A drained nutrient-poor peatland forest in boreal Sweden constitutes a net carbon sink after integrating terrestrial and aquatic fluxes

Northern peatlands provide a globally important carbon (C) store. Since the beginning of the 20th century, however, large areas of natural peatlands have been drained for biomass production across Fennoscandia. Today, drained peatland forests constitute a common feature of the managed boreal landscape, yet their ecosystem C balance and associated climate impact are not well understood, particularly within the nutrient-poor boreal region. In this study, we estimated the net ecosystem carbon balance (NECB) from a nutrient-poor drained peatland forest and an adjacent natural mire in northern Sweden by integrating terrestrial carbon dioxide (CO2 ) and methane (CH4 ) fluxes with aquatic losses of dissolved organic C (DOC) and inorganic C based on eddy covariance and stream discharge measurements, respectively, over two hydrological years. Since the forest included a dense spruce-birch area and a sparse pine area, we were able to further evaluate the effect of contrasting forest structure on the NECB and component fluxes. We found that the drained peatland forest was a net C sink with a 2-year mean NECB of -115 ± 5 g C m-2  year-1 while the adjacent mire was close to C neutral with 14.6 ± 1.7 g C m-2  year-1 . The NECB of the drained peatland forest was dominated by the net CO2 exchange (net ecosystem exchange [NEE]), whereas NEE and DOC export fluxes contributed equally to the mire NECB. We further found that the C sink strength in the sparse pine forest area (-153 ± 8 g C m-2  year-1 ) was about 1.5 times as high as in the dense spruce-birch forest area (-95 ± 8 g C m-2  year-1 ) due to enhanced C uptake by ground vegetation and lower DOC export. Our study suggests that historically drained peatland forests in nutrient-poor boreal regions may provide a significant net ecosystem C sink and associated climate benefits.

Microbial activity contributes to spatial heterogeneity of wetland methane fluxes

The emission of methane from wetlands is spatially heterogeneous, as concurrently measured surface fluxes can vary by orders of magnitude within the span of a few meters. Despite extensive study and the climatic significance of these emissions, it remains unclear what drives large, within-site variations. While geophysical factors (e.g., soil temperature) are known to correlate with methane (CH4) flux, measurable variance in these parameters often declines as spatial and temporal scales become finer. As methane emitted from wetlands is the direct, net product of microbial metabolisms which both produce and degrade CH4, it stands to reason that characterizing the spatial variability of microbial communities within a wetland-both horizontally and vertically-may help explain observed variances in flux. To that end, we surveyed microbial communities to a depth of 1 m across an ombrotrophic peat bog in Maine, USA using amplicon sequencing and gene expression techniques. Surface methane fluxes and geophysical factors were concurrently measured. Across the first meter of peat at the site, we observed significant changes in the abundance and composition of methanogenic taxa at every depth sampled, with variance in methanogen abundance explaining 70% of flux heterogeneity at a subset of plots. Among measured environmental factors, only peat depth emerged as correlated with flux, and had significant impact on the abundance and composition of methane-cycling communities. These conclusions suggest that a heightened awareness of how microbial communities are structured and spatially distributed within wetlands could offer improved insights into predicting CH4 flux dynamics. IMPORTANCE Globally, wetlands are one of the largest sources of methane (CH4), a greenhouse gas with a warming impact significantly greater than CO2. Methane produced in wetlands is the byproduct of a group of microorganisms which convert organic carbon into CH4. Despite our knowledge of how this process works, it is still unclear what drives dramatic, localized (<10 m) variance in emission rates from the surface of wetlands. While environmental conditions, like soil temperature or water table depth, correlate with methane flux when variance in these factors is large (e.g., spring vs fall), the explanatory power of these variables decline when spatial and temporal scales become smaller. As methane fluxes are the direct product of microbial activity, we profiled how the microbial community varied, both horizontally and vertically, across a peat bog in Maine, USA, finding that variance in microbial communities was likely contributing to much of the observed variance in flux.

