60 research outputs found
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Respiration of aged soil carbon during fall in permafrost peatlands enhanced by active layer deepening following wildfire but limited following thermokarst
Permafrost peatlands store globally significant amounts of soil organic carbon (SOC) that may be vulnerable to climate change. Permafrost thaw exposes deeper, older SOC to microbial activity, but SOC vulnerability to mineralization and release as carbon dioxide is likely influenced by the soil environmental conditions that follow thaw. Permafrost thaw in peat plateaus, the dominant type of permafrost peatlands in North America, occurs both through deepening of the active layer and through thermokarst. Active layer deepening exposes aged SOC to predominately oxic conditions, while thermokarst is associated with complete permafrost thaw which leads to ground subsidence, inundation and soil anoxic conditions. Thermokarst often follows active layer deepening, and wildfire is an important trigger of this sequence. We compared the mineralization rate of aged SOC at an intact peat plateau (∼70 cm oxic active layer), a burned peat plateau (∼120 cm oxic active layer), and a thermokarst bog (∼550 cm anoxic peat profile) by measuring respired 14C-CO2. Measurements were done in fall when surface temperatures were near-freezing while deeper soil temperatures were still close to their seasonal maxima. Aged SOC (1600 yrs BP) contributed 22.1 ± 11.3% and 3.5 ± 3.1% to soil respiration in the burned and intact peat plateau, respectively, indicating a fivefold higher rate of aged SOC mineralization in the burned than intact peat plateau (0.15 ± 0.07 versus 0.03 ± 0.03 g CO2-C m−2 d−1). None or minimal contribution of aged SOC to soil respiration was detected within the thermokarst bog, regardless of whether thaw had occurred decades or centuries ago. While more data from other sites and seasons are required, our study provides strong evidence of substantially increased respiration of aged SOC from burned peat plateaus with deepened active layer, while also suggesting inhibition of aged SOC respiration under anoxic conditions in thermokarst bogs
Energy input is primary controller of methane bubbling in subarctic lakes
Emission of methane (CH4) from surface waters is often dominated by ebullition (bubbling), a transport mode with high‐spatiotemporal variability. Based on new and extensive CH4 ebullition data, we demonstrate striking correlations (r2 between 0.92 and 0.997) when comparing seasonal bubble CH4 flux from three shallow subarctic lakes to four readily measurable proxies of incoming energy flux and daily flux magnitudes to surface sediment temperature (r2 between 0.86 and 0.94). Our results after continuous multiyear sampling suggest that CH4 ebullition is a predictable process, and that heat flux into the lakes is the dominant driver of gas production and release. Future changes in the energy received by lakes and ponds due to shorter ice‐covered seasons will predictably alter the ebullitive CH4 flux from freshwater systems across northern landscapes. This finding is critical for our understanding of the dynamics of radiatively important trace gas sources and associated climate feedback
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Aged soils contribute little to contemporary carbon cycling downstream of thawing permafrost peatlands
Funder: Department for Business, Energy and Industrial Strategy, UK Government; Id: http://dx.doi.org/10.13039/100011693Abstract: Vast stores of millennial‐aged soil carbon (MSC) in permafrost peatlands risk leaching into the contemporary carbon cycle after thaw caused by climate warming or increased wildfire activity. Here we tracked the export and downstream fate of MSC from two peatland‐dominated catchments in subarctic Canada, one of which was recently affected by wildlife. We tested whether thermokarst bog expansion and deepening of seasonally thawed soils due to wildfire increased the contributions of MSC to downstream waters. Despite being available for lateral transport, MSC accounted for ≤6% of dissolved organic carbon (DOC) pools at catchment outlets. Assimilation of MSC into the aquatic food web could not explain its absence at the outlets. Using δ13C‐Δ14C‐δ15N‐δ2H measurements, we estimated only 7% of consumer biomass came from MSC by direct assimilation and algal recycling of heterotrophic respiration. Recent wildfire that caused seasonally thawed soils to reach twice as deep in one catchment did not change these results. In contrast to many other Arctic ecosystems undergoing climate warming, we suggest waterlogged peatlands will protect against downstream delivery and transformation of MSC after climate‐ and wildfire‐induced permafrost thaw
Opposing Effects of Climate and Permafrost Thaw on CH4 and CO2 Emissions From Northern Lakes
