Environmental and vegetation controls on the spatial variability of CH4 emission from wet-sedge and tussock tundra ecosystems in the Arctic

Warming effects on the flux of CH4 from peatland mesocosms are regulated by plant species composition: Richness and functional types

Peatlands in northeast China are experiencing severe climate warming. Most studies on peatlands focus on the responses of CH₄ dynamics to temperature. However, they rarely consider the synchronous changes in the composition of plant communities caused by the expansion of vascular plants. In this study, an experiment combined warming with the manipulation of plants to examine the concentrations of CH₄ porewater and its fluxes in the mesocosm. We found that warming increased the concentration of CH₄ and its fluxes relative to the control treatments, and it was strongly modulated by plant richness and functional types. The average CH₄ fluxes in the warming and non-warming mesocosms varied from 72.10 to 119.44 and 97.95 to 194.43 mg m^-2 h^-1, respectively. Plant species richness significantly increased CH₄ flux at the warming level of 3.2 °C (P < 0.01). The presence of vascular plants, such as Carex globularis and Vaccinium uliginosum, significantly increased the CH₄ fluxes after warming had occurred. Our results suggest that the distinct response of CH₄ to richness and species primarily stemmed from the direct or indirect effects of plant biomass and functional characteristics. Therefore, more consideration should be given to the diversity changes caused by vascular plant expansion when estimating CH₄ flux in boreal peatland, especially in the context of future climate warming.

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.

Organohalide-Respiring Bacteria at the Heart of Anaerobic Metabolism in Arctic Wet Tundra Soils

Recent work revealed an active biological chlorine cycle in coastal Arctic tundra of northern Alaska. This raised the question of whether chlorine cycling was restricted to coastal areas or if these processes extended to inland tundra. The anaerobic process of organohalide respiration, carried out by specialized bacteria like Dehalococcoides, consumes hydrogen gas and acetate using halogenated organic compounds as terminal electron acceptors, potentially competing with methanogens that produce the greenhouse gas methane. We measured microbial community composition and soil chemistry along an ∼262-km coastal-inland transect to test for the potential of organohalide respiration across the Arctic Coastal Plain and studied the microbial community associated with Dehalococcoides to explore the ecology of this group and its potential to impact C cycling in the Arctic. Concentrations of brominated organic compounds declined sharply with distance from the coast, but the decrease in organic chlorine pools was more subtle. The relative abundances of Dehalococcoides were similar across the transect, except for being lower at the most inland site. Dehalococcoides correlated with other strictly anaerobic genera, plus some facultative ones, that had the genetic potential to provide essential resources (hydrogen, acetate, corrinoids, or organic chlorine). This community included iron reducers, sulfate reducers, syntrophic bacteria, acetogens, and methanogens, some of which might also compete with Dehalococcoides for hydrogen and acetate. Throughout the Arctic Coastal Plain, Dehalococcoides is associated with the dominant anaerobes that control fluxes of hydrogen, acetate, methane, and carbon dioxide. Depending on seasonal electron acceptor availability, organohalide-respiring bacteria could impact carbon cycling in Arctic wet tundra soils.IMPORTANCE Once considered relevant only in contaminated sites, it is now recognized that biological chlorine cycling is widespread in natural environments. However, linkages between chlorine cycling and other ecosystem processes are not well established. Species in the genus Dehalococcoides are highly specialized, using hydrogen, acetate, vitamin B₁₂-like compounds, and organic chlorine produced by the surrounding community. We studied which neighbors might produce these essential resources for Dehalococcoides species. We found that Dehalococcoides species are ubiquitous across the Arctic Coastal Plain and are closely associated with a network of microbes that produce or consume hydrogen or acetate, including the most abundant anaerobic bacteria and methanogenic archaea. We also found organic chlorine and microbes that can produce these compounds throughout the study area. Therefore, Dehalococcoides could control the balance between carbon dioxide and methane (a more potent greenhouse gas) when suitable organic chlorine compounds are available to drive hydrogen and acetate uptake.