Spatio-Temporal Variability of Methane Fluxes in Boreo-Nemoral Alder Swamp (European Russia)

In 1995–1998 and 2013–2016, we measured methane fluxes (1Q-median-3Q, mgC m−2 h−1) in the Petushikha black alder swamp of the boreo-nemoral zone of European Russia. At microelevations (EL sites), flat surfaces (FL), microdepressions (DEP), and water surfaces of streams and channels (STR) sites, the fluxes comprised 0.01–0.03–0.09, 0.02–0.06–0.19, 0.04–0.14–0.43, and 0.10–0.21–0.44, respectively. The biggest uncertainty of methane fluxes was caused by seasonal variability (the level of relative variability of fluxes is a nonparametric analogue of the coefficient of variation) which comprised 144%, then by spatial variability—105%, and the smallest by interannual variability—75%. Both spatial and temporal variability of methane fluxes at different elements of the microrelief is heterogeneous: the most variable are communities that are “unstable” in terms of hydrological conditions, such as FL and DEP, and the least variable are the most drained EL and the most moistened STR (“stable” in terms of hydrological conditions). The obtained data on the fluxes and their spatial and temporal variability are consistent with the literature data and can be used to optimize the process of planning studies of the methane budget of “sporadic methane sources”, such as waterlogged forests. This is especially relevant for an adequate assessment of the role of methane fluxes in the formation of the waterlogged forests carbon budget and a changing climate.

Spatial and temporal heterogeneity of methane ebullition in lowland headwater streams and the impact on sampling design

Headwater streams are known sources of methane (CH₄) to the atmosphere, but their contribution to global scale budgets remains poorly constrained. While efforts have been made to better understand diffusive fluxes of CH₄ in streams, much less attention has been paid to ebullitive fluxes. We examine the temporal and spatial heterogeneity of CH₄ ebullition from four lowland headwater streams in the temperate northeastern United States over a 2-yr period. Ebullition was observed in all monitored streams with an overall mean rate of 1.00 ± 0.23 mmol CH₄ m^-2 d^-1, ranging from 0.01 to 1.79 to mmol CH₄ m^-2 d^-1 across streams. At biweekly timescales, rates of ebullition tended to increase with temperature. We observed a high degree of spatial heterogeneity in CH₄ ebullition within and across streams. Yet, catchment land use was not a simple predictor of this heterogeneity, and instead patches scale variability weakly explained by water depth and sediment organic matter content and quality. Overall, our results support the prevalence of CH₄ ebullition from streams and high levels of variability characteristic of this process. Our findings also highlight the need for robust temporal and spatial sampling of ebullition in lotic ecosystems to account for this high level of heterogeneity, where multiple sampling locations and times are necessary to accurately represent the mean rate of flux in a stream. The heterogeneity observed likely indicates a complex set of drivers affect CH₄ ebullition from streams which must be considered when upscaling site measurements to larger spatial scales.

Methane production and oxidation potentials along a fen-bog gradient from southern boreal to subarctic peatlands in Finland

Methane (CH₄ ) emissions from northern peatlands are projected to increase due to climate change, primarily because of projected increases in soil temperature. Yet, the rates and temperature responses of the two CH₄ emission-related microbial processes (CH₄ production by methanogens and oxidation by methanotrophs) are poorly known. Further, peatland sites within a fen-bog gradient are known to differ in the variables that regulate these two mechanisms, yet the interaction between peatland type and temperature lacks quantitative understanding. Here, we investigated potential CH₄ production and oxidation rates for 14 peatlands in Finland located between c. 60 and 70°N latitude, representing bogs, poor fens, and rich fens. Potentials were measured at three different temperatures (5, 17.5, and 30℃) using the laboratory incubation method. We linked CH₄ production and oxidation patterns to their methanogen and methanotroph abundance, peat properties, and plant functional types. We found that the rich fen-bog gradient-related nutrient availability and methanogen abundance increased the temperature response of CH₄ production, with rich fens exhibiting the greatest production potentials. Oxidation potential showed a steeper temperature response than production, which was explained by aerenchymous plant cover, peat water holding capacity, peat nitrogen, and sulfate content. The steeper temperature response of oxidation suggests that, at higher temperatures, CH₄ oxidation might balance increased CH₄ production. Predicting net CH₄  fluxes as an outcome of the two mechanisms is complicated due to their different controls and temperature responses. The lack of correlation between field CH₄  fluxes and production/oxidation potentials, and the positive correlation with aerenchymous plants points toward the essential role of CH₄ transport for emissions. The scenario of drying peatlands under climate change, which is likely to promote Sphagnum establishment over brown mosses in many places, will potentially reduce the predicted warming-related increase in CH₄ emissions by shifting rich fens to Sphagnum-dominated systems.

