 and N deposition. The decomposition of litter is influenced by 1) the quality of the litter, and 2) the microenvironmental conditions in which the litter decomposes. This study aims at experimentally testing the effects of these two drivers in the context of global change. We studied the in situ litter decomposition from three common peatland species (Eriophorum vaginatum, Polytrichum strictum and Sphagnum fallax) collected after one year of litter production under pre-treatment conditions (elevated CO2: 560 ppm or enhanced N: 3 g m−2 y−1 NH4NO3) and decomposed the following year under treatment conditions (same as pre-treatment). By considering the cross-effects between pre-treatments and treatments, we distinguished between the effects on mass loss of 1) the pre-treatment-induced litter quality and 2) the treatment conditions under which the litters were decomposing. The combination between CO2 pre-treatment and CO2 treatment reduced Polytrichum decomposition by −24% and this can be explained by litter quality-driven decomposition changes brought by the pre-treatment. CO2 pre-treatment reduced Eriophorum litter quality, although this was not sufficient to predict decomposition. The N addition pre-treatment reduced the decomposition of Eriophorum, due to enhanced lignin and soluble phenols concentrations in the initial litter, and reduced litter-driven losses of starch and enhanced litter-driven losses of soluble phenols. While decomposition indices based on initial litter quality provide a broad explanation of quantitative and qualitative decomposition, they can only be taken as first approximations. Indeed, the microbial ATP activity, the litter N loss and resulting litter quality, were strongly altered irrespective of the compounds'' initial concentration and by means of processes that occurred independently of the initial litter-qualitative changes. The experimental design was valuable to assess litter- and ecosystem-driven decomposition pathways simultaneously or independently. The ability to separate these two drivers makes it possible to attest the presence of litter-qualitative changes even without any litter biochemical determinations, and shows the screening potential of this approach for future experiments dealing with multiple plant species. [Copyright &y& Elsevier]

Background: Northern peatlands have accumulated vast amounts of carbon (C) as peat. Warming temperatures may affect peatland C stores by increasing microbial decomposition of ancient peat through enhanced input of labile root exudates by expansion of vascular plants, thereby accelerating atmospheric warming. Aims: We set out to explore how much freshly assimilated C is allocated belowground by vascular plants, and if the above‐ to belowground allocation is affected by temperature and plant functional types. Methods: We traced the C allocation pathways of two dominant plant functional types (i.e., sedges and shrubs) in two peatlands under different temperature regimes by combining selective plant removal in mixed sedge‐shrub vegetation and in situ 13C pulse‐labelling. Aboveground to belowground C allocation as well as the C turnover were assessed by quantifying 13C in plant leaves and soil respiration and by measuring δ13C in dissolved organic C. A depth‐resolved quantification of 13C in the peat soil gave additional insight into belowground C allocation patterns. Results: Temperature did not affect the rate at which 13C was assimilated into shoots, but higher temperature decreased the fraction of assimilated C that was allocated belowground by vascular plants. Sedges assimilated CO2 faster into their shoot biomass (faster depletion in 13C in shoots) and allocated more of the assimilated 13C belowground than shrubs. Conversely, sedges retained this belowground allocated C better than shrubs, leading to lower 13C in soil respiration measured under sedges. Conclusions: Climate induced vascular plant expansion will increase input of fresh assimilates into the peat substantially, even though part of this effect will be offset by reduced above‐ to belowground allocation rates. If shrub density increases relative to sedges, fresh assimilates are more likely to be respired than translocated to roots where they could reach and, potentially mobilize, ancient C stored in deeper peat layers. [ABSTRACT FROM AUTHOR]