Much stronger tundra methane emissions during autumn freeze than spring thaw

Warming in the Arctic has been more apparent in the non-growing season than in the typical growing season. In this context, methane (CH₄ ) emissions in the non-growing season, particularly in the shoulder seasons, account for a substantial proportion of the annual budget. However, CH₄ emissions in spring and autumn shoulders are often underestimated by land models and measurements due to limited data availability and unknown mechanisms. This study investigates CH₄ emissions during spring thaw and autumn freeze using eddy covariance CH₄ measurements from three Arctic sites with multi-year observations. We find that the shoulder seasons contribute to about a quarter (25.6 ± 2.3%, mean ± SD) of annual total CH₄ emissions. Our study highlights the three to four times higher contribution of autumn freeze CH₄ emission to total annual emission than that of spring thaw. Autumn freeze exhibits significantly higher CH₄ flux (0.88 ± 0.03 mg m^-2  hr^-1 ) than spring thaw (0.48 ± 0.04 mg m^-2  hr^-1 ). The mean duration of autumn freeze (58.94 ± 26.39 days) is significantly longer than that of spring thaw (20.94 ± 7.79 days), which predominates the much higher cumulative CH₄ emission during autumn freeze (1,212.31 ± 280.39 mg m^-2  year^-1 ) than that during spring thaw (307.39 ± 46.11 mg m^-2  year^-1 ). Near-surface soil temperatures cannot completely reflect the freeze-thaw processes in deeper soil layers and appears to have a hysteresis effect on CH₄ emissions from early spring thaw to late autumn freeze. Therefore, it is necessary to consider commonalities and differences in CH₄ emissions during spring thaw versus autumn freeze to accurately estimate CH₄ source from tundra ecosystems for evaluating carbon-climate feedback in Arctic.

Spatial heterogeneity of belowground microbial communities linked to peatland microhabitats with different plant dominants

Peatland vegetation is composed mostly of mosses, graminoids and ericoid shrubs, and these have a distinct impact on peat biogeochemistry. We studied variation in soil microbial communities related to natural peatland microhabitats dominated by Sphagnum, cotton-grass and blueberry. We hypothesized that such microhabitats will be occupied by structurally and functionally different microbial communities, which will vary further during the vegetation season due to changes in temperature and photosynthetic activity of plant dominants. This was addressed using amplicon-based sequencing of prokaryotic and fungal rDNA and qPCR with respect to methane-cycling communities. Fungal communities were highly microhabitat-specific, while prokaryotic communities were additionally directed by soil pH and total N content. Seasonal alternations in microbial community composition were less important; however, they influenced the abundance of methane-cycling communities. Cotton-grass and blueberry bacterial communities contained relatively more α-Proteobacteria but less Chloroflexi, Fibrobacteres, Firmicutes, NC10, OD1 and Spirochaetes than in Sphagnum. Methanogens, syntrophic and anaerobic bacteria (i.e. Clostridiales, Bacteroidales, Opitutae, Chloroflexi and Syntrophorhabdaceae) were suppressed in blueberry indicating greater aeration that enhanced abundance of fungi (mainly Archaeorhizomycetes) and resulted in the highest fungi-to-bacteria ratio. Thus, microhabitats dominated by different vascular plants are inhabited by unique microbial communities, contributing greatly to spatial functional diversity within peatlands.

Impact of fertiliser, water table, and warming on celery yield and CO2 and CH4 emissions from fenland agricultural peat

Peatlands are globally important areas for carbon preservation; although covering only 3% of global land area, they store 30% of total soil carbon. Lowland peat soils can also be very productive for agriculture, but their cultivation requires drainage as most crops are intolerant of root-zone anoxia. This leads to the creation of oxic conditions in which organic matter becomes vulnerable to mineralisation. Given the demand for high quality agricultural land, 40% of the UK's peatlands have been drained for agricultural use. In this study we present the outcomes of a controlled environment experiment conducted on agricultural fen peat to examine possible trade-offs between celery growth (an economically important crop on the agricultural peatlands of eastern England) and emissions of greenhouse gases (carbon dioxide (CO₂) and methane (CH₄)) at different temperatures (ambient and ambient +5 °C), water table levels (-30 cm, and -50 cm below the surface), and fertiliser use. Raising the water table from -50 cm to -30 cm depressed yields of celery, and at the same time decreased the entire ecosystem CO₂ loss by 31%. A 5 °C temperature increase enhanced ecosystem emissions of CO₂ by 25% and increased celery dry shoot weight by 23% while not affecting the shoot fresh weight. Fertiliser addition increased both celery yields and soil respiration by 22%. Methane emissions were generally very low and not significantly different from zero. Our results suggest that increasing the water table can lower emissions of greenhouse gases and reduce the rate of peat wastage, but reduces the productivity of celery. If possible, the water table should be raised to -30 cm before and after cultivation, and only decreased during the growing season, as this would reduce the overall greenhouse gas emissions and peat loss, potentially not affecting the production of vegetable crops.