Wetland Heterogeneity Determines Methane Emissions: A Pan-Arctic Synthesis

Methane (CH₄) emissions from pan-Arctic wetlands provide a potential positive feedback to global warming. However, the differences in CH₄ emissions across wetland types in these regions have not been well understood. We synthesized approximately 9000 static chamber CH₄ measurements during the growing season from 83 sites across pan-Arctic regions. We highlighted spatial variations of CH₄ emissions corresponding to environmental heterogeneity across wetland types. CH₄ emission is the highest in fens, followed by marshes, bogs, and the lowest in swamps. This gradient is controlled by the water table, soil temperature, and dominant plant functional types and their interactions. The water table position for maximum CH₄ emission is below, close to, and above the ground surface in bogs, marshes/fens, and swamps, respectively. The temperature sensitivity (Q₁₀) of CH₄ emissions varied among different wetland types, ranging from the lowest in swamps to the highest in fens. The interactive impact of temperature and the water table positions on CH₄ emissions are regulated with dominant plant functional types. CH₄ emissions from wetlands dominated by vascular plants rely more on species composition than that dominated by non-vascular plants. Wetlands with greater abundance of graminoids (e.g., fens) have higher CH₄ emissions than tree-dominated wetlands (e.g., swamps). This synthesis emphasizes the role of wetland heterogeneity in determining the strength of CH₄ emissions.

Methane dynamics and thermal response in impoundments of the Rhine River, Germany

River impoundments have been identified as important emission hot spots of the greenhouse gas methane. In this study, we investigated methane dynamics of five river impoundments within a two year period using a variety of methods ranging from sediment incubations for measuring methane formation rates (MF), automatic bubble-traps and echo-sounding surveys to assess ebullition fluxes, and estimated diffusive methane fluxes via dissolved concentrations in the water and calculated transport coefficients via wind speed. MF was found to be predominantly acetoclastic, and higher porewater acetate concentrations were associated with higher MF. Moreover, sediment MF showed consistent depth profiles, and when depth-integrated, MF was comparable to bubble-trap ebullition time-series measurements. Thermal response analysed for our systems and a wide range of literature data demonstrated a consistent mean value, but a large range of temperature coefficient Q₁₀ (1.6 to 7.0) for different studies. Annual mean ebullition rates varied over more than one order of magnitude from site to site (0.03 to 1.85 mgCH₄ l^-1 d^-1), demonstrating that river impoundments are not all hot-spots. Future work should investigate the role of sediment delivery, deposition patterns and management on methane emissions by ebullition.

Multi-year net ecosystem carbon balance of a restored peatland reveals a return to carbon sink

