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[Display omitted] • Freeze-thaw perturbation led to enhanced qCO 2 as less CO 2 emitted from even less SMBC. • First thawed layer and coarse aggregates experienced more intensified freeze-thaw. • Lagged freezing and fine aggregates preserved microbes from cryogenic perturbation. • Coarse aggregates survived with higher F/B ratio but lower CO 2 emissions after freeze-thaw. • Fine aggregates were more resilient to freeze-thaw with lower F/B ratio and stable CO 2. Currently available investigations often fail to adequately recognize that soil does not freeze or thaw instantly nor uniformly, but unidirectionally and progressively. Unidirectional freeze-thaw can re-organize soil physio-, chemical- and biological-properties across layers as well among aggregates, thus posing far-reaching yet undervalued impacts on soil carbon biogeochemical cycling, especially in eroding settings. In this study, a Mollisol was refilled into soil columns, and respectively experienced complete freeze-thaw (i.e., no layer distinguished), and progressive freeze-thaw (i.e., from outer layer T1, to inner core T3). After thawing, all soils were fractionated into six size classes, and each size class was then incubated at 10℃ over 7 days to measure daily CO 2 emission rate. Our results show that: (1) The soil microbial biomass carbon (SMBC) was generally reduced by 25 % after freeze-thaw, but only the finest aggregates ≤ 32 μm maintained comparable CO 2 emission rates. (2) The inner core T3 thawed last and survived with significantly more SMBC and CO 2 , but lower respiratory quotient (qCO 2), than the outer layer T1 thawed first. (3) The fine aggregates survived with more SMBC and total CO 2 emissions, but significantly lower qCO 2 and smaller ratios of fungal to bacterial biomass carbon, than the coarse aggregates. The disproportionally less respiratory carbon losses (i.e., lower qCO 2) in T3 was probably because the lagged freezing and thus weakened water-ice phase changes in the inner core helped to better preserve the soil microbes. The fungal-dominant coarse aggregates were very responsive to freezing and evidently more respiratory active, whereas the fine aggregates were more resilient as the enhanced freezing point depression and the thus prolonged unfrozen state, fostering a better microbial survival with bacteria-dominant communities and robust basal respiration. Although limited to controlled simulation, this study may help to disentangle the role of unidirectional freeze-thaw in perturbing slope-scale soil carbon biogeochemical cycling. [ABSTRACT FROM AUTHOR]

Permafrost regions play an important role in global carbon and nitrogen cycling, storing enormous amounts of organic carbon and preserving a delicate balance of nutrient dynamics. However, the increasing frequency and severity of wildfires in these regions pose significant challenges to the stability of these ecosystems. This review examines the effects of fire on chemical, biological, and physical properties of permafrost regions. The physical, chemical, and pedological properties of frozen soil are impacted by fires, leading to changes in soil structure, porosity, and hydrological functioning. The combustion of organic matter during fires releases carbon and nitrogen, contributing to greenhouse gas emissions and nutrient loss. Understanding the interactions between fire severity, ecosystem processes, and the implications for permafrost regions is crucial for predicting the impacts of wildfires and developing effective strategies for ecosystem protection and agricultural productivity in frozen soils. By synthesizing available knowledge and research findings, this review enhances our understanding of fire severity's implications for permafrost ecosystems and offers insights into effective fire management strategies. [Display omitted] • Fire effects on soil nutrients and microbial communities have been discussed. • Fire-related modifications in physical properties of soil have been highlighted. • Effects of fire on active layer depth, soil moisture content and hydrological processes. • Mitigation techniques to deal with soil problems due to fire have been discussed. [ABSTRACT FROM AUTHOR]

Scientific grazing management is essential to maintain soil organic carbon (SOC) stability in subalpine meadows on the Qinghai-Tibet Plateau (QTP). We studied the stability of SOC in Carex subalpine meadows in QTP with five rest-grazing treatments. (ST1: soil surfaces started thawing; ST2: soil thawing depth > 10 cm; RG1: re-greening coverage reached 30–40%; RG2: re-greening coverage reached 80%; CK: local traditional rest-grazing time). The results showed that rest-grazing at different times in spring affected the active SOC composition by affecting the stability of physical structure. The underground organic carbon storage was dominated by the root carbon storage, while the active organic carbon component had a significant correlation with the litter content. Compared with the traditional rest-grazing, ST1, ST2, RG1 and RG2 increased the topsoil SOC storage by 5.87%, 7.24%, 13.55%, 2.17%, and the topsoil root carbon storage by 32.77%, 36.25%, 9.91%, −0.79%. Rest-grazing at ST1 promoted the production of soil macro-aggregates, increased the hydrolase activity and active SOC contents in topsoil, and promoted the soil nutrient cycling. Continuing grazing after the plant re-green increased the physical structure stability and oxidase activity but decreased the active SOC and SOC storage. Although a late rest-grazing in spring has increased the SOC stability, it reduced soil carbon use efficiency and sequestration. It is necessary to rest-grazing subalpine QTP Carex meadows before soil thaw, which can promote SOC accumulation and nutrient cycling, and transport plant carbon into the deep soil through roots, which is beneficial for grassland productivity and grassland carbon sequestration. [ABSTRACT FROM AUTHOR]

Northern permafrost soils store more than half of the global soil carbon. Frozen for at least two consecutive years, but often for millennia, permafrost temperatures have increased drastically in the last decades. The resulting thermal erosion leads not only to gradual thaw, resulting in an increase of seasonally thawing soil thickness, but also to abrupt thaw events, such as sudden collapses of the soil surface. These could affect 20% of the permafrost zone and half of its organic carbon, increasing accessibility for deeper rooting vegetation and microbial decomposition into greenhouse gases. Knowledge gaps include the impact of permafrost thaw on the soil microfauna as well as key taxa to change the microbial mineralization of ancient permafrost carbon stocks during erosion. Here, we present the first sequencing study of an abrupt permafrost erosion microbiome in Northeast Greenland, where a thermal erosion gully collapsed in the summer of 2018, leading to the thawing of 26,500-year-old permafrost material. We investigated which soil parameters (pH, soil carbon content, age and moisture, organic and mineral horizons, and permafrost layers) most significantly drove changes of taxonomic diversity and the abundance of soil microorganisms in two consecutive years of intense erosion. Sequencing of the prokaryotic 16S rRNA and fungal ITS2 gene regions at finely scaled depth increments revealed decreasing alpha diversity with depth, soil age, and pH. The most significant drivers of variation were found in the soil age, horizons, and permafrost layer for prokaryotic and fungal beta diversity. Permafrost was mainly dominated by Proteobacteria and Firmicutes, with Polaromonas identified as the most abundant taxon. Thawed permafrost samples indicated increased abundance of several copiotrophic phyla, such as Bacteroidia, suggesting alterations of carbon utilization pathways within eroding permafrost. [ABSTRACT FROM AUTHOR]

Large-herbivore grazing has been shown to substantially alter tundra soil and vegetation properties as well as carbon fluxes, yet observational evidence to quantify the impact of herbivore introduction into Arctic permafrost ecosystems remains sparse. In this study we investigated growing-season CO2 and CH4 fluxes with flux chambers on a former wet tussock tundra inside Pleistocene Park, a landscape experiment in northeast Siberia with a 22-year history of grazing. Reference data for an undisturbed system were collected on a nearby ungrazed tussock tundra. Linked to a reduction in soil moisture, topsoil temperatures at the grazed site reacted 1 order of magnitude faster to changes in air temperatures compared to the ungrazed site and were significantly higher, and the difference strongly decreased with depth. Overall, both GPP (gross primary productivity, i.e., CO2 uptake by photosynthesis) and Reco (ecosystem respiration, i.e., CO2 release from the ecosystem) were significantly higher at the grazed site with notable variations across plots at each site. The increases in CO2 component fluxes largely compensated for each other, leaving NEE (net ecosystem exchange) similar across grazed and ungrazed sites for the observation period. Soil moisture and CH4 fluxes at the grazed site decreased over the observation period, while in contrast the constantly waterlogged soils at the ungrazed site kept CH4 fluxes at significantly higher levels. Our results indicate that grazing of large herbivores may promote topsoil warming and drying, in this way effectively accelerating CO2 turnover while decreasing methane emissions in the summer months of peak ecosystem activity. Since we lack quantitative information on the pre-treatment status of the grazed ecosystem, however, these findings need to be considered qualitative trends for the peak growing season, and absolute differences between treatments are subject to elevated uncertainty. Moreover, our experiment did not include autumn and winter fluxes, and thus no inferences can be made for the annual NEE and CH4 budgets in tundra ecosystems. [ABSTRACT FROM AUTHOR]

A warming climate combined with frequent and severe fires cause permafrost to thaw, especially in the region of discontinuous permafrost, where soil temperatures may only be a few degrees below 0 °C. Soil thaw releases carbon and nitrogen into the actively cycling pools, and whereas C emissions following permafrost thaw are well documented, the fates of N remain unclear. Denitrification could release N from ecosystems as nitrous oxide or nitrogen gas, but the contributions of these processes to the high-latitude N cycle remain uncertain. We quantified microbial capacity for denitrification and N2O production in boreal soils, lakes, and streams using anoxic C- and N-amended assays, and assessed correlates of denitrifying enzyme activity in Interior Alaska. Riparian soils and stream sediments supported the highest potential rates of denitrification, upland soils were intermediate, and lakes supported lower rates, whereas deep permafrost soils supported little denitrification. Time since fire had no effect on denitrification potential in upland soils. Across all landscape positions, DEA was negatively correlated with ammonium pools. Within each landscape position, potential rate of denitrification increased with soil or sediment organic matter content. Widespread N loss to denitrification in boreal forests could constrain the capacity for N-limited primary producers to maintain C stocks in soils following permafrost thaw. [ABSTRACT FROM AUTHOR]

Northern peatlands are a major component of the global carbon (C) cycle. Widespread climate‐driven ecohydrological changes in these ecosystems can have major consequences on their C sequestration function. Here, we synthesize plant macrofossil data from 33 surficial peat cores from different ecoclimatic regions, with high‐resolution chronologies. The main objectives were to document recent ecosystem state shifts and explore their impact on C sequestration in high‐latitude undisturbed peatlands of northeastern Canada. Our synthesis shows widespread recent ecosystem shifts in peatlands, such as transitions from oligotrophic fens to bogs and Sphagnum expansion, coinciding with climate warming which has also influenced C accumulation during the last ~100 years. The rapid shifts towards drier bog communities and an expansion of Sphagnum sect. Acutifolia after 1980 CE were most pronounced in the northern subarctic sites and are concurrent with summer warming in northeastern Canada. These results provide further evidence of a northward migration of Sphagnum‐dominated peatlands in North America in response to climate change. The results also highlight differences in the timing of ecosystem shifts among peatlands and regions, reflecting internal peatland dynamics and varying responses of vegetation communities. Our study suggests that the recent rapid climate‐driven shifts from oligotrophic fen to drier bog communities have promoted plant productivity and thus peat C accumulation. We highlight the importance of considering recent ecohydrological trajectories when modelling the potential contribution of peatlands to climate change. Our study suggests that, contrary to expectations, peat C sequestration could be promoted in high‐latitude non‐permafrost peatlands where wet sedge fens may transition to drier Sphagnum bog communities due to warmer and longer growing seasons. [ABSTRACT FROM AUTHOR]

This paper presents the results of a community survey that was designed to better understand the effects of permafrost degradation and coastal erosion on civil infrastructure. Observations were collected from residents in four Arctic coastal communities: Point Lay, Wainwright, Utqiaġvik, and Kaktovik. All four communities are underlain by continuous ice-rich permafrost with varying degrees of degradation and coastal erosion. The types, locations, and periods of observed permafrost thaw and coastal erosion were elicited. Survey participants also reported the types of civil infrastructure being affected by permafrost degradation and coastal erosion and any damage to residential buildings. Most survey participants reported that coastal erosion has been occurring for a longer period than permafrost thaw. Surface water ponding, ground surface collapse, and differential ground settlement are the three types of changes in ground surface manifested by permafrost degradation that are most frequently reported by the participants, while houses are reported as the most affected type of infrastructure in the Arctic coastal communities. Wall cracking and house tilting are the most commonly reported types of residential building damage. The effects of permafrost degradation and coastal erosion on civil infrastructure vary between communities. Locations of observed permafrost degradation and coastal erosion collected from all survey participants in each community were stacked using heatmap data visualization. The heatmaps constructed using the community survey data are reasonably consistent with modeled data synthesized from the scientific literature. This study shows a useful approach to coproduce knowledge with Arctic residents to identify locations of permafrost thaw and coastal erosion at higher spatial resolution as well as the types of infrastructure damage of most concern to Arctic residents. [ABSTRACT FROM AUTHOR]

Global warming has reduced the extent of permafrost, increased permafrost temperatures, and deepened the active layer across the Arctic. Permafrost degradation has detrimental effects on infrastructure and negative impacts on ecosystem services for many Arctic communities. This study examines the adaptive capacity for managing permafrost degradation in Northwest Greenland. The methods are based on questionnaire and interview data from fieldwork, frozen ground temperature records and published data forecasting the deepening of the active layer. Results illustrate the impact of permafrost degradation on the physical environment, hunting and harvesting, housing, and the economy in Northwest Greenland. House owners are mending damage caused by ground movement, and local institutions are concerned with the maintenance of roads and other public infrastructure impacted by permafrost. The scientific knowledge needed to inform decision-making is useful for identifying overall changes, but existing data sources are scarce, and more detailed permafrost maps are needed for long-term town planning. The study concludes that many individuals and institutions engage in autonomous adaptation on an ad hoc basis, rather than pursuing an overall strategy to increase the adaptive capacity in advance of future permafrost degradation in Northwest Greenland. [ABSTRACT FROM AUTHOR]

Effects of permafrost degradation on carbon (C) and nitrogen (N) cycling on the Qinghai‐Tibetan Plateau (QTP) have rarely been analyzed. This study used a revised process‐based biogeochemical model to quantify the effects in the region during the 21st century. We found that permafrost degradation would expose 0.61 ± 0.26 (mean ± SD) and 1.50 ± 0.15 Pg C of soil organic carbon under the representative concentration pathway (RCP) 4.5 and the RCP 8.5, respectively. Among them, more than 20% will be decomposed, enhancing heterotrophic respiration by 8.62 ± 4.51 (RCP 4.5) and 33.66 ± 14.03 (RCP 8.5) Tg C/yr in 2099. Deep soil N supply due to thawed permafrost is not accessible to plants, only stimulating net primary production by 7.15 ± 4.83 (RCP 4.5) and 24.27 ± 9.19 (RCP 8.5) Tg C/yr in 2099. As a result, the single effect of permafrost degradation would cumulatively weaken the regional C sink by 209.44 ± 137.49 (RCP 4.5) and 371.06 ± 151.70 (RCP 8.5) Tg C during 2020–2099. However, when factors of climate change, CO2 increasing and permafrost degradation are all considered, the permafrost region on the QTP would be a stronger C sink in the 21st century. Permafrost degradation has a greater influence on C balance of alpine meadows than alpine steppes on the QTP. The shallower active layer, higher soil C and N stocks, and wetter environment in alpine meadows are responsible for its stronger response to permafrost degradation. This study highlights that permafrost degradation could continue to release large amounts of C to the atmosphere irrespective of potentially more nitrogen available from deep soils. Key Points: Soil carbon release due to permafrost degradation overwhelms net primary production on the Qinghai‐Tibetan PlateauDeep soil nitrogen addition from thawing permafrost has a limited benefit to plant carbon uptakePermafrost degradation has more pronounced influences on carbon cycling of alpine meadows than alpine steppes [ABSTRACT FROM AUTHOR]

Permafrost thaw increases active layer thickness, changes landscape hydrology and influences vegetation species composition. These changes alter belowground microbial and geochemical processes, affecting production, consumption and net emission rates of climate forcing trace gases. Net carbon dioxide (CO2) and methane (CH4) fluxes determine the radiative forcing contribution from these climate-sensitive ecosystems. Permafrost peatlands may be amosaic of dry frozen hummocks, semi-thawed or perched sphagnum dominated areas, wet permafrostfree sedge dominated sites and open water ponds. We revisited estimates of climate forcing made for 1970 and 2000 for Stordalen Mire in northern Sweden and found the trend of increasing forcing continued into 2014. TheMire continued to transition from dry permafrost to sedge and open water areas, increasing by 100% and 35%, respectively, over the 45-year period, causing the net radiative forcing of Stordalen Mire to shift fromnegative to positive. This trend is driven by transitioning vegetation community composition, improved estimates of annual CO2 and CH4 exchange and a 22% increase in the IPCC's 100-year global warming potential (GWP_100) value for CH4. These results indicate that discontinuous permafrost ecosystems, while still remaining a net overall sink of C, can become a positive feedback to climate change on decadal timescales. [ABSTRACT FROM AUTHOR]

Background: Soil microorganisms in the thawing permafrost play key roles in the maintenance of ecosystem function and regulation of biogeochemical cycles. However, our knowledge of patterns and drivers of permafrost microbial communities is limited in northeastern China. Therefore, we investigated the community structure of soil bacteria in the active, transition and permafrost layers based on 90 soil samples collected from 10 sites across the continuous permafrost region using high-throughput Illumina sequencing. Results: Proteobacteria (31.59%), Acidobacteria (18.63%), Bacteroidetes (9.74%), Chloroflexi (7.01%) and Actinobacteria (6.92%) were the predominant phyla of the bacterial community in all soil layers; however, the relative abundances of the dominant bacterial taxa varied with soil depth. The bacterial community alpha-diversity based on the Shannon index and the phylogenetic diversity index both decreased significantly with depth across the transition from active layer to permafrost layer. Nonmetric multidimensional scaling analysis and permutation multivariate analysis of variance revealed that microbial community structures were significantly different among layers. Redundancy analysis and Spearman's correlation analysis showed that soil properties differed between layers such as soil nutrient content, temperature and moisture mainly drove the differentiation of bacterial communities. Conclusions: Our results revealed significant differences in bacterial composition and diversity among soil layers. Our findings suggest that the heterogeneous environmental conditions between the three soil horizons had strong influences on microbial niche differentiation and further explained the variability of soil bacterial community structures. This effort to profile the vertical distribution of bacterial communities may enable better evaluations of changes in microbial dynamics in response to permafrost thaw, which would be beneficial to ecological conservation of permafrost ecosystems. [ABSTRACT FROM AUTHOR]

In the context of global warming, the melting of Arctic permafrost raises the threat of a reemergence of microorganisms some of which were shown to remain viable in ancient frozen soils for up to half a million years. In order to evaluate this risk, it is of interest to acquire a better knowledge of the composition of the microbial communities found in this understudied environment. Here, we present a metagenomic analysis of 12 soil samples from Russian Arctic and subarctic pristine areas: Chukotka, Yakutia and Kamchatka, including nine permafrost samples collected at various depths. These large datasets (9.2 × 1011 total bp) were assembled (525 313 contigs > 5 kb), their encoded protein contents predicted, and then used to perform taxonomical assignments of bacterial, archaeal and eukaryotic organisms, as well as DNA viruses. The various samples exhibited variable DNA contents and highly diverse taxonomic profiles showing no obvious relationship with their locations, depths or deposit ages. Bacteria represented the largely dominant DNA fraction (95%) in all samples, followed by archaea (3.2%), surprisingly little eukaryotes (0.5%), and viruses (0.4%). Although no common taxonomic pattern was identified, the samples shared unexpected high frequencies of β-lactamase genes, almost 0.9 copy/bacterial genome. In addition to known environmental threats, the particularly intense warming of the Arctic might thus enhance the spread of bacterial antibiotic resistances, today's major challenge in public health. β-Lactamases were also observed at high frequency in other types of soils, suggesting their general role in the regulation of bacterial populations. Permafrost bacteria appear to constitute an enormous reservoir of antibiotic resistance genes. [ABSTRACT FROM AUTHOR]

In remote communities in the Canadian Arctic, petroleum hydrocarbons supply most household energy needs. Their transportation and use frequently incurs small volume spills in populated areas. The remediation method that is currently used when such spills affect the soil under northern villages' stilted buildings is expensive and not well suited to local conditions. Here, we review local constraints and environmental considerations and select the best remediation technology for this context: in situ chemical oxidation, involving sodium persulfate (SPS) alkali activated with calcium peroxide (CP). Activated SPS presents a good reactivity and amenability to compounds found in diesel. Its high persistence allows a gradual contaminant degradation, regulating heat release from exothermic reactions associated with the oxidative reactions. CP provides suitable alkali activation, acts itself as an oxidant and provides O2 into the subsurface, which may favour a final smoothing bioremediation step. The SPS properties and the contaminant amenability mean that diesel is removed relatively efficiently, while the subsurface temperature increase is limited, thus preserving the residual permafrost. The solid form of the chemicals offers safe and economic transportation and operation, along with versatility regarding the preparation and distribution of the oxidizing solution into the subsurface. Finally, the oxidation by-products resulting from this method are not considered to be environmentally problematic in the context of the application, and they can be partly confined during the treatment. [ABSTRACT FROM AUTHOR]

Stordalen Mire is a peatland in the discontinuous permafrost zone in arctic Sweden that exhibits a habitat gradient from permafrost palsa, to Sphagnum bog underlain by permafrost, to Eriophorum‐dominated fully thawed fen. We used three independent approaches to evaluate the annual, multi‐decadal, and millennial apparent carbon accumulation rates (aCAR) across this gradient: seven years of direct semi‐continuous measurement of CO2 and CH4 exchange, and 21 core profiles for 210Pb and 14C peat dating. Year‐round chamber measurements indicated net carbon balance of −13 ± 8, −49 ± 15, and −91 ± 43 g C m−2 y−1 for the years 2012–2018 in palsa, bog, and fen, respectively. Methane emission offset 2%, 7%, and 17% of the CO2 uptake rate across this gradient. Recent aCAR indicates higher C accumulation rates in surface peats in the palsa and bog compared to current CO2 fluxes, but these assessments are more similar in the fen. aCAR increased from low millennial‐scale levels (17–29 g C m−2 y−1) to moderate aCAR of the past century (72–81 g C m−2 y−1) to higher recent aCAR of 90–147 g C m−2 y−1. Recent permafrost collapse, greater inundation and vegetation response has made the landscape a stronger CO2 sink, but this CO2 sink is increasingly offset by rising CH4 emissions, dominated by modern carbon as determined by 14C. The higher CH4 emissions result in higher net CO2‐equivalent emissions, indicating that radiative forcing of this mire and similar permafrost ecosystems will exert a warming influence on future climate. Key Points: Stordalen Mire, a permafrost peatland, has become a stronger carbon sink due to shifts in vegetation and inundation accompanying thawThis stronger carbon sink is offset by an increase in methane emission, resulting in higher net CO2‐equivalent emissionsAnalysis of respiration products indicates that methane emission from the fen and bog are dominated by modern carbon [ABSTRACT FROM AUTHOR]

Abrupt thaw could cause permafrost ecosystems to release more carbon than is predicted from gradual thaw alone. However, thermokarst feature mapping is limited in scope, and observed responses of carbon fluxes to abrupt thaw are variable. We developed a thermokarst detection algorithm that identifies thermokarst features from a single elevation dataset with 71.5 percent accuracy and applied it in Healy, Alaska. Additionally, we investigated the landscape-level variation in carbon dioxide and methane fluxes by extent of abrupt thaw using eddy covariance. Seven percent of the site was classified as thermokarst. Water tracks were the most extensive form of thermokarst, although small pits were much more numerous. Abrupt thaw was positively correlated with carbon uptake during the growing season, when increases in gross primary productivity outpaced increases in ecosystem respiration in vegetation-dense water tracks. However, this was outweighed by higher carbon release in thermokarst features during the nongrowing season. Additionally, abrupt thaw was positively correlated with methane production nearly year-round. Our findings support the hypothesis that abrupt thaw of permafrost carbon will contribute to the permafrost climate feedback above and beyond that associated with gradual thaw and highlights the need to map thermokarst and incorporate abrupt thaw into Earth System Models. [ABSTRACT FROM AUTHOR]

As a buffer layer for the energy and water exchange between atmosphere and permafrost, the active layer is sensitive to climate warming. Changes in the thermal state in active layer can alter soil organic carbon (SOC) dynamics. It is critical to identify the response of soil microbial communities to warming to better predict the regional carbon cycle under the background of global warming. Here, the active layer soils collected from a wetland-forest ecotone in the continuous permafrost region of Northeastern China were incubated at 5 and 15°C for 45 days. High-throughput sequencing of the 16S rRNA gene was used to examine the response of bacterial community structure to experimental warming. A total of 4148 OTUs were identified, which followed the order 15°C > 5°C > pre-incubated. Incubation temperature, soil layer and their interaction have significant effects on bacterial alpha diversity (Chao index). Bacterial communities under different temperature were clearly distinguished. Chloroflexi, Actinobacteria, Proteobacteria, and Acidobacteria accounted for more than 80% of the community abundance at the phylum level. Warming decreased the relative abundance of Chloroflexi and Acidobacteria, while Actinobacteria and Proteobacteria exhibited increasing trend. At family level, the abundance of norank_o__norank_c__AD3 and Ktedonobacteraceae decreased significantly with the increase of temperature, while Micrococcaccac increased. In addition, the amount of SOC mineralization were positively correlated with the relative abundances of most bacterial phyla and SOC content. SOC content was positively correlated with the relative abundance of most bacterial phyla. Results indicate that the SOC content was the primary explanatory variable and driver of microbial regulation for SOC mineralization. Our results provide a new perspective for understanding the microbial mechanisms that accelerates SOC decomposition under warming conditions in the forest-wetland ecotone of permafrost region. [ABSTRACT FROM AUTHOR]

The bacterial composition of permafrost samples taken during drilling of frozen marine sediments in the area of Barentsburg coal mine on the east coast of Grønfjord Bay of Western Spitsbergen has been studied. The study was based on the analysis of the V4 region of the 16S rRNA gene, carried out using next generation sequencing, as well as using classical microbiological methods (direct luminescence microscopy and aerobic cultivation).The total cell number in permafrost samples ranges from 6.73 ± 0.73 × 106 to 3.37 ± 0.19 107 cells per g. The number of cultivable aerobic bacteria in frozen samples on 1/5 TSA and R2A media ranges from 0 to 6.20 ± 0.45 × 104 CFU/g. Isolates of aerobic bacteria were identified by 16S rRNA gene analysis as representatives of the genera Arthrobacter, Pseudarthrobacter, Psychrobacter, and Rhodoferax. The dominant phyla of the domain Bacteria were Actinobacteria, Proteobacteria, Chloroflexi, Nitrospirae and Firmicutes. As a result of phylogenetic analysis of the dominant operational taxonomic units, representatives of methane oxidizing, sulfate reducing bacteria, as well as heterotrophic bacteria involved in the transformation of organic matter were found. [ABSTRACT FROM AUTHOR]

Fires are predicted to increase in Arctic regions due to ongoing climate change. Tundra fires can alter carbon and nutrient cycling and release a substantial amount of greenhouse gases with global consequences. Yet, the long-term effects of tundra fires on carbon (C) and nitrogen (N) stocks and cycling are still unclear. Here we used a space-for-time approach to investigate the long-term fire effects on C and N stocks and cycling in soil and aboveground living biomass. We collected data from three large fire scars (>44, 28 and 12 years old) and corresponding control areas and used linear mixed-effects models in a Bayesian framework to analyse how the stocks and cycling were influenced by fire. We found that tundra fires did not affect total C and N stocks because a major part of the stocks was located belowground in soils, which were largely unaltered by fire. However, fire had a strong effect on stocks in the aboveground vegetation, mainly due to the reduction of the lichen layer. Fire reduced N concentrations in graminoids and herbs on the younger fire scars, which affected respective C/N ratios and indicated an increased post-fire competition between vascular plants. Aboveground plant biomass was depleted in 13C in all three fire scars. This could be related to a lower 13C abundance in CO[sub 2] in the ambient air because of increased post-fire decomposition, providing a source of 13C-depleted CO[sub 2]. In soil, the relative abundance of 13C changed with time after fire because of the combined effects of microbial decomposition and plant-related fractionation processes. Our results indicate that in lichen-rich subarctic tundra ecosystems, the contribution of fires to the release of additional carbon to the atmosphere might be relatively small as soil stocks appear to be resilient. [ABSTRACT FROM AUTHOR]

Rapid environmental change occurring in northern permafrost regions may have profound implications for fish biodiversity but remains poorly understood. Climate change, increasing human development, and resultant permafrost thaw may combine to alter the quality and quantity of fish habitat including reductions in preferred thermal habitat, changes in water quality, and modified drainage patterns.Our study objective was to understand how lake fish communities residing on permafrost landscapes may be responding to climate change and land use disturbance. We investigated the drivers of freshwater fish community health in lakes of the lower Mackenzie River basin, an ice‐rich permafrost region that is experiencing substantial warming, permafrost thaw, and new major highway development. We collected lake morphometry, water quality, and fish community data from 50 lakes and derived several indicators of aquatic health including fish species richness, relative abundance, and the occurrence of three culturally important fish species.We found that water quality and lake size were significant co‐drivers of fish community health whereas relationships with summer thermal habitat, as represented by July air temperature, were relatively negligible. Dissolved organic carbon (an indicator for lake browning) emerged as a particularly important driver of fish community structure, and fish community health steeply declined when dissolved organic carbon concentrations exceeded 17–18 mg/L. We suggest potential mechanisms for these declines including light inhibition during summer and a reduced capacity for overwintering in smaller and murkier lakes that may experience faster oxygen depletion rates.Using a more expansive regional water quality database of 203 lakes, we observed potential supporting evidence that warming and new road development increased dissolved organic carbon and total phosphorus concentrations, possibly reducing fish habitat quality in this region.Together, these results highlight how fishes relying on the numerous small and shallow lakes that dominate permafrost landscapes may be vulnerable to the combined effects of rapid warming and new infrastructure. [ABSTRACT FROM AUTHOR]

The ongoing disproportionate increases in temperature and precipitation over the Arctic region may greatly alter the latitudinal gradients in greenup and snowmelt timings as well as associated carbon dynamics of tundra ecosystems. Here we use remotely-sensed and ground-based datasets and model results embedding snowmelt timing in phenology at seven tundra flux tower sites in Alaska during 2001–2018, showing that the carbon response to early greenup or delayed snowmelt varies greatly depending upon local climatic limits. Increases in net ecosystem productivity (NEP) due to early greenup were amplified at the higher latitudes where temperature and water strongly colimit vegetation growth, while NEP decreases due to delayed snowmelt were alleviated by a relief of water stress. Given the high likelihood of more frequent delayed snowmelt at higher latitudes, this study highlights the importance of understanding the role of snowmelt timing in vegetation growth and terrestrial carbon cycles across warming Arctic ecosystems. The ongoing disproportionate increases in temperature and precipitation in the Alaska may alter the latitudinal gradients in greenup and snowmelt timings as well as carbon dynamics. With a broad range of datasets and model results, the authors show that the carbon response to early greenup or delayed snowmelt varies greatly depending upon local climatic limits. [ABSTRACT FROM AUTHOR]

Permafrost is experiencing climate warming at a rate that is two times faster than the rest of the Earth's surface. However, it is still lack of a quantitative basis for predicting the functional stability of permafrost ecosystems in carbon (C) and nutrient cycling. We compiled the data of 708 observations from 89 air-warming experiments in the Northern Hemisphere and characterized the general effects of temperature increase on permafrost C exchange and balance, biomass production, microbial biomass, soil nutrients, and vegetation N dynamics through a meta-analysis. Also, an investigation was made on how responses might change with habitat-specific (e.g., plant functional groups and soil moisture status) conditions and warming variables (e.g., warming phases, levels, and timing). The net ecosystem C exchange (NEE) was found to be downregulated by warming as a result of a stronger sensitivity to warming in respiration (15.6%) than in photosynthesis (6.2%). Vegetation usually responded to warming by investing more C to the belowground, as belowground biomass increased much more (30.1%) than aboveground biomass (2.9%). Warming had a minor effect on microbial biomass. Warming increased soil ammonium and nitrate concentrations. What's more, a synthesis of 70 observations from 11 herbs and 9 shrubs revealed a 2.5% decline of N in green leaves. Compared with herbs, shrubs had a stronger response to respiration and had a decline in green leaf N to a greater extent. Not only in dry condition did green leaf N decline with warming but also in wet conditions. Warming in nongrowing seasons would negatively affect soil water, C uptake, and biomass production during growing seasons. Permafrost C loss and vegetation N decline may increase with warming levels and timing. Overall, these findings suggest that besides a positive C cycling-climate feedback, there will be a negative feedback between permafrost nutrient cycling and climate warming. [ABSTRACT FROM AUTHOR]

Climate change–driven permafrost thaw has a strong influence on pan‐Arctic regions, via, for example, the formation of thermokarst ponds. These ponds are hotspots of microbial carbon cycling and greenhouse gas production, and efforts have been put on disentangling the role of bacteria and archaea in recycling the increasing amounts of carbon arriving to the ponds from degrading watersheds. However, despite the well‐established role of fungi in carbon cycling in the terrestrial environments, the interactions between permafrost thaw and fungal communities in Arctic freshwaters have remained unknown. We integrated data from 60 ponds in Arctic hydro‐ecosystems, representing a gradient of permafrost integrity and spanning over five regions, namely Alaska, Greenland, Canada, Sweden, and Western Siberia. The results revealed that differences in pH and organic matter quality and availability were linked to distinct fungal community compositions and that a large fraction of the community represented unknown fungal phyla. Results display a 16%–19% decrease in fungal diversity, assessed by beta diversity, across ponds in landscapes with more degraded permafrost. At the same time, sites with similar carbon quality shared more species, aligning a shift in species composition with the quality and availability of terrestrial dissolved organic matter. We demonstrate that the degradation of permafrost has a strong negative impact on aquatic fungal diversity, likely via interactions with the carbon pool released from ancient deposits. This is expected to have implications for carbon cycling and climate feedback loops in the rapidly warming Arctic. [ABSTRACT FROM AUTHOR]

Ecosystem carbon (C) dynamics after permafrost thaw depends on more than just climate change since soil nutrient status may also impact ecosystem C balance. It has been advocated that nitrogen (N) release upon permafrost thaw could promote plant growth and thus offset soil C loss. However, compared with the widely accepted C‐N interactions, little is known about the potential role of soil phosphorus (P) availability. We combined 3‐year field observations along a thaw sequence (constituted by four thaw stages, i.e., non‐collapse and 5, 14, and 22 years since collapse) with an in‐situ fertilization experiment (included N and P additions at the level of 10 g N m−2 year−1 and 10 g P m−2 year−1) to evaluate ecosystem C‐nutrient interactions upon permafrost thaw. We found that changes in soil P availability rather than N availability played an important role in regulating gross primary productivity and net ecosystem productivity along the thaw sequence. The fertilization experiment confirmed that P addition had stronger effects on plant growth than N addition in this permafrost ecosystem. These two lines of evidence highlight the crucial role of soil P availability in altering the trajectory of permafrost C cycle under climate warming. [ABSTRACT FROM AUTHOR]

The abundance of greater white‐fronted geese (Anser albifrons frontalis) on the Arctic Coastal Plain (ACP) of northern Alaska, USA, has more than tripled since the late 1990s; however, recent rate of annual population growth has declined as population size increased, which may indicate white‐fronted geese on the ACP are approaching carrying capacity. We examined rates of gosling growth in greater white‐fronted geese at 3 sites on the ACP during 2012–2014 to assist with predictions of future population trends and assess evidence for density‐dependent constraints on recruitment. We marked goslings at hatch with individually coded webtags and conducted brood drives during early August to capture, measure, and weigh goslings. Annual estimates of gosling mass at 32 days old (range = 1,190–1,685) indicate that goslings had obtained >60% of asymptotic size. This rate of growth corresponds with that of other goose species and populations with access to high‐quality forage and no limitations on forage availability, and is consistent with the overall increase in abundance of white‐fronted geese at the ACP scale. Contrary to most previous investigations, age‐adjusted mass of goslings did not decline with hatch date. Goslings grew faster in coastal areas than at inland freshwater sites. Taken together, these findings suggest forage was not limiting gosling growth rates in either ecosystem, but forage was of greater quality in coastal areas where goose foraging habitat is expanding because of permafrost subsidence. Spatial patterns of gosling growth corresponded with local‐scale patterns of population density and population change; the areas with greatest rates of gosling growth were those with the greatest population density and rates of population increase. We found little evidence to suggest forage during brood rearing was limiting population increase of white‐fronted geese on the ACP. Factors responsible for the apparent slowing of ACP‐wide population growth are likely those that occur in stages of the annual cycle outside of the breeding grounds. Published 2021. This article is a U.S. Government work and is in the public domain in the USA. Gosling white‐fronted geese had high growth rates and our data did not suggest forage during brood rearing was limiting population increase of white‐fronted geese on the Arctic Coastal Plain of northern Alaska. Spatial patterns of variation in gosling growth corresponded with patterns of population growth and density. [ABSTRACT FROM AUTHOR]

Temperature is an important factor governing microbe‐mediated carbon feedback from permafrost soils. The link between taxonomic and functional microbial responses to temperature change remains elusive due to the lack of studies assessing both aspects of microbial ecology. Our previous study reported microbial metabolic and trophic shifts in response to short‐term temperature increases in Arctic peat soil, and linked these shifts to higher CH4 and CO2 production rates (Proceedings of the National Academy of Sciences of the United States of America, 112, E2507–E2516). Here, we studied the taxonomic composition and functional potential of samples from the same experiment. We see that along a high‐resolution temperature gradient (1–30°C), microbial communities change discretely, but not continuously or stochastically, in response to rising temperatures. The taxonomic variability may thus in part reflect the varied temperature responses of individual taxa and the competition between these taxa for resources. These taxonomic responses contrast the stable functional potential (metagenomic‐based) across all temperatures or the previously observed metabolic or trophic shifts at key temperatures. Furthermore, with rising temperatures we observed a progressive decrease in species diversity (Shannon Index) and increased dispersion of greenhouse gas (GHG) production rates. We conclude that the taxonomic variation is decoupled from both the functional potential of the community and the previously observed temperature‐dependent changes in microbial function. However, the reduced diversity at higher temperatures might help explain the higher variability in GHG production at higher temperatures. [ABSTRACT FROM AUTHOR]

Permafrost soils store approximately twice the amount of carbon currently present in Earth’s atmosphere and are acutely impacted by climate change due to the polar amplification of increasing global temperature. Many organic-rich permafrost sediments are located on large river floodplains, where river channel migration periodically erodes and redeposits the upper tens of meters of sediment. Channel migration exerts a first-order control on the geographic distribution of permafrost and floodplain stratigraphy and thus may affect microbial habitats. To examine how river channel migration in discontinuous permafrost environments affects microbial community composition, we used amplicon sequencing of the 16S rRNA gene on sediment samples from floodplain cores and exposed riverbanks along the Koyukuk River, a large tributary of the Yukon River in west-central Alaska. Microbial communities are sensitive to permafrost thaw: communities found in deep samples thawed by the river closely resembled near-surface active-layer communities in nonmetric multidimensional scaling analyses but did not resemble floodplain permafrost communities at the same depth. Microbial communities also displayed lower diversity and evenness in permafrost than in both the active layer and permafrost-free point bars recently deposited by river channel migration. Taxonomic assignments based on 16S and quantitative PCR for the methyl coenzyme M reductase functional gene demonstrated that methanogens and methanotrophs are abundant in older permafrost-bearing deposits but not in younger, nonpermafrost point bar deposits. The results suggested that river migration, which regulates the distribution of permafrost, also modulates the distribution of microbes potentially capable of producing and consuming methane on the Koyukuk River floodplain. [ABSTRACT FROM AUTHOR]

Permafrost thaw induces soil hydrological changes which in turn affects carbon cycle processes in the Arctic terrestrial ecosystems. However, hydrological impacts of thawing permafrost on microbial processes and greenhouse gas (GHG) dynamics are poorly understood. This study examined changes in microbial communities using gene and genome‐centric metagenomics on an Arctic floodplain subject to decadal drainage, and linked them to CO2 and CH4 flux and soil chemistry. Decadal drainage led to significant changes in the abundance, taxonomy, and functional potential of microbial communities, and these modifications well explained the changes in CO2 and CH4 fluxes between ecosystem and atmosphere—increased fungal abundances potentially increased net CO2 emission rates and highly reduced CH4 emissions in drained sites corroborated the marked decrease in the abundance of methanogens and methanotrophs. Interestingly, various microbial taxa disproportionately responded to drainage: Methanoregula, one of the key players in methanogenesis under saturated conditions, almost disappeared, and also Methylococcales methanotrophs were markedly reduced in response to drainage. Seven novel methanogen population genomes were recovered, and the metabolic reconstruction of highly correlated population genomes revealed novel syntrophic relationships between methanogenic archaea and syntrophic partners. These results provide a mechanistic view of microbial processes regulating GHG dynamics in the terrestrial carbon cycle, and disproportionate microbial responses to long‐term drainage provide key information for understanding the effects of warming‐induced soil drying on microbial processes in Arctic wetland ecosystems. [ABSTRACT FROM AUTHOR]

The permafrost of Mohe County and its suburbs in the Daxing'an Mountains has been influenced by the urbanization. Remote sensing, GIS technology and numerical simulation was used to study the temperature variations of permafrost with the changes in surface vegetation that cover Mohe County and suburban areas, and the law of permafrost degradation on the study area was analyzed. The research results show that the urban area of the study area increased 114.42% from 2000 to 2016, and the urbanization process is continuing to accelerate. The Normalized Difference Vegetation Index map of 2017 in Mohe County and its suburbs was studied and the maximum proportion of vegetation coverage was different in the four seasons. The numerical calculation model results show that the permafrost temperature change in the study area cyclically fluctuates in a cosine form. The annual variation curve of permafrost temperature gradually decreased and its accompanying phase lag increased with depth. The annual temperature change value with the different depths of the town was greater than the natural ground. The maximum permafrost thawing depths of the town and natural ground were 4.2 m and 2.82 m in 50 a, and the degradation rates of the two permafrost are, respectively, 0.88 cm/a and 0.46 cm/a. These results show that urbanization has accelerated the degradation of permafrost. [ABSTRACT FROM AUTHOR]

Permafrost (PF)‐affected soils are widespread in the Arctic and store about half the global soil organic carbon. This large carbon pool becomes vulnerable to microbial decomposition through PF warming and deepening of the seasonal thaw layer (active layer [AL]). Here we combined greenhouse gas (GHG) production rate measurements with a metagenome‐based assessment of the microbial taxonomic and metabolic potential before and after 5 years of incubation under anoxic conditions at a constant temperature of 4°C in the AL, PF transition layer, and intact PF. Warming led to a rapid initial release of CO2 and, to a lesser extent, CH4 in all layers. After the initial pulse, especially in CO2 production, GHG production rates declined and conditions became more methanogenic. Functional gene‐based analyses indicated a decrease in carbon‐ and nitrogen‐cycling genes and a community shift to the degradation of less‐labile organic matter. This study reveals low but continuous GHG production in long‐term warming scenarios, which coincides with a decrease in the relative abundance of major metabolic pathway genes and an increase in carbohydrate‐active enzyme classes. [ABSTRACT FROM AUTHOR]

This article is a Commentary on Wei et al. (2023), 240: 1802–1816. [ABSTRACT FROM AUTHOR]

Permafrost thaw is a challenge in many Arctic regions, one that modifies ecosystems and affects infrastructure and livelihoods. To date, there have been no demographic studies of the population on permafrost. We present the first estimates of the number of inhabitants on permafrost in the Arctic Circumpolar Permafrost Region (ACPR) and project changes as a result of permafrost thaw. We combine current and projected populations at settlement level with permafrost extent. Key findings indicate that there are 1162 permafrost settlements in the ACPR, accommodating 5 million inhabitants, of whom 1 million live along a coast. Climate-driven permafrost projections suggest that by 2050, 42% of the permafrost settlements will become permafrost-free due to thawing. Among the settlements remaining on permafrost, 42% are in high hazard zones, where the consequences of permafrost thaw will be most severe. In total, 3.3 million people in the ACPR live currently in settlements where permafrost will degrade and ultimately disappear by 2050. [ABSTRACT FROM AUTHOR]

The metacommunity concept, defined as a set of local communities connected by species dispersal, provides deep understanding of large‐scale ecological processes. The elements of the metacommunity structure (EMS) framework use occurrence data to differentiate among different patterns (i.e., checkerboard, nestedness, species turnover).Metacommunities of tropical Odonata show species turnover following latitude and temperature gradients but there are no such large‐scale studies for other regions. We performed EMS analysis with data for the Odonata of British Columbia, Canada, testing the role of five environmental variables (temperature, latitude, altitude, precipitation, landcover typology) in structuring the metacommunities and their turnover.The suborder Anisoptera drives the general pattern, with the communities showing a Clementsian‐type response (groups of species that replace each other along the environmental gradients) following temperature and latitude ordering. The Clementsian pattern determined by site temperatures reflects the turnover from a group of cold‐adapted species to one of warm‐adapted species, separated by many species with more generalised temperature requirements. Similarly, the Clementsian pattern associated with the latitude gradient indicates the substitution of a low‐latitude group in the south with a high‐latitude group in the north. The sites ordered by landcover did not show significant coherence and turnover.In a macro‐scale framework, Odonata species assemblages seem to be sensitive to the climatic and geographic variables of local sites (i.e., temperature and latitude), regardless of the surrounding habitat typology. The role of such variables in shaping the assembly of Odonata communities should be considered in large‐scale management and conservation projects. [ABSTRACT FROM AUTHOR]

The regional variability in tundra and boreal carbon dioxide (CO2) fluxes can be high, complicating efforts to quantify sink‐source patterns across the entire region. Statistical models are increasingly used to predict (i.e., upscale) CO2 fluxes across large spatial domains, but the reliability of different modeling techniques, each with different specifications and assumptions, has not been assessed in detail. Here, we compile eddy covariance and chamber measurements of annual and growing season CO2 fluxes of gross primary productivity (GPP), ecosystem respiration (ER), and net ecosystem exchange (NEE) during 1990–2015 from 148 terrestrial high‐latitude (i.e., tundra and boreal) sites to analyze the spatial patterns and drivers of CO2 fluxes and test the accuracy and uncertainty of different statistical models. CO2 fluxes were upscaled at relatively high spatial resolution (1 km2) across the high‐latitude region using five commonly used statistical models and their ensemble, that is, the median of all five models, using climatic, vegetation, and soil predictors. We found the performance of machine learning and ensemble predictions to outperform traditional regression methods. We also found the predictive performance of NEE‐focused models to be low, relative to models predicting GPP and ER. Our data compilation and ensemble predictions showed that CO2 sink strength was larger in the boreal biome (observed and predicted average annual NEE −46 and −29 g C m−2 yr−1, respectively) compared to tundra (average annual NEE +10 and −2 g C m−2 yr−1). This pattern was associated with large spatial variability, reflecting local heterogeneity in soil organic carbon stocks, climate, and vegetation productivity. The terrestrial ecosystem CO2 budget, estimated using the annual NEE ensemble prediction, suggests the high‐latitude region was on average an annual CO2 sink during 1990–2015, although uncertainty remains high. [ABSTRACT FROM AUTHOR]

The oxygen isotopic composition of tree‐ring cellulose (δ18Ocell) has been widely used to reconstruct historical environmental changes; however, the control factors on δ18Ocell have not been fully constrained—especially in high latitudes. To evaluate the influence of metabolic processes and related environmental factors on δ18Ocell, we analyzed the δ18O values of soil water (1998–2015), stem water (1997–2016), leaf water (3 days in each 2014 and 2015), and tree‐ring cellulose (1981–2016), on a dominant larch species (Larix cajanderi) in an eastern Siberian boreal forest. We determined that the δ18O variability of water sources is dependent on the precipitation, and the 18O enrichment in leaf water is determined by relative humidity. These findings suggest that both water source uptake and leaf water enrichment processes can affect the δ18O values of oxygen‐containing compounds in larch trees in the study site. However, the δ18Ocell (one of the end oxygen‐containing products) was found dependent on water sources on which was related to the amount of summer rainfall in the previous year. This finding significantly differs from that of studies in other areas, which infer the positive correlation between δ18Ocell and leaf water enrichment rather than precipitation or water sources. These differences are predominantly due to the specific conditions of high‐latitude areas, such as the large seasonal δ18O difference in precipitation, the existence of permafrost, and the low growth rate. Our findings contribute toward the development of tree‐ring paleoclimate reconstructions—especially in eastern Siberia. Plain Language Summary: Scientists often find correlations between the oxygen isotopic composition of tree‐ring cellulose (δ18Ocell) and environmental parameters, such as temperature, precipitation, and relative humidity (RH), etc, and δ18Ocell become a good indicator for paleoclimate reconstruction. However, the trend of these correlations is not always the same among different sites. What and why has not been clearly clarified, especially in high latitudes. In this study, we focused on the δ18Ocell of larch trees and most related environmental factors in eastern Siberia. We also attempted to explain why the correlations occurred through the aspects of isotopic fractionation processes and compounds related to oxygen utilization. For the most important result, we found positive correlations between pre‐rain (previous summer rainfall) and δ18Ocell. We also found source water δ18O was positively dependent on the amount of pre‐rain, meanwhile, leaf water δ18O was negatively related to RH. These suggest that δ18Ocell are highly dependent on water sources stored in permafrost from previous year rather than leaf water evapotranspiration. The contradicts to the previous study is considered as the particular environmental conditions in our study site, including the large temperature variability, low snow amount, and slow growth. The conclusion can significantly contribute toward paleoclimate reconstructions in close sites. [ABSTRACT FROM AUTHOR]

Boreal peatlands are critical ecosystems globally because they house 30%–40% of terrestrial carbon (C), much of which is stored in permafrost soil vulnerable to climate warming‐induced thaw. Permafrost thaw leads to thickening of the active (seasonally thawed) layer and alters nutrient and light availability. These physical changes may influence community‐level plant functional traits through intraspecific trait variation and/or species turnover. As permafrost thaw is expected to cause an efflux of carbon dioxide (CO2) and methane (CH4) from the soil to the atmosphere, it is important to understand thaw‐induced changes in plant community productivity to evaluate whether these changes may offset some of the anticipated increases in C emissions. To this end, we collected vascular plant community composition and foliar functional trait data along gradients in aboveground tree biomass and active layer thickness (ALT) in a rapidly thawing boreal peatland, with the expectation that changes in above‐ and belowground conditions are indicative of altered resource availability. We aimed to determine whether community‐level traits vary across these gradients, and whether these changes are dominated by intraspecific trait variation, species turnover, or both. Our results highlight that variability in community‐level traits was largely attributable to species turnover and that both community composition and traits were predominantly driven by ALT. Specifically, thicker active layers associated with permafrost‐free peatlands (i.e., bogs and fens) shifted community composition from slower‐growing evergreen shrubs to faster‐growing graminoids and forbs with a corresponding shift toward more productive trait values. The results from this rapidly thawing peatland suggest that continued warming‐induced permafrost thaw and thermokarst development alter plant community composition and community‐level traits and thus ecosystem productivity. Increased productivity may help to mitigate anticipated CO2 efflux from thawing permafrost, at least in the short term, though this response may be swamped by increase CH4 release. [ABSTRACT FROM AUTHOR]

Le CEN est ensuite reconnu officiellement le 2 août 1961 par un arrêté en conseil de la Chambre du Conseil exécutif du gouvernement du Québec qui confirme la toute première subvention du CEN pour la réalisation de travaux de recherche dans le Nord du Québec. Photographies: D. Sarrazin Présence physique du CEN dans le Nord En plus de ce regroupement stratégique de scientifiques à expertises multiples et de partenaires communautaires, le CEN a mis en place un réseau unique de stations de recherche ainsi qu'un réseau de suivi environnemental le long d'un gradient sud-nord, de la forêt boréale à l'Arctique, sur près de 26 degrés de latitude (Figure 2). Ce bateau a été nommé le Louis-Edmond Hamelin en l'honneur du fondateur du CEN Aujourd'hui, la mission du CEN est de relever les défis de la recherche sur la nordicité en partenariat avec les communautés autochtones en contribuant au développement durable des régions nordiques par l'amélioration de la compréhension des environnements froids et de la capacité à prédire les changements qui les affectent (CEN [3]). The CEN was then officially recognized on 2 August 1961 by an Order in Council of the Executive Council Chamber of the Government of Quebec, which provided the very first CEN grant to fund research in northern Quebec. Following an exhausting process undertaken by Hamelin ([8]), the council of Laval University founded the Centre d'études nordiques (CEN) in April 1961. [Extracted from the article]

Large herbivore grazing has been shown to substantially alter tundra soil and vegetation properties as well as carbon fluxes, yet observational evidence to quantify the impact of herbivore introduction into Arctic permafrost ecosystems remains sparse. In this study we investigated growing season CO2 and CH4 fluxes with flux chambers on a former wet tussock tundra inside Pleistocene Park, a landscape experiment in Northeast Siberia with a 22 year history of grazing. Reference data for an undisturbed system were collected on a nearby ungrazed tussock tundra. Linked to a reduction in soil moisture, topsoil temperatures at the grazed site reacted one order of magnitude faster to changes in air temperatures compared to the ungrazed site and were significantly higher, while the difference strongly decreased with depth. Overall, both GPP (gross primary productivity, i.e. CO2 uptake by photosynthesis) and Reco (ecosystem respiration, i.e. CO2 release from the ecosystem) were significantly higher at the grazed site with notable variations across plots at each site. The increases in CO2 component fluxes largely compensated each other, leaving NEE (net ecosystem exchange) similar across grazed and ungrazed sites for the observation period. Soil moisture and CH4 fluxes at the grazed site decreased over the observation period, while in contrast the constantly water- logged soils at the ungrazed site kept CH4 fluxes at significantly higher levels. Our results indicate that grazing of large herbivores promotes topsoil warming and drying, effectively accelerating CO2 turnover while decreasing methane emissions. Our experiment did not include autumn and winter fluxes, and thus no inferences can be made for the annual NEE and CH4 budgets at tundra ecosystems. [ABSTRACT FROM AUTHOR]

Climate change is the greatest global threat to ecosystems on the Earth. Previous studies assessed the impacts of climate change on sensitive tropical coral reefs, extreme environments in European Alps and the Arctic with a focus on snow and permafrost. This article reflects on the past developments and future challenges for scientific research and policy response relating to these topics from a peer's perspective. This leads to the identification of several warning signs for potentially dangerous developments in these sensitive system and extreme environments as well as opportunities for research and policy in the future. While urgent actions are required to limit global warming, science-based policy can provide needed guidance. [ABSTRACT FROM AUTHOR]

The subnivium is the seasonal microhabitat at the snow–ground interface and serves as a refuge for a diversity of species. Increasingly warmer winters are disrupting the continuity of snow cover, and likely the stability of the subnivium. To examine how the extent and duration of this sensitive and widespread below‐snow habitat will shift under warming winters, we deployed fully automated winter greenhouses across latitudinal and land‐cover gradients to monitor subnivium conditions under different climate‐change scenarios and predicted subnivium occurrence across the Great Lakes region. In a +3°C warming scenario, we found little change in subnivium extent or duration, whereas a +5°C scenario produced widespread and marked reductions in both extent (loss of 200,000 km2, a 45% decline) and duration (>1 month decline). Although the subnivium appears resilient to moderate future warming, we predict abrupt and extensive changes in response to increased winter warming, which will have sweeping ecological and environmental consequences in high‐latitude ecosystems. [ABSTRACT FROM AUTHOR]

Conserving biodiversity in an era of rapid climate change requires understanding the mechanisms that influence dispersal, gene flow and, ultimately, species persistence. This information is becoming critical for conserving key species in rapidly warming places such as the Arctic. Arctic freshwater fish not only face warmer conditions, but also the drying of tundra streams due to climate change. Here, we examined population structure, gene flow, and the influence of landscape features on the neutral genetic variation of the Arctic grayling on Alaska's North Slope. We estimated the number of genetically distinct clusters and determined effective population sizes for and patterns of gene flow among geographic regions. We predicted that river distance, river drying, distance to the coast, and elevational gradient would influence genetic differentiation for Arctic grayling. Bayesian clustering and discriminant analysis of principal components found support for five or six genetic clusters roughly corresponding to downstream and headwater subwatersheds. Estimates of gene flow revealed asymmetric downstream bias. River distance and river dry zones were significantly associated with increasing genetic differentiation among sampling locations despite this species' high dispersal capability and the temporary nature of dry zones. Isolation and downstream-biased dispersal could contribute to high levels of inter-population genetic variation among the headwaters of the North Slope Arctic grayling metapopulation, which might be particularly important for species conservation during rapid climate change. More generally, small, isolated populations might drive particular alleles to higher frequencies due to selection or drift, thus promoting the genetic potential for rapid evolutionary changes under future climate change. [ABSTRACT FROM AUTHOR]

Human development and warming in the Arctic are increasingly putting new pressures on the abundant freshwater ecosystems in the region. In particular, recent and rapid expansion of thermokarst ponds in permafrost regions has been associated with these stresses. Our goal was to examine the relative impacts of pond enrichment by urban encroachment and thermokarsting, compared to relatively unimpacted regions, on the environmental conditions and zooplankton communities of tundra ponds near Utqiaġvik, AK, USA. Both urban ponds and thermokarst ponds were found to have substantially different environmental conditions, as well as distinctive zooplankton communities relative to reference sites. Thermokarst ponds presented an environment rich in DOC and nutrients (N, P, and Si) and phytoplankton biomass. Enrichment by nutrients and DOC in thermokarst ponds led to significantly greater zooplankton abundance composed largely of Daphnia middendorffiana (66 individuals L−1) and cyclopoid copepods (65 individuals L−1). Urban ponds had moderate nutrient levels, with elevated benthic algal biomass. These ponds had intermediate numbers of D. middendorffianna (14 individuals L−1), as well as significantly higher numbers of diaptomid copepods (8 individuals L−1) and chydorids (11 individuals L−1), as compared to one or both reference regions. The two reference regions [Barrow Environmental Observatory (BEO) and International Biological Program (IBP)] had consistently lower nutrient concentrations, algal levels, and zooplankton abundances (less than 3 individuals L−1). Finally, harpacticoid copepods and Bosmina longirostris were new records for the zooplankton in the region. Further collections are required to determine the drivers for these new observations. These results identify the large potential relative contributions of urban inputs and thermokarsting to enrichment of Arctic aquatic ecosystems and point to thermokarsting, which will likely increase with warming, as a major contributor to change in these systems. [ABSTRACT FROM AUTHOR]

The cryosphere (i.e. glaciers and permafrost) and its related landforms offer a wide range of ecosystem services, thus they have strong relationships with human population. Even if these harsh environments have often been regarded as inhospitable, there is a growing amount of literature on glacial biodiversity, specifically concerning European mountains. Glaciers and permafrost-related landforms (e.g. rock glaciers) host a variety of cold-adapted taxa, from bacteria to vertebrates. They have been included in the Natura 2000 network, specifically in the habitat type: Permanent Glaciers (code 8340), but their biodiversity is still poorly known. Even if local extinctions and population reductions of cold-adapted species due to glacier and permafrost shrinking have been already documented, none of the species living in this habitat type are listed in the Habitat Directive Annexes. With this commentary, we call for urgent actions for an ecological characterization of this habitat type in order to plan monitoring and management of the biodiversity hosted by them. An increased knowledge of this no longer permanent habitat appears particularly urgent, because it is not replaceable and is likely to go extinct in the next decades. [ABSTRACT FROM AUTHOR]

Cajander larch (Larix cajanderi Mayr.) forests of the Siberian Arctic are experiencing increased wildfire activity in conjunction with climate warming. These shifts could affect postfire variation in the density and arrangement of trees and understory plant communities. To better understand how understory plant composition, abundance, and diversity vary with tree density, we surveyed understory plant communities and stand characteristics (e.g., canopy cover, active layer depth, and soil organic layer depth) within 25 stands representing a density gradient of similarly-aged larch trees that established following a 1940 fire near Cherskiy, Russia. Understory plant diversity and mean total plant abundance decreased with increased canopy cover. Canopy cover was also the most important variable affecting individual species' abundances. In general, tall shrubs (e.g., Betula nana subsp. exilis) were more abundant in low-density stands with high light availability, and mosses (e.g., Sanionia spp.) were more abundant in high-density stands with low light availability. These results provide evidence that postfire variation in tree recruitment affects understory plant community composition and diversity as stands mature. Therefore, projected increases in wildfire activity in the Siberian Arctic could have cascading impacts on forest structure and composition in both overstory and understory plant communities. [ABSTRACT FROM AUTHOR]

Climate change is rapidly altering the composition and availability of snow, with implications for snow-affected ecological processes, including reproduction, predation, habitat selection, and migration. How snowpack changes influence these ecological processes is mediated by physical snowpack properties, such as depth, density, hardness, and strength, each of which is in turn affected by climate change. Despite this, it remains difficult to obtain meaningful snow information relevant to the ecological processes of interest, precluding a mechanistic understanding of these effects. This problem is acute for species that rely on particular attributes of the subnivean space, for example depth, thermal resistance, and structural stability, for key life-history processes like reproduction, thermoregulation, and predation avoidance. We used a spatially explicit snow evolution model to investigate how habitat selection of a species that uses the subnivean space, the wolverine, is related to snow depth, snow density, and snow melt on Arctic tundra. We modeled these snow properties at a 10 m spatial and a daily temporal resolution for 3 years, and used integrated step selection analyses of GPS collar data from 21 wolverines to determine how these snow properties influenced habitat selection and movement. We found that wolverines selected deeper, denser snow, but only when it was not undergoing melt, bolstering the evidence that these snow properties are important to species that use the Arctic snowpack for subnivean resting sites and dens. We discuss the implications of these findings in the context of climate change impacts on subnivean species. [ABSTRACT FROM AUTHOR]

Summary: Due to climate warming, alpine ecosystems are changing rapidly. Ongoing upward migrations of plants and thus an increase of easily decomposable substrates will strongly affect the soil microbiome. To understand how belowground communities will respond to such changes, we set up an incubation experiment with permafrost and active soil layers from northern (NW) and southern (SE) slopes of a mountain ridge on Muot da Barba Peider in the Swiss Alps and incubated them with or without artificial root exudates (AREs) at two temperatures, 4°C or 15°C. The addition of AREs resulted in elevated respiration across all soil types. Bacterial and fungal alpha diversity decreased significantly, coinciding with strong shifts in microbial community structure in ARE‐treated soils. These shifts in bacterial community structure were driven by an increased abundance of fast‐growing copiotrophic taxa. Fungal communities were predominantly affected by AREs in SE active layer soils and shifted towards fast‐growing opportunistic yeast. In contrast, in the colder NW facing active layer and permafrost soils fungal communities were more influenced by temperature changes. These findings demonstrate the sensitivity of soil microbial communities in high alpine ecosystems to climate change and how shifts in these communities may lead to functional changes impacting biogeochemical processes. [ABSTRACT FROM AUTHOR]

Producer-decomposer interactions within aquatic biofilms can range from mutualistic associations to competition depending on available resources. The outcomes of such interactions have implications for biogeochemical cycling and, as such, may be especially important in northern peatlands, which are a global carbon sink and are expected to experience changes in resource availability with climate change. The purpose of this study was to evaluate the effects of nutrients and organic carbon on the relative proportion of primary producers (microalgae) and heterotrophic decomposers (bacteria and fungi) during aquatic biofilm development in a boreal peatland. Given that decomposers are often better competitors for nutrients than primary producers in aquatic ecosystems, we predicted that labile carbon subsidies would shift the biofilm composition towards heterotrophy owing to the ability of decomposers to outcompete primary producers for available nutrients in the absence of carbon limitation. We manipulated nutrients (nitrate and phosphate) and organic carbon (glucose) in a full factorial design using nutrient-diffusing substrates in an Alaskan fen. Heterotrophic bacteria were limited by organic carbon and algae were limited by inorganic nutrients. However, the outcomes of competitive interactions depended on background nutrient levels. Heterotrophic bacteria were able to outcompete algae for available nutrients when organic carbon was elevated and nutrient levels remained low, but not when organic carbon and nutrients were both elevated through enrichment. Fungal biomass was significantly lower in the presence of glucose alone, possibly owing to antagonistic interactions with heterotrophic bacteria. In contrast to bacteria, fungi were stimulated along with algae following nutrient enrichment. The decoupling of algae and heterotrophic bacteria in the presence of glucose alone shifted the biofilm trophic status towards heterotrophy. This effect was overturned when nutrients were enriched along with glucose, owing to a subsequent increase in algal biomass in the absence of nutrient limitation. By measuring individual components of the biofilm and obtaining data on the trophic status, we have begun to establish a link between resource availability and biofilm formation in northern peatlands. Our results show that labile carbon subsidies from outside sources have the potential to disrupt microbial coupling and shift the metabolic balance in favour of heterotrophy. The extent to which this occurs in the future will probably depend on the timing and composition of bioavailable nutrients delivered to surface waters with environmental change (e.g. permafrost thaw). [ABSTRACT FROM AUTHOR]

• A halogen-free additive, dibenzyl ether, was successfully introduced into PSCs. • The dibenzyl ether additive could boost the phase composition distribution. • The morphology, exciton dissociation, charge transport ability can be improved. Conventional additives like 1,8-diiodooctane (DIO) and 1-chloronaphthalene (CN) play an essential role in optimization of blend film morphology of polymer solar cells (PSCs). However, they have halogenated elements in their end groups, which are harmful to the environment and humans, limiting large-scale applications. In this work, a novel non-toxic and non-halogen additive, dibenzyl ether (DE), was introduced into PTB7-Th:PC 71 BM based PSCs, and the role of the DE additive was systemically investigated. The results showed that DE additive can not only adjust phase separation and optimize the morphology of blend film, but also promote the mobility of hole and electron. The grazing-incidence small-angle X-ray scattering (GISAXS) showed that the introduction of DE increased the interface area between PTB7-Th and PC 71 BM, which can promote exciton dissociation, and achieve better charge transport. Transient photovoltage (TPV) and transient photocurrent (TPC) measurements show that carrier recombination of the additive-containing devices can be effectively inhibited with improved charge transport efficiency of the active layer. Consequently, the devices with DE additive achieve power conversion efficiency (PCE) of 9.53% and a fill factor (FF) of 70.41%. Our non-toxic and non-halogen additive provides a potential solution for the preparation of environmental-friendly PSCs in the future. [ABSTRACT FROM AUTHOR]