Funder: Natural Sciences and Engineering Research CouncilFunder: Northern Scientific Training Program, University of AlbertaFunder: UAlberta North, Vanier Canada Graduate ScholarshipW. Garfield Weston FoundationAbstract: Small, organic‐rich lakes are important sources of methane (CH4) and carbon dioxide (CO2) to the atmosphere, yet the sensitivity of emissions to climate warming is poorly constrained and potentially influenced by permafrost thaw. Here, we monitored emissions from 20 peatland lakes across a 1,600 km permafrost transect in boreal western Canada. Contrary to expectations, we observed a shift from source to sink of CO2 for lakes warmer regions, driven by greater primary productivity associated with greater hydrological connectivity to lakes and nutrient availability in the absence of permafrost. Conversely, an 8‐fold increase in CH4 emissions in warmer regions was associated with water temperature and shifts in microbial communities and dominant anaerobic processes. Our results suggest that the net radiative forcing from altered greenhouse gas emissions of northern peatland lakes this century will be dominated by increasing CH4 emissions and only partially offset by reduced CO2 emissions
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Permafrost and Climate Change: Carbon Cycle Feedbacks From the Warming Arctic
Rapid Arctic environmental change affects the entire Earth system as thawing permafrost ecosystems release greenhouse gases to the atmosphere. Understanding how much permafrost carbon will be released, over what time frame, and what the relative emissions of carbon dioxide and methane will be is key for understanding the impact on global climate. In addition, the response of vegetation in a warming climate has the potential to offset at least some of the accelerating feedback to the climate from permafrost carbon. Temperature, organic carbon, and ground ice are key regulators for determining the impact of permafrost ecosystems on the global carbon cycle. Together, these encompass services of permafrost relevant to global society as well as to the people living in the region and help to determine the landscape-level response of this region to a changing climate
Biomass offsets little or none of permafrost carbon release from soils, streams, and wildfire : an expert assessment
As the permafrost region warms, its large organic carbon pool will be increasingly vulnerable to decomposition, combustion, and hydrologic export. Models predict that some portion of this release will be offset by increased production of Arctic and boreal biomass; however, the lack of robust estimates of net carbon balance increases the risk of further overshooting international emissions targets. Precise empirical or model-based assessments of the critical factors driving carbon balance are unlikely in the near future, so to address this gap, we present estimates from 98 permafrost-region experts of the response of biomass, wildfire, and hydrologic carbon flux to climate change. Results suggest that contrary to model projections, total permafrost-region biomass could decrease due to water stress and disturbance, factors that are not adequately incorporated in current models. Assessments indicate that end-of-the-century organic carbon release from Arctic rivers and collapsing coastlines could increase by 75% while carbon loss via burning could increase four-fold. Experts identified water balance, shifts in vegetation community, and permafrost degradation as the key sources of uncertainty in predicting future system response. In combination with previous findings, results suggest the permafrost region will become a carbon source to the atmosphere by 2100 regardless of warming scenario but that 65%-85% of permafrost carbon release can still be avoided if human emissions are actively reduced.Peer reviewe
Quantity and composition of waterborne carbon transport in subarctic catchments containing peatlands and permafrost
Both quantity and composition of waterborne transport of dissolved organic carbon (DOC) from peatlands has been hypothesized to be affected by permafrost thaw. Changes in DOC can impact the carbon (C) balance of peatlands directly, but also the carbon balances and metabolism of downstream aquatic environments. In this study I have investigated the DOC export from different peatland types in the Stordalen catchment of northern Sweden (68.20N, 19.03E). The research was performed at various spatial and temporal scales in order to assess the importance of peatland permafrost thaw for both peatland and catchment DOC export. In the Stordalen catchment, peatland permafrost thaw leads to the conversion of palsas (a rain-fed peatland type with a permafrost core) into bogs dominated by Sphagnum mosses or into fens of varying nutrient status depending on their hydrological setting. The palsas were found to have low DOC export rates, at between 2.5 and 3.5 g C m-2 yr-1, and its composition characterized by several bulk DOC indices was of poor substrate quality for microbial degradation. The DOC export from the bogs was not found to differ in quantity to that of the palsas, but its DOC composition had lower aromaticity and a larger fraction derived from microbial sources. Snowmelt runoff occurred early on the palsa and bog, and mass flux