Peatlands after drainage and extraction are large sources of carbon (C) to the atmosphere. Restoration, through re-wetting and revegetation, aims to return the C sink function by re-establishing conditions similar to that of an undrained peatland. However, the time needed to re-establish C sequestration is not well constrained due to the lack of multi-year measurements. We measured over 3 years the net ecosystem exchange of CO₂ (NEE), methane ( F CH 4 ), and dissolved organic carbon (DOC) at a restored post-extraction peatland (RES) in southeast Canada (restored 14 years prior to the start of the study) and compared our observations to the C balance of an intact reference peatland (REF) that has a long-term continuous flux record and is in the same climate zone. Small but significant differences in winter respiration driven by temperature were mainly responsible for differences in cumulative NEE between years. Low growing season inter-annual variability was linked to constancy of the initial spring water table position, controlled by the blocked drainage ditches and the presence of water storage structures (bunds and pools). Half-hour F CH 4 at RES was small except when Typha latifolia-invaded drainage ditches were in the tower footprint; this effect at the ecosystem level was small as ditches represent a minor fraction of RES. The restored peatland was an annual sink for CO₂ (-90 ± 18 g C m^-2  year^-1 ), a source of CH₄ (4.4 ± 0.2 g C m^-2  year^-1 ), and a source of DOC (6.9 ± 2.2 g C m^-2  year^-1 ), resulting in mean net ecosystem uptake of 78 ± 17 g C m^-2  year^-1 . Annual NEE at RES was most similar to wetter, more productive years at REF. Integrating structures to increase water retention, alongside re-establishing key species, have been effective at re-establishing the net C sink rate to that of an intact peatland.

Nongrowing season methane emissions-a significant component of annual emissions across northern ecosystems

Wetlands are the single largest natural source of atmospheric methane (CH₄ ), a greenhouse gas, and occur extensively in the northern hemisphere. Large discrepancies remain between "bottom-up" and "top-down" estimates of northern CH₄ emissions. To explore whether these discrepancies are due to poor representation of nongrowing season CH₄ emissions, we synthesized nongrowing season and annual CH₄ flux measurements from temperate, boreal, and tundra wetlands and uplands. Median nongrowing season wetland emissions ranged from 0.9 g/m² in bogs to 5.2 g/m² in marshes and were dependent on moisture, vegetation, and permafrost. Annual wetland emissions ranged from 0.9 g m^-2  year^-1 in tundra bogs to 78 g m^-2  year^-1 in temperate marshes. Uplands varied from CH₄ sinks to CH₄ sources with a median annual flux of 0.0 ± 0.2 g m^-2  year^-1 . The measured fraction of annual CH₄ emissions during the nongrowing season (observed: 13% to 47%) was significantly larger than that was predicted by two process-based model ensembles, especially between 40° and 60°N (modeled: 4% to 17%). Constraining the model ensembles with the measured nongrowing fraction increased total nongrowing season and annual CH₄ emissions. Using this constraint, the modeled nongrowing season wetland CH₄ flux from >40° north was 6.1 ± 1.5 Tg/year, three times greater than the nongrowing season emissions of the unconstrained model ensemble. The annual wetland CH₄ flux was 37 ± 7 Tg/year from the data-constrained model ensemble, 25% larger than the unconstrained ensemble. Considering nongrowing season processes is critical for accurately estimating CH₄ emissions from high-latitude ecosystems, and necessary for constraining the role of wetland emissions in a warming climate.

Near-zero methane emission from an abandoned boreal peatland pasture based on eddy covariance measurements

Although estimates of the annual methane (CH₄) flux from agriculturally managed peatlands exist, knowledge of controls over the variation of CH₄ at different time-scales is limited due to the lack of high temporal-resolution data. Here we present CH₄ fluxes measured from May 2014 to April 2016 using the eddy covariance technique at an abandoned peatland pasture in western Newfoundland, Canada. The goals of the study were to identify the controls on the seasonal variations in CH₄ flux and to quantify the annual CH₄ flux. The seasonal variation in daily CH₄ flux was not strong in the two study years, however a few periods of pronounced emissions occurred in the late growing season. The daily average CH₄ flux was small relative to other studies, ranging from -4.1 to 9.9 nmol m^-2 s^-1 in 2014–15 and from -7.1 to 12.1 nmol m^-2 s^-1 in 2015–16. Stepwise multiple regression was used to investigate controls on CH₄ flux and this analysis found shifting controls on CH₄ flux at different periods of the growing season. During the early growing season CH₄ flux was closely related to carbon dioxide fixation rates, suggesting substrate availability was the main control. The peak growing season CH₄ flux was principally controlled by the CH₄ oxidation in 2014, where the CH₄ flux decreased and increased with soil temperature at 50 cm and soil water content at 10 cm, but a contrasting temperature-CH₄ relation was found in 2015. The late growing season CH₄ flux was found to be regulated by the variation in water table level and air temperature in 2014. The annual CH₄ emission was near zero in both study years (0.36 ± 0.30 g CH₄ m^-2 yr^-1 in 2014–15 and 0.13 ± 0.38 g CH₄ m^-2 yr^-1 in 2015–16), but fell within the range of CH₄ emissions reported for agriculturally managed peatlands elsewhere.

