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Permafrost thaw causes the seasonally thawed active layer to deepen, causing the Arctic to shift toward carbon release as soil organic matter becomes susceptible to decomposition. Ground subsidence initiated by ice loss can cause these soils to collapse abruptly, rapidly shifting soil moisture as microtopography changes and also accelerating carbon and nutrient mobilization. The uncertainty of soil moisture trajectories during thaw makes it difficult to predict the role of abrupt thaw in suppressing or exacerbating carbon losses. In this study, we investigated the role of shifting soil moisture conditions on carbon dioxide fluxes during a 13-year permafrost warming experiment that exhibited abrupt thaw. Warming deepened the active layer differentially across treatments, leading to variable rates of subsidence and formation of thermokarst depressions. In turn, differential subsidence caused a gradient of moisture conditions, with some plots becoming consistently inundated with water within thermokarst depressions and others exhibiting generally dry, but more variable soil moisture conditions outside of thermokarst depressions. Experimentally induced permafrost thaw initially drove increasing rates of growing season gross primary productivity (GPP), ecosystem respiration (Reco ), and net ecosystem exchange (NEE) (higher carbon uptake), but the formation of thermokarst depressions began to reverse this trend with a high level of spatial heterogeneity. Plots that subsided at the slowest rate stayed relatively dry and supported higher CO2 fluxes throughout the 13-year experiment, while plots that subsided very rapidly into the center of a thermokarst feature became consistently wet and experienced a rapid decline in growing season GPP, Reco , and NEE (lower carbon uptake or carbon release). These findings indicate that Earth system models, which do not simulate subsidence and often predict drier active layer conditions, likely overestimate net growing season carbon uptake in abruptly thawing landscapes.

Currently, Arctic sea ice thickness (SIT) data with extensive spatiotemporal coverage primarily comes from satellite observations, including CryoSat-2, Soil Moisture and Ocean Salinity (SMOS), and the Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2). The studies of the intercomparison and evaluation of multi-source satellite products in recent years are limited. In this study, three latest version products of ICESat-2, CryoSat-2, and CS2SMOS (a merged product of CryoSat-2 and SMOS) were examined from October to April, between 2018 and 2022. Three types of observation including airborne data from the Operation IceBridge (OIB) and IceBird, and in situ data from Beaufort Gyre Exploration Project (BGEP) are selected as the reference in the evaluation. The intercomparison results show that the mean SIT is generally largest in ICESat-2, second largest in CryoSat-2, and smallest in CS2SMOS. The SIT in CryoSat-2 is closer to the SIT in ICESat-2. The thickness displayed by the three satellite products starts to increase at different freezing months, varying between October and November. The three satellite products demonstrated the strongest agreements in SIT in the Beaufort Sea and Central Arctic regions, and exhibited the most distinct differences in the Barents Sea. In the evaluation with OIB data, three satellite-derived SIT were generally underestimated and CS2SMOS demonstrates the closest match. The evaluation using IceBird data indicates an underestimation for all satellites, with CryoSat-2 showing the best agreement. In the assessment with BGEP data, ICESat-2 displayed a more pronounced degree of overestimation or underestimation compared to the other two satellites, and CS2SMOS exhibited the optimal agreement. Based on the comprehensive consideration, CS2SMOS demonstrated the best performance with the airborne and in situ observational data, followed by CryoSat-2 and ICESat-2. The intercomparison and evaluation results of satellite products can contribute to a further understanding of the accuracies and uncertainties of the latest version SIT retrieval and the appropriate selection and utilization of satellite products. [ABSTRACT FROM AUTHOR]

The importance of uptake of atmospheric methane (CH4) in dry Arctic soils for the total Arctic CH4 budget is unresolved. This is partly due to lack of data on the spatial variability of net CH4 consumption and understanding of the main process drivers. We measured net CH4 consumption in Arctic and subarctic landscapes located in in Disko Bay Area and Kangerlussuaq in Western Greenland and in the St. Elias Range in the Yukon, Canada, respectively. Our aim was to characterize the in situ spatial variability of net CH4 uptake in hitherto unexplored Arctic dry upland soils to explore possible limits and environmental drivers across the Arctic geodiversity. Furthermore, we sampled soil for incubation experiments to investigate how net CH4 oxidation responded to changes in soil moisture in contrasting geomorphic settings and parent geological parent materials. We used a laser-based fast deployable chamber system for flux measurements. All studied sites were net sinks of atmospheric CH4 with an average flux −7.5 ± 5.6 μmol CH4 m-2h−1, that are in the upper range of reported net CH4 uptake fluxes in similar soils of the Arctic. The large observed spatial variability within all studied sites (coefficient of variation 50 – 120 %) highlights the need for careful research design allowing for many spatial replicates to achieve representative values. Sites with an active geomorphic environment (abrasion plateau, riverbeds, mountain tops) generally had lower than average net CH4 uptake. Our incubation studies revealed that subsurface CH4 oxidation is the main driver of net surface-atmosphere exchange and that net CH4 oxidation in these layers responded more to changes to soil moisture than in near surface layers. Our study shows surprisingly similar flux magnitudes of net CH4 uptake across widely different landscape forms and geologic parent material, but responding similarly to soil hydrology and geomorphic disturbance, indicating global controls on the net CH4 oxidation in these dry upland environments.

Permafrost-affected ecosystems of the Arctic-boreal zone in northwestern North America are undergoing profound transformation due to rapid climate change. NASA's Arctic Boreal Vulnerability Experiment (ABoVE) is investigating characteristics that make these ecosystems vulnerable or resilient to this change. ABoVE employs airborne synthetic aperture radar (SAR) as a powerful tool to characterize tundra, taiga, peatlands, and fens. Here, we present an annotated guide to the L-band and P band airborne SAR data acquired during the 2017, 2018, 2019, and 2022 ABoVE airborne campaigns. We summarize the ~80 SAR flight lines and how they fit into the ABoVE experimental design. We provide hyperlinks to extensive maps, tables, and every flight plan as well as individual flight lines. We illustrate the interdisciplinary nature of airborne SAR data with examples of preliminary results from ABoVE studies including: boreal forest canopy structure from tomoSAR data over Delta Junction, AK and the BERMS site in northern Saskatchewan and active layer thickness and soil moisture data product validation. This paper is presented as a guide to enable interested readers to fully explore the ABoVE L55 and P-band SAR data. [ABSTRACT FROM AUTHOR]

Using L-band microwave radiative transfer theory to retrieve ice and snow parameters is one of the focuses of Arctic research. At present, due to limitations of frequency and substrates, few operational microwave radiative transfer models can be used to simulate L-band brightness temperature (TB) in Arctic sea ice. The snow microwave radiative transfer (SMRT) model, developed with the support of the European Space Agency in 2018, has been used to simulate high-frequency TB in polar regions and has obtained good results, but no studies have shown whether it can be used appropriately in the L-band. Therefore, in this study, we systematically evaluate the ability of the SMRT model to simulate L-band TB in the Arctic sea ice and snow environment, and we show that the results are significantly optimized by improving the simulation method. In this paper, we first consider the thermal insulation effect of snow by adding the thermodynamic equation, then use a reasonable salinity profile formula for multi-layer model simulation to solve the problem of excessive L-band penetration in the SMRT single-layer model, and finally add ice lead correction to resolve the large influence it has on the results. The improved SMRT model is evaluated using Operation IceBridge (OIB) data from 2012 to 2015 and compared with the snow-corrected classical L-band radiative transfer model for Arctic sea ice proposed in 2010 (KA2010). The results show that the SMRT model has better simulation results, and the correlation coefficient (R) between SMRT-simulated TB and Soil Moisture and Ocean Salinity (SMOS) satellite TB is 0.65, and the RMSE is 3.11 K. Finally, the SMRT model with the improved simulation method is applied to the whole Arctic from November 2014 to April 2015, and the simulated R is 0.63, and the RMSE is 5.22 K. The results show that the SMRT multi-layer model is feasible for simulating L-band TB in the Arctic sea ice and snow environment, which provides a basis for the retrieval of Arctic parameters. [ABSTRACT FROM AUTHOR]

The Arctic soil communities play a vital role in stabilizing and decomposing soil carbon, which affects the global carbon cycling. Studying the food web structure is critical for understanding biotic interactions and the functioning of these ecosystems. Here, we studied the trophic relationships of (microscopic) soil biota of two different Arctic spots in Ny-Ålesund, Svalbard, within a natural soil moisture gradient by combining DNA analysis with stable isotopes as trophic tracers. The results of our study suggested that the soil moisture strongly influenced the diversity of soil biota, with the wetter soil, having a higher organic matter content, hosting a more diverse community. Based on a Bayesian mixing model, the community of wet soil formed a more complex food web, in which bacterivorous and detritivorous pathways were important in supplying carbon and energy to the upper trophic levels. In contrast, the drier soil showed a less diverse community, lower trophic complexity, with the green food web (via unicellular green algae and gatherer organisms) playing a more important role in channelling energy to higher trophic levels. These findings are important to better understand the soil communities inhabiting the Arctic, and for predicting how the ecosystem will respond to the forthcoming changes in precipitation regimes. [ABSTRACT FROM AUTHOR]

Climate-driven permafrost thaw alters the strongly coupled carbon and nitrogen cycles within the Arctic tundra, influencing the availability of limiting nutrients including nitrate (NO3−). Researchers have identified two primary mechanisms that increase nitrogen and NO3− availability within permafrost soils: (1) the ‘frozen feast’, where previously frozen organic material becomes available as it thaws, and (2) ‘shrubification’, where expansion of nitrogen-fixing shrubs promotes increased soil nitrogen. Through the synthesis of original and previously published observational data, and the application of multiple geospatial approaches, this study investigates and highlights a third mechanism that increases NO3− availability: the hydrogeomorphic evolution of polygonal permafrost landscapes. Permafrost thaw drives changes in microtopography, increasing the drainage of topographic highs, thus increasing oxic conditions that promote NO3− production and accumulation. We extrapolate relationships between NO3− and soil moisture in elevated topographic features within our study area and the broader Alaskan Coastal Plain and investigate potential changes in NO3− availability in response to possible hydrogeomorphic evolution scenarios of permafrost landscapes. These approximations indicate that such changes could increase Arctic tundra NO3− availability by ~250–1000%. Thus, hydrogeomorphic changes that accompany continued permafrost degradation in polygonal permafrost landscapes will substantially increase soil pore water NO3− availability and boost future fertilization and productivity in the Arctic.

In reconstructing past changes in precipitation or evaporation from the geologic record, paleoclimatologists sometimes employ the dual‐biomarker isotope method. This method requires that two co‐occurring sedimentary lipids are derived from different biological sources, and that their compound‐specific 2H/1H ratios record different aspects of the hydrologic cycle. Several studies have used this approach, typically by comparing the 2H/1H ratios of n‐alkyl lipids (δ2Hwax) thought to be sourced from aquatic versus terrestrial plants that gain their hydrogen atoms from lake water and soil water, respectively. Yet confidently fingerprinting n‐alkyl lipid sources continues to be a challenge because the wax inputs of different plant types vary across biomes, lake types, and time. New research in the Journal of Geophysical Research‐Biogeosciences by Hollister et al. (2022, https://doi.org/10.1029/2022jg006903) utilizes a combination of three independent metrics to demonstrate that for Arctic lakes, mid‐chain n‐alkanoic acids can be sourced principally from aquatic plants whereas long‐chain n‐alkanoic acids derive from a mixture of aquatic and terrestrial plants. By cataloging wax compound distributions and compound‐specific H and C isotopes of many new plant species, their efforts will strengthen future biomarker paleoclimatology and reinforce previous applications of the dual‐biomarker approach in high‐latitude lakes. The identification of a lake system with a strong aquatic plant wax signal in the sediments should motivate future targeting of similar lakes for reconstructing past moisture with the dual‐biomarker method. Plain Language Summary: Lake sediments accumulate leaf wax molecules made by plants that climate scientists can use to reconstruct past changes in precipitation and evaporation, key climate variables in the Earth system. For scientists to leverage information from these chemical fossils, they need to understand which plants are producing the waxes, and whether those plants gain their water from soils, from lakes, or from another water source. While land plants using soil water can dominate sedimentary wax signals, a recently published study indicates that exceptions occur in Arctic lakes with abundant aquatic plant growth. These advances support paleoclimate research and demonstrate an approach for fingerprinting the source of plant waxes in Arctic lakes. Key Points: Comparison of 2H/1H among differing sedimentary lipids can reveal paleohydrologic insights that single compounds cannotThe biological sources of mid‐chain n‐alkyl lipids to Arctic lake sediments remains debated because of variations between study sitesA new article by Hollister et al. (2022, https://doi.org/10.1029/2022jg006903) documents the contribution of aquatic plants to sedimentary waxes in Qaupat Lake, Baffin Island [ABSTRACT FROM AUTHOR]

Root-zone soil moisture exerts a fundamental control on vegetation, energy balance, and the carbon cycle in Arctic ecosystems, but it is still not well understood in vast, remote, and understudied regions of discontinuous permafrost. The root-zone soil moisture product (30 m resolution) used in this analysis was retrieved from a time-series P-Band (420–440 MHz) synthetic aperture radar (SAR) backscatter observations (August 2017 & October 2017). While similar approaches have been taken to retrieve surface (0 cm to 5 cm) soil moisture from L-Band (1.2 GHz) SAR backscatter, this is one of the first known attempts at reaching the root-zone in permafrost regions. Here, we analyze secondary factors (excluding primary factors, such as precipitation) controlling summer (August) soil moisture at depths of 6 cm, 12 cm, and 20 cm over a 4500 km2 area on the Seward Peninsula of Alaska. Using a random forest model, we quantify the impact of topography, vegetation, and meteorological factors on soil moisture distributions. In developing the random forest model, we explore a variety of feature scales (30 m, 60 m, 90 m, 120 m, 180 m, and 240 m), tune hyperparameters (the structure of individual decision trees making up the ensemble including the number and depth of trees), and perform the final feature selection using cross-validated recursive feature elimination. Results suggest that root-zone soil moisture on the Seward Peninsula is primarily controlled by vegetation at 6 cm, but deeper in the soil column topography and meteorological factors, such as predominant winter wind direction and summer insolation, play a larger role. The random forest model accounts for 40% to 60% of the variation observed (R2 = 0.44 at 6 cm, R2 = 0.52 at 12 cm, R2 = 0.58 at 20 cm). These results indicate that vegetation is the dominant control on soil moisture shallow in the soil column, but the impact of vegetation does not extend to deeper layers retrieved from P-Band SAR backscatter. [ABSTRACT FROM AUTHOR]

Climate-driven permafrost thaw alters the strongly coupled carbon and nitrogen cycles within the Arctic tundra, influencing the availability of limiting nutrients including nitrate (NO3−). Researchers have identified two primary mechanisms that increase nitrogen and NO3− availability within permafrost soils: (1) the 'frozen feast', where previously frozen organic material becomes available as it thaws, and (2) 'shrubification', where expansion of nitrogen-fixing shrubs promotes increased soil nitrogen. Through the synthesis of original and previously published observational data, and the application of multiple geospatial approaches, this study investigates and highlights a third mechanism that increases NO3− availability: the hydrogeomorphic evolution of polygonal permafrost landscapes. Permafrost thaw drives changes in microtopography, increasing the drainage of topographic highs, thus increasing oxic conditions that promote NO3− production and accumulation. We extrapolate relationships between NO3− and soil moisture in elevated topographic features within our study area and the broader Alaskan Coastal Plain and investigate potential changes in NO3− availability in response to possible hydrogeomorphic evolution scenarios of permafrost landscapes. These approximations indicate that such changes could increase Arctic tundra NO3− availability by ~250–1000%. Thus, hydrogeomorphic changes that accompany continued permafrost degradation in polygonal permafrost landscapes will substantially increase soil pore water NO3− availability and boost future fertilization and productivity in the Arctic. [ABSTRACT FROM AUTHOR]

An algorithmic approach, based on satellite-derived sea-surface ("skin") salinities (SSS), is proposed to correct for errors in SSS retrievals and convert these skin salinities into comparable in-situ ("bulk") salinities for the top-5 m of the subpolar and Arctic Oceans. In preparation for routine assimilation into operational ocean forecast models, Soil Moisture Active Passive (SMAP) satellite Level-2 SSS observations are transformed using Argo float data from the top-5 m of the ocean to address the mismatch between the skin depth of satellite L-band SSS measurements (∼1 cm) and the thickness of top model layers (typically at least 1 m). Separate from the challenge of Argo float availability in most of the subpolar and Arctic Oceans, satellite-derived SSS products for these regions currently are not suitable for assimilation for a myriad of other reasons, including erroneous ancillary air-sea forcing/flux products. In the subpolar and Arctic Oceans, the root-mean-square error (RMSE) between the SMAP SSS product and several in-situ salinity observational data sets for the top-5 m is greater than 1.5 pss (Practical Salinity Scale), which can be larger than their temporal variability. Thus, we train a machine-learning algorithm (called a Generalized Additive Model) on in-situ salinities from the top-5 m and an independent air-sea forcing/flux product to convert the SMAP SSS into bulk-salinities, correct biases, and quantify their standard errors. The RMSE between these corrected bulk-salinities and in-situ measurements is less than 1 pss in open ocean regions. Barring persistently problematic data near coasts and ice-pack edges, the corrected bulk-salinity data are in better agreement with in-situ data than their SMAP SSS equivalent. [ABSTRACT FROM AUTHOR]

The importance of uptake of atmospheric methane (CH 4) in dry Arctic soils for the total Arctic CH 4 budget is unresolved. This is partly due to lack of data on the spatial variability of net CH 4 consumption and understanding of the main process drivers. We measured net CH 4 consumption in Arctic and subarctic landscapes located in in Disko Bay Area and Kangerlussuaq in Western Greenland and in the St. Elias Range in the Yukon, Canada, respectively. Our aim was to characterize the in situ spatial variability of net CH 4 uptake in hitherto unexplored Arctic dry upland soils to explore possible limits and environmental drivers across the Arctic geodiversity. Furthermore, we sampled soil for incubation experiments to investigate how net CH 4 oxidation responded to changes in soil moisture in contrasting geomorphic settings and parent geological parent materials. We used a laser-based fast deployable chamber system for flux measurements. All studied sites were net sinks of atmospheric CH 4 with an average flux −7.5 ± 5.6 μmol CH 4 m-2h−1, that are in the upper range of reported net CH 4 uptake fluxes in similar soils of the Arctic. The large observed spatial variability within all studied sites (coefficient of variation 50 – 120 %) highlights the need for careful research design allowing for many spatial replicates to achieve representative values. Sites with an active geomorphic environment (abrasion plateau, riverbeds, mountain tops) generally had lower than average net CH 4 uptake. Our incubation studies revealed that subsurface CH4 oxidation is the main driver of net surface-atmosphere exchange and that net CH 4 oxidation in these layers responded more to changes to soil moisture than in near surface layers. Our study shows surprisingly similar flux magnitudes of net CH 4 uptake across widely different landscape forms and geologic parent material, but responding similarly to soil hydrology and geomorphic disturbance, indicating global controls on the net CH 4 oxidation in these dry upland environments. [ABSTRACT FROM AUTHOR]

Soil moisture has a fundamental influence on the processes and functions of tundra ecosystems. Yet, the local dynamics of soil moisture are often ignored, due to the lack of fine resolution, spatially extensive data. In this study, we modelled soil moisture with two mechanistic models, SpaFHy (a catchment‐scale hydrological model) and JSBACH (a global land surface model), and examined the results in comparison with extensive growing‐season field measurements over a mountain tundra area in northwestern Finland. Our results show that soil moisture varies considerably in the study area and this variation creates a mosaic of moisture conditions, ranging from dry ridges (growing season average 12 VWC%, Volumetric Water Content) to water‐logged mires (65 VWC%). The models, particularly SpaFHy, simulated temporal soil moisture dynamics reasonably well in parts of the landscape, but both underestimated the range of variation spatially and temporally. Soil properties and topography were important drivers of spatial variation in soil moisture dynamics. By testing the applicability of two mechanistic models to predict fine‐scale spatial and temporal variability in soil moisture, this study paves the way towards understanding the functioning of tundra ecosystems under climate change. [ABSTRACT FROM AUTHOR]

Root-zone soil moisture exerts a fundamental control on vegetation, energy balance, and the carbon cycle in Arctic ecosystems, but it is still not well understood in vast, remote, and understudied regions of discontinuous permafrost. The root-zone soil moisture product (30 m resolution) used in this analysis was retrieved from a time-series P-Band (420–440 MHz) synthetic aperture radar (SAR) backscatter observations (August 2017 & October 2017). While similar approaches have been taken to retrieve surface (0 cm to 5 cm) soil moisture from L-Band (1.2 GHz) SAR backscatter, this is one of the first known attempts at reaching the root-zone in permafrost regions. Here, we analyze secondary factors (excluding primary factors, such as precipitation) controlling summer (August) soil moisture at depths of 6 cm, 12 cm, and 20 cm over a 4500 km2 area on the Seward Peninsula of Alaska. Using a random forest model, we quantify the impact of topography, vegetation, and meteorological factors on soil moisture distributions. In developing the random forest model, we explore a variety of feature scales (30 m, 60 m, 90 m, 120 m, 180 m, and 240 m), tune hyperparameters (the structure of individual decision trees making up the ensemble including the number and depth of trees), and perform the final feature selection using cross-validated recursive feature elimination. Results suggest that root-zone soil moisture on the Seward Peninsula is primarily controlled by vegetation at 6 cm, but deeper in the soil column topography and meteorological factors, such as predominant winter wind direction and summer insolation, play a larger role. The random forest model accounts for 40% to 60% of the variation observed (R2 = 0.44 at 6 cm, R2 = 0.52 at 12 cm, R2 = 0.58 at 20 cm). These results indicate that vegetation is the dominant control on soil moisture shallow in the soil column, but the impact of vegetation does not extend to deeper layers retrieved from P-Band SAR backscatter.

Warming environmental conditions are often credited with increasing Arctic shrub growth and altering abundance and distribution, yet it is unclear whether tundra shrub expansion will continue into future decades. Water availability may begin to limit Arctic shrub growth if increasing air temperatures create drier soil conditions due to increased evapotranspiration and permafrost-thaw-induced soil drainage. However, few studies have effectively considered how dominant tundra shrub species respond to variations in both temperature and moisture. To better understand the key effects of temperature variation and soil moisture on two dominant circumpolar deciduous shrubs, we studied shrub growth along a natural landscape gradient in West Greenland, which is a region observed to be drying due to ongoing warming. We found that the growth forms of both grey willow (Salix glauca) and dwarf birch (Betula nana) were sensitive to warmer and drier conditions. For both species, increases in air temperature positively correlated with greater shrub volume, with the doubling of canopy volume due to increased woody biomass. Leaf biomass was best predicted by edaphic features including extractable ammonium, which was positively related to soil moisture, and bulk density. Warmer soils tended to be drier, suggesting that ongoing warming in the area could lead to significant water limitation. Our findings suggest that drier soil conditions might be limiting foliar production despite warming temperatures for two circumpolar dominant shrubs, Betula nana and Salix glauca, which could have wide-ranging, biome-level consequences about ongoing and predicted shrub growth and expansion. [ABSTRACT FROM AUTHOR]

Potential land–climate feedbacks in subarctic regions, where rapid warming is driving forest expansion into the tundra, may be mediated by differences in transpiration of different plant functional types. Here, we assess the environmental controls of overstorey transpiration and its relevance for ecosystem evapotranspiration in subarctic deciduous woodlands. We measured overstorey transpiration of mountain birch canopies and ecosystem evapotranspiration in two locations in northern Fennoscandia, having dense (Abisko) and sparse (Kevo) overstories. For Kevo, we also upscale chamber‐measured understorey evapotranspiration from shrubs and lichen using a detailed land cover map. Subdaily evaporative fluxes were not affected by soil moisture and showed similar controls by vapour pressure deficit and radiation across sites. At the daily timescale, increases in evaporative demand led to proportionally higher contributions of overstorey transpiration to ecosystem evapotranspiration. For the entire growing season, the overstorey transpired 33% of ecosystem evapotranspiration in Abisko and only 16% in Kevo. At this latter site, the understorey had a higher leaf area index and contributed more to ecosystem evapotranspiration compared with the overstorey birch canopy. In Abisko, growing season evapotranspiration was 27% higher than precipitation, consistent with a gradual soil moisture depletion over the summer. Our results show that overstorey canopy transpiration in subarctic deciduous woodlands is not the dominant evaporative flux. However, given the observed environmental sensitivity of evapotranspiration components, the role of deciduous trees in driving ecosystem evapotranspiration may increase with the predicted increases in tree cover and evaporative demand across subarctic regions. [ABSTRACT FROM AUTHOR]

Although cryptogamic covers are important ecosystem engineers in high Arctic tundra, they were often neglected in vegetation surveys. Hence we conducted a systematic survey of cryptogamic cover and vascular plant coverage and composition at two representative, but differing Arctic sites (Ny-Ålesund, Svalbard) along catenas with a natural soil moisture gradient, and integrated these data with physical–chemical soil properties. Soil samples were taken for comprehensive pedological and mineralogical analyses. Vegetation surveys were conducted based on classification by functional groups. Vascular plants were identified to species level. Correlation and multivariate statistical analysis were applied to determine the key environmental factors explaining vegetation patterns along the soil moisture gradients. We observed significant differences in gravimetric water, soil organic matter and nutrient contents along the moisture gradients. These differences were coincident with a shift in vegetation cover and species composition. While chloro- and cyanolichens were abundant at the drier sites, mosses dominated the wetter and vascular plants the intermediate plots. Twenty four vascular plant species could be identified, of which only six were present at both sites. Cryptogamic covers generally dominated with maximum areal coverage up to 70% and hence should be considered as a new additional syntaxon in future ground-truth and remote sensing based vegetation surveys of Svalbard. Multivariate analysis revealed that soil moisture showed the strongest relation between vegetation patterns, together with NH4–N and pH. In conclusion, soil moisture is a key driver in controlling cryptogamic cover and vegetation coverage and vascular plant species composition in high Arctic tundra. [ABSTRACT FROM AUTHOR]

Nitrogen availability in the Arctic strongly influences plant productivity and distribution, and in permafrost systems with patterned ground, ecosystem carbon and nutrient cycling can vary substantially over short distances. Improved understanding of fine-scale spatial and temporal variation in soil N availability is needed to better predict tundra responses to a warming climate. We quantified plant-available inorganic nitrogen at multiple soil depths in 12 microhabitats associated with a gradient from low-center ice-wedge polygons to high-center polygons in coastal tundra at Utqiaġvik (formerly Barrow), Alaska. We measured vegetation composition, biomass, N content, and rooting depth distribution, as well as soil temperature, moisture, pH, and thaw depth to determine relationships between the spatial and temporal variability in N availability and environmental and vegetation drivers. Soil moisture varied across the microhabitats of the polygonal terrain and was the most important variable linked to distribution of both ammonium and nitrate, with ammonium predominating in wetter areas and nitrate predominating in drier areas. Total inorganic N availability increased as the soil in the active layer thawed, but the newly available N near the permafrost boundary late in the season was apparently not available to roots and did not contribute to plant N content. Nitrate in the drier sites also was not associated with plant N content, raising the possibility of N losses from this N-limited ecosystem. The strong relationship between soil moisture, inorganic N availability, and plant N content implies that understanding hydrological changes that may occur in a warming climate is key to determining nutrient cycling responses in complex polygonal tundra landscapes. [ABSTRACT FROM AUTHOR]

Biological soil crusts (biocrusts) naturally coexist with vascular plants in many dryland ecosystems. Although most studies of dryland biocrusts have been conducted in warm deserts, dryland biocrusts also exist in the Arctic, where they may be an important source of nitrogen (N) and carbon (C) to nutrient-limited environments. In Kangerlussuaq, Greenland, wind-driven soil erosion has created a heterogeneous landscape where biocrusts dominate distinct patches of soil but are absent from the surrounding shrub and graminoid tundra. Prior to this study, little was known about the physical development and nutrient cycling of west Greenland biocrusts and their role in maintaining landscape heterogeneity. We characterized the physical properties, lichen assemblages, and nutrient concentrations of biocrusts and underlying soils along gradients in biocrust development and age. We found that biocrusts took 180 ± 40 years to fully develop and that biocrusts became thicker and soil penetration resistance increased as they developed. The N-fixing lichen Stereocaulon sp. was found throughout the study region at all stages of biocrust development. Natural 15N abundance suggests that Stereocaulon sp. obtains about half of its N from biological fixation and that some biologically fixed N