Seagrass and macrophyte mediated CO2 and CH4 dynamics in shallow coastal waters

Seagrass meadows are among the most important coastal/ marine ecosystems for long-term carbon storage and conditioning of coastal waters. A combined air-water flux of CO2 and CH4 from the seagrass meadows was studied for the first time from Asia's largest brackish-water lagoon, Chilika, India. Ecosystem-based comparisons were carried out during two hydrologically different conditions of dry and wet seasons in the seagrass dominated southern sector (SS); macrophyte-dominated northern sector (NS); the largely un-vegetated central sector (CS) and the tidally active outer channel (OC) of the lagoon. The mean fluxes of CO2 from SS, NS, CS and OC were 9.8, 146.6, 48.4 and 33.0mM m-2d-1, and that of CH4 were 0.12, 0.11, 0.05 and 0.07mM m-2d-1, respectively. The net emissions (in terms of CO2 equivalents), considering the global warming potential of CO2 (GWP: 1) and CH4 (GWP: 28) from seagrass meadows were over 14 times lower compared to the macrophyte-dominated sector of the lagoon. Contrasting emissivity characteristics of CO2 and CH4 were observed between macrophytes and seagrass, with the former being a persistent source of CO2. It is inferred that although seagrass meadows act as a weak source of CH4, they could be effective sinks of CO2 if land-based pollution sources are minimized.

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.

Tundra water budget and implications of precipitation underestimation

Difficulties in obtaining accurate precipitation measurements have limited meaningful hydrologic assessment for over a century due to performance challenges of conventional snowfall and rainfall gauges in windy environments. Here, we compare snowfall observations and bias adjusted snowfall to end-of-winter snow accumulation measurements on the ground for 16 years (1999-2014) and assess the implication of precipitation underestimation on the water balance for a low-gradient tundra wetland near Utqiagvik (formerly Barrow), Alaska (2007-2009). In agreement with other studies, and not accounting for sublimation, conventional snowfall gauges captured 23-56% of end-of-winter snow accumulation. Once snowfall and rainfall are bias adjusted, long-term annual precipitation estimates more than double (from 123 to 274 mm), highlighting the risk of studies using conventional or unadjusted precipitation that dramatically under-represent water balance components. Applying conventional precipitation information to the water balance analysis produced consistent storage deficits (79 to 152 mm) that were all larger than the largest actual deficit (75 mm), which was observed in the unusually low rainfall summer of 2007. Year-to-year variability in adjusted rainfall (±33 mm) was larger than evapotranspiration (±13 mm). Measured interannual variability in partitioning of snow into runoff (29% in 2008 to 68% in 2009) in years with similar end-of-winter snow accumulation (180 and 164 mm, respectively) highlights the importance of the previous summer's rainfall (25 and 60 mm, respectively) on spring runoff production. Incorrect representation of precipitation can therefore have major implications for Arctic water budget descriptions that in turn can alter estimates of carbon and energy fluxes.

Plants, microorganisms, and soil temperatures contribute to a decrease in methane fluxes on a drained Arctic floodplain

As surface temperatures are expected to rise in the future, ice-rich permafrost may thaw, altering soil topography and hydrology and creating a mosaic of wet and dry soil surfaces in the Arctic. Arctic wetlands are large sources of CH₄ , and investigating effects of soil hydrology on CH₄ fluxes is of great importance for predicting ecosystem feedback in response to climate change. In this study, we investigate how a decade-long drying manipulation on an Arctic floodplain influences CH₄ -associated microorganisms, soil thermal regimes, and plant communities. Moreover, we examine how these drainage-induced changes may then modify CH₄ fluxes in the growing and nongrowing seasons. This study shows that drainage substantially lowered the abundance of methanogens along with methanotrophic bacteria, which may have reduced CH₄ cycling. Soil temperatures of the drained areas were lower in deep, anoxic soil layers (below 30 cm), but higher in oxic topsoil layers (0-15 cm) compared to the control wet areas. This pattern of soil temperatures may have reduced the rates of methanogenesis while elevating those of CH₄ oxidation, thereby decreasing net CH₄ fluxes. The abundance of Eriophorum angustifolium, an aerenchymatous plant species, diminished significantly in the drained areas. Due to this decrease, a higher fraction of CH₄ was alternatively emitted to the atmosphere by diffusion, possibly increasing the potential for CH₄ oxidation and leading to a decrease in net CH₄ fluxes compared to a control site. Drainage lowered CH₄ fluxes by a factor of 20 during the growing season, with postdrainage changes in microbial communities, soil temperatures, and plant communities also contributing to this reduction. In contrast, we observed CH₄ emissions increased by 10% in the drained areas during the nongrowing season, although this difference was insignificant given the small magnitudes of fluxes. This study showed that long-term drainage considerably reduced CH₄ fluxes through modified ecosystem properties.

Effect of water table management and elevated CO2 on radish productivity and on CH4 and CO2 fluxes from peatlands converted to agriculture

Anthropogenic activity is affecting the global climate through the release of greenhouse gases (GHGs) e.g. CO₂ and CH₄. About a third of anthropogenic GHGs are produced from agriculture, including livestock farming and horticulture. A large proportion of the UK's horticultural farming takes place on drained lowland peatlands, which are a source of significant amounts of CO₂ into the atmosphere. This study set out to establish whether raising the water table from the currently used -50cm to -30cm could reduce GHGs emissions from agricultural peatlands, while simultaneously maintaining the current levels of horticultural productivity. A factorial design experiment used agricultural peat soil collected from the Norfolk Fens (among the largest of the UK's lowland peatlands under intensive cultivation) to assess the effects of water table levels, elevated CO₂, and agricultural production on GHG fluxes and crop productivity of radish, one of the most economically important fenland crops. The results of this study show that a water table of -30cm can increase the productivity of the radish crop while also reducing soil CO₂ emissions but without a resultant loss of CH₄ to the atmosphere, under both ambient and elevated CO₂ concentrations. Elevated CO₂ increased dry shoot biomass, but not bulb biomass nor root biomass, suggesting no immediate advantage of future CO₂ levels to horticultural farming on peat soils. Overall, increasing the water table could make an important contribution to global warming mitigation while not having a detrimental impact on crop yield.

Sunlight stimulates methane uptake and nitrous oxide emission from the High Arctic tundra

Many environmental factors affecting methane (CH₄) and nitrous oxide (N₂O) fluxes have been investigated during the processes of carbon and nitrogen transformation in the boreal tundra. However, effects of sunlight on CH₄ and N₂O fluxes and their budgets were neglected in the boreal tundra. Here, summertime CH₄ and N₂O fluxes in the presence and total absence of sunlight were investigated at the six tundra sites (DM1-DM6) on Ny-Ålesund in the High Arctic. The mean CH₄ fluxes at the tundra sites ranged from -4.7 to -158.6μg CH₄ m^-2h^-1 in the presence of light, indicating that a large CH₄ sink occurred in the tundra soils. However, enhanced CH₄ emission in total absence of light occurred at all the tundra sites. The mean N₂O fluxes ranged from 7.4 to 14.6μg N₂O m^-2h^-1 in the presence of light, whereas in the absence of light all the tundra sites generally released less N₂O, and even significant N₂O uptake occurred there. Soil temperature, chamber temperature and soil moisture showed no significant correlations with tundra CH₄ and N₂O flux. The presence of sunlight increased tundra CH₄ uptake by 114.2μg CH₄ m^-2h^-1 and N₂O emission by 10.9μg N₂O m^-2h^-1 compared with total absence of light. Overall our results showed that tundra ecosystem switched from CH₄ sink and N₂O emission source in the presence of light to CH₄ emission source and N₂O sink in the absence of light. Therefore sunlight had an important effect on CH₄ and N₂O budgets in the High Arctic tundra. The exclusion of sunlight might overestimate CH₄ budgets, but underestimate N₂O budgets in the Arctic tundra ecosystem.