The positive net radiative greenhouse gas forcing of increasing methane emissions from a thawing boreal forest-wetland landscape

At the southern margin of permafrost in North America, climate change causes widespread permafrost thaw. In boreal lowlands, thawing forested permafrost peat plateaus ('forest') lead to expansion of permafrost-free wetlands ('wetland'). Expanding wetland area with saturated and warmer organic soils is expected to increase landscape methane (CH₄ ) emissions. Here, we quantify the thaw-induced increase in CH₄ emissions for a boreal forest-wetland landscape in the southern Taiga Plains, Canada, and evaluate its impact on net radiative forcing relative to potential long-term net carbon dioxide (CO₂ ) exchange. Using nested wetland and landscape eddy covariance net CH₄ flux measurements in combination with flux footprint modeling, we find that landscape CH₄ emissions increase with increasing wetland-to-forest ratio. Landscape CH₄ emissions are most sensitive to this ratio during peak emission periods, when wetland soils are up to 10 °C warmer than forest soils. The cumulative growing season (May-October) wetland CH₄ emission of ~13 g CH₄  m^-2 is the dominating contribution to the landscape CH₄ emission of ~7 g CH₄  m^-2 . In contrast, forest contributions to landscape CH₄ emissions appear to be negligible. The rapid wetland expansion of 0.26 ± 0.05% yr^-1 in this region causes an estimated growing season increase of 0.034 ± 0.007 g CH₄  m^-2  yr^-1 in landscape CH₄ emissions. A long-term net CO₂ uptake of >200 g CO₂  m^-2  yr^-1 is required to offset the positive radiative forcing of increasing CH₄ emissions until the end of the 21st century as indicated by an atmospheric CH₄ and CO₂ concentration model. However, long-term apparent carbon accumulation rates in similar boreal forest-wetland landscapes and eddy covariance landscape net CO₂ flux measurements suggest a long-term net CO₂ uptake between 49 and 157 g CO₂  m^-2  yr^-1 . Thus, thaw-induced CH₄ emission increases likely exert a positive net radiative greenhouse gas forcing through the 21st century.

Emissions of methane from northern peatlands: a review of management impacts and implications for future management options

Northern peatlands constitute a significant source of atmospheric methane (CH₄). However, management of undisturbed peatlands, as well as the restoration of disturbed peatlands, will alter the exchange of CH₄ with the atmosphere. The aim of this systematic review and meta‐analysis was to collate and analyze published studies to improve our understanding of the factors that control CH₄ emissions and the impacts of management on the gas flux from northern (latitude 40° to 70°N) peatlands. The analysis includes a total of 87 studies reporting measurements of CH₄ emissions taken at 186 sites covering different countries, peatland types, and management systems. Results show that CH₄ emissions from natural northern peatlands are highly variable with a 95% CI of 7.6–15.7 g C m⁻² year⁻¹ for the mean and 3.3–6.3 g C m⁻² year⁻¹ for the median. The overall annual average (mean ± SD) is 12 ± 21 g C m⁻² year⁻¹ with the highest emissions from fen ecosystems. Methane emissions from natural peatlands are mainly controlled by water table (WT) depth, plant community composition, and soil pH. Although mean annual air temperature is not a good predictor of CH₄ emissions by itself, the interaction between temperature, plant community cover, WT depth, and soil pH is important. According to short‐term forecasts of climate change, these complex interactions will be the main determinant of CH₄ emissions from northern peatlands. Drainage significantly (p < .05) reduces CH₄ emissions to the atmosphere, on average by 84%. Restoration of drained peatlands by rewetting or vegetation/rewetting increases CH₄ emissions on average by 46% compared to the original premanagement CH₄ fluxes. However, to fully evaluate the net effect of management practice on the greenhouse gas balance from high latitude peatlands, both net ecosystem exchange (NEE) and carbon exports need to be considered.

Field Test of a Remote Multi-Path CLaDS Methane Sensor

Existing technologies for quantifying methane emissions are often limited to single point sensors, making large area environmental observations challenging. We demonstrate the operation of a remote, multi-path system using Chirped Laser Dispersion Spectroscopy (CLaDS) for quantification of atmospheric methane concentrations over extended areas, a technology that shows potential for monitoring emissions from wetlands.

Variations in Temperature Sensitivity (Q10) of CH4 Emission from a Subtropical Estuarine Marsh in Southeast China

Understanding the functional relationship between greenhouse gas fluxes and environmental variables is crucial for predicting the impacts of wetlands on future climate change in response to various perturbations. We examined the relationships between methane (CH₄) emission and temperature in two marsh stands dominated by the Phragmites australis and Cyperus malaccensis, respectively, in a subtropical estuarine wetland in southeast China based on three years of measurement data (2007–2009). We found that the Q₁₀ coefficient of CH₄ emission to soil temperature (Q_s10) from the two marsh stands varied slightly over the three years (P > 0.05), with a mean value of 3.38 ± 0.46 and 3.89 ± 0.41 for the P. australis and C. malaccensis stands, respectively. On the other hand, the three-year mean Q_a10 values (Q₁₀ coefficients of CH₄ emission to air temperature) were 3.39 ± 0.59 and 4.68 ± 1.10 for the P. australis and C. malaccensis stands, respectively, with a significantly higher Q_a10 value for the C. malaccensis stand in 2008 (P < 0.05). The seasonal variations of Q₁₀ (Q_s10 and Q_a10) differed among years, with generally higher values in the cold months than those in the warm months in 2007 and 2009. We found that the Q_s10 values of both stands were negatively correlated with soil conductivity, but did not obtain any conclusive results about the difference in Q₁₀ of CH₄ emission between the two tidal stages (before flooding and after ebbing). There were no significant differences in both Q_s10 and Q_a10 values of CH₄ emission between the P. australis stand and the C. malaccensis stands (P > 0.05). Our results show that the Q₁₀ values of CH₄ emission in this estuarine marsh are highly variable across space and time. Given that the overall CH₄ flux is governed by a suite of environmental factors, the Q₁₀ values derived from field measurements should only be considered as a semi-empirical parameter for simulating CH₄ emissions.

A synthesis of methane emissions from 71 northern, temperate, and subtropical wetlands

Wetlands are the largest natural source of atmospheric methane. Here, we assess controls on methane flux using a database of approximately 19 000 instantaneous measurements from 71 wetland sites located across subtropical, temperate, and northern high latitude regions. Our analyses confirm general controls on wetland methane emissions from soil temperature, water table, and vegetation, but also show that these relationships are modified depending on wetland type (bog, fen, or swamp), region (subarctic to temperate), and disturbance. Fen methane flux was more sensitive to vegetation and less sensitive to temperature than bog or swamp fluxes. The optimal water table for methane flux was consistently below the peat surface in bogs, close to the peat surface in poor fens, and above the peat surface in rich fens. However, the largest flux in bogs occurred when dry 30-day averaged antecedent conditions were followed by wet conditions, while in fens and swamps, the largest flux occurred when both 30-day averaged antecedent and current conditions were wet. Drained wetlands exhibited distinct characteristics, e.g. the absence of large flux following wet and warm conditions, suggesting that the same functional relationships between methane flux and environmental conditions cannot be used across pristine and disturbed wetlands. Together, our results suggest that water table and temperature are dominant controls on methane flux in pristine bogs and swamps, while other processes, such as vascular transport in pristine fens, have the potential to partially override the effect of these controls in other wetland types. Because wetland types vary in methane emissions and have distinct controls, these ecosystems need to be considered separately to yield reliable estimates of global wetland methane release.

Diversity and community structure of Archaea inhabiting the rhizoplane of two contrasting plants from an acidic bog

Plant root exudates increase nutrient availability and influence microbial communities including archaeal members. We examined the archaeal community inhabiting the rhizoplane of two contrasting vascular plants, Dulichium arundinaceum and Sarracenia purpurea, from an acidic bog in upstate NY. Multiple archaeal 16S rRNA gene libraries showed that methanogenic Archaea were dominant in the rhizoplane of both plants. In addition, the community structure (evenness) of the rhizoplane was found markedly different from the bulk peat. The archaeal community in peat from the same site has been found dominated by the E2 group, meanwhile the rhizoplane communities on both plants were co-dominated by Methanosarcinaceae (MS), rice cluster (RC)-I, and E2. Complementary T-RFLP analysis confirmed the difference between bulk peat and rhizoplane, and further characterized the dominance pattern of MS, RC-I, and E2. In the rhizoplane, MS was dominant on both plants although as a less variable fraction in S. purpurea. RC-I was significantly more abundant than E2 on S. purpurea, while the opposite was observed on D. arundinaceum, suggesting a plant-specific enrichment. Also, the statistical analyses of T-RFLP data showed that although both plants overlap in their community structure, factors such as plant type, patch location, and time could explain nearly a third of the variability in the dataset. Other factors such as water table, plant replicate, and root depth had a low contribution to the observed variance. The results of this study illustrate the general effects of roots and the specific effects of plant types on their nearby archaeal communities which in bog-inhabiting plants were mainly composed by methanogenic groups.

Rapid deglacial and early Holocene expansion of peatlands in Alaska

Northern peatlands represent one of the largest biospheric carbon (C) reservoirs; however, the role of peatlands in the global carbon cycle remains intensely debated, owing in part to the paucity of detailed regional datasets and the complexity of the role of climate, ecosystem processes, and environmental factors in controlling peatland C dynamics. Here we used detailed C accumulation data from four peatlands and a compilation of peatland initiation ages across Alaska to examine Holocene peatland dynamics and climate sensitivity. We find that 75% of dated peatlands in Alaska initiated before 8,600 years ago and that early Holocene C accumulation rates were four times higher than the rest of the Holocene. Similar rapid peatland expansion occurred in West Siberia during the Holocene thermal maximum (HTM). Our results suggest that high summer temperature and strong seasonality during the HTM in Alaska might have played a major role in causing the highest rates of C accumulation and peatland expansion. The rapid peatland expansion and C accumulation in these vast regions contributed significantly to the peak of atmospheric methane concentrations in the early Holocene. Furthermore, we find that Alaskan peatlands began expanding much earlier than peatlands in other regions, indicating an important contribution of these peatlands to the pre-Holocene increase in atmospheric methane concentrations.

Recent peat accumulation rates in minerotrophic peatlands of the Bay James region, Eastern Canada, inferred by 210Pb and 137Cs radiometric techniques

(210)Pb and (137)Cs dating techniques are used to characterise recent peat accumulation rates of two minerotrophic peatlands located in the La Grande Rivière hydrological watershed, in the James Bay region (Canada). Several cores were collected during the summer 2005 in different parts of the two selected peatlands. These minerotrophic patterned peatlands are presently affected by erosion processes, expressed by progressive mechanical destruction of their pools borders. This erosion process is related to a water table rise induced by a regional increase of humidity since the last century. The main objective of the present paper is to (1) evaluate if (210)Pb and (137)Cs dating techniques can be applied to build accurate chronologies in these environments and (2) detect changes in the peat accumulation rates in regard to this amplification of humidity. In both sites, unsupported (210)Pb shows an exponential decreasing according to the depth. Chronologies inferred from (210)Pb allow to reconstruct peat accumulation rates since ca. 1855 AD. The (137)Cs data displayed evident mobility and diffusion, preventing the establishment of any sustained chronology based on these measurements. In the two sites, peat accumulation rates inferred from (210)Pb chronologies fluctuate between 0.005 and 0.038 g cm(-2) yr(-1). As a result, the rise of the water table during the last decade has not yet affected peat accumulation rates.