at this time of years was responsible for >70% of the annual DOC export – causing the palsas and bogs to significantly affect catchment DOC export patterns during early snowmelt runoff, despite covering <4% of the catchments. Partly due to the restricted DOC export, the Stordalen palsa/bog complex was found to be net annual C sinks despite its low ecosystem productivity. In contrast, the fens were found to export 2 to 4 times as much DOC as the palsas and bogs, with a majority of the DOC export occurring outside the snowmelt runoff period. Several pieces of evidence show that fens not only act as catchment sources of DOC, but through selective degradation they transform DOC that reach the fens from upstream sources, causing the fens to regulate catchment DOC composition and concentrations during summer and fall low runoff periods despite covering only a small fraction of the catchment area. The impact on peatland DOC export from permafrost thaw is primarily dependent on the hydrological setting after thaw in Stordalen, where a conversion of palsas into fens has the greatest potential for the alteration of catchment DOC export patterns.L'hypothèse a été émise que la quantité et la composition du carbone organique dissous (COD) des tourbières, transporté par l'eau, est affecté par le dégel du pergélisol. Les changements de COD peuvent influer sur l'équilibre du carbone (C) directement, mais aussi les bilans de C et le métabolisme des milieux aquatiques en aval. Dans cette étude, j'ai étudié l'exportation de COD provenant de différents types de tourbières dans le bassin versant Stordalen au nord de la Suède (68.20N, 19.03E). La recherche a été réalisée à différentes échelles spatiales et temporelles afin d'évaluer l'importance du dégel du pergélisol des tourbières sur la tourbière elle-même et sur l'exportation du COD dans le bassin versant. Dans le bassin versant Stordalen, le dégel du pergélisol des tourbières a conduit à la conversion des palses (un type de tourbière pluviale avec un noyau de pergélisol) vers des tourbières dominées par les sphaignes ou en tourbières de carex avec des statuts d'éléments nutritifs variés en fonction de leur contexte hydrologique. Les palses se sont avérés avoir un faible taux d'exportation de COD, entre 2,5 et 3,5 gC m-2 an-1, et sa composition se caractérise par plusieurs indices de COD qui étaient de mauvaise qualité de substrat pour la dégradation microbienne. L'exportation du COD dans les tourbières n'a pas montré de différences quant à la quantité par rapport à celles de la palse, mais sa composition avait une plus faible aromaticité et une fraction plus importante provenant de sources microbiennes. La fonte des neiges s'est produite tôt dans la palse et les tourbières, et le flux de COD à ce moment de l'année a été responsable de plus de 70% de l'exportation annuelle de COD – provoquant la palse et les tourbières de modifier sensiblement l'exportation du COD du bassin versant malgré que ces dernières ne couvrent que moins de 4% du bassin versant. En raison notamment de l'exportation restreinte de COD, le complexe palse/bassin versant Stordalen a été déterminé être un puit net annuel de C en dépit de sa faible productivité de son écosystème. En revanche, les tourbières à carex on montré une exportation de 2 à 4 fois plus élevée que le COD des palse et des tourbières à sphaigne, avec une majorité de l'exportation du COD survenant en dehors de la période de fonte des neiges. Plusieurs preuves ont montrés que les tourbières à carex n'agissaient pas uniquement en tant que source de captage de COD, mais par la dégradation sélective, elles transforment le COD qui atteint les tourbières à carex à partir de sources en amont, ce qui provoque les tourbières à carex de modifier la composition du COD et les concentrations du bassin versant, en été et en automne, période de faible hydraulicité, malgré que ces dernières ne couvrent qu'une petite fraction du bassin versant. L'impact de l'exportation de COD des tourbières durant le dégel du pergélisol a été montré dépendant des conditions hydrologiques après le dégel, durant laquelle une conversion en tourbières à carex montre un plus grand potentiel pour la modification des patrons de transport de COD
Total waterborne carbon export and DOC composition from ten nested subarctic peatland catchments—importance of peatland cover, groundwater influence, and inter-annual variability of precipitation patterns
Waterborne carbon (C) export from terrestrial ecosystems is a potentially important flux for the net catchment C balance and links the biogeochemical C cycling of terrestrial ecosystems to their downstream aquatic ecosystems. We have monitored hydrology and stream chemistry over 3 years in ten nested catchments (0.6–15.1 km2) with variable peatland cover (0%–22%) and groundwater influence in subarctic Sweden. Total waterborne C export, including dissolved and particulate organic carbon (DOC and POC) and dissolved inorganic carbon (DIC), ranged between 2.8 and 7.3 g m–2 year–1, representing ~10%–30% of catchment net ecosystem exchange of CO2. Several characteristics of catchment waterborne C export were affected by interacting effects of peatland cover and groundwater influence, including magnitude and timing, partitioning into DOC, POC, and DIC and chemical composition of the exported DOC. Waterborne C export was greater during the wetter years, equivalent to an average change in export of ~2 g m–2 year–1 per 100 mm of precipitation. Wetter years led to a greater relative increase in DIC export than DOC export due to an inferred relative shift in dominance from shallow organic flow pathways to groundwater sources. Indices of DOC composition (SUVA254 and a250/a365) indicated that DOC aromaticity and average molecular weight increased with catchment peatland cover and decreased with increased groundwater influence. Our results provide examples on how waterborne C export and DOC composition might be affected by climate change. Copyright © 2012 John Wiley & Sons, Ltd
Lability of dissolved organic carbon from boreal peatlands: interactions between permafrost thaw, wildfire, and season
Boreal peatlands are major sources of dissolved organic carbon (DOC) to downstream aquatic ecosystems, where it influences carbon cycling and food web structure. Wildfire and permafrost thaw alter peatland vegetation and hydrology and may affect the quantity and chemical composition of exported DOC. We studied the influence of wildfire and thaw on microbial and photochemical lability of near-surface porewater DOC, assessed through 7 d incubations. We carried out these incubations in spring, summer, and fall but only found differences in spring when DOC biodegradability (% loss during dark incubations) increased with lower DOC aromaticity and C/N ratios. During spring, the most labile DOC was found in recently formed thermokarst bogs along collapsing peat plateau edges (25% loss), which was greater than in mature sections of thermokarst bogs (3%), and peat plateaus with intact permafrost (9%). Increased DOC lability following thaw was likely linked to high DOC production and turnover associated with productive hydrophilic Sphagnum mosses and sedges, rather than thawed permafrost peat. A wildfire (3 yr prior) reduced DOC biodegradability in both peat plateaus (4%) and rapidly collapsing peat plateau edges (14%). Biodegradability of DOC in summer and fall was low across all sites; 2% and 4%, respectively. Photodegradation was shown to potentially contribute significantly to downstream DOC degradation but did not vary across peatland sites. We show that disturbances such as permafrost thaw and wildfire have the potential to affect downstream carbon cycling, particularly as the largest influences were found in spring when peatlands are well connected to downstream aquatic ecosystems.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author
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Climate warming has direct and indirect effects on microbes associated with carbon cycling in northern lakes
Northern lakes disproportionately influence the global carbon cycle, and may do so more in the future depending on how their microbial communities respond to climate warming. Microbial communities can change because of the direct effects of climate warming on their metabolism and the indirect effects of climate warming on groundwater connectivity from thawing of surrounding permafrost, especially at lower landscape positions. Here we used shotgun metagenomics to compare the taxonomic and functional gene composition of sediment microbes in 19 peatland lakes across a 1600-km permafrost transect in boreal western Canada. We found microbes responded differently to the loss of regional permafrost cover than to increases in local groundwater connectivity. These results suggest that both the direct and indirect effects of climate warming, which were respectively associated with loss of permafrost thaw and subsequent changes in groundwater connectivity interact to change microbial composition and function. Archaeal methanogens and genes involved in all major methanogenesis pathways were more abundant in warmer regions with less permafrost, but higher groundwater connectivity partly offset these effects. Bacterial community composition and methanotrophy genes did not vary with regional permafrost cover, and the latter changed similarly to methanogenesis with groundwater connectivity. Finally, we found an increase in sugar utilisation genes in regions with less permafrost, which may further fuel methanogenesis. These results provide the microbial mechanism for observed increases in methane emissions associated with loss of permafrost cover in this region and suggest that future emissions will primarily be controlled by archaeal methanogens over methanotrophic bacteria as northern lakes warm. Our study more generally suggests that future predictions of aquatic carbon cycling will be improved by considering how climate warming exerts both direct effects associated with regional-scale permafrost thaw and indirect effects associated with local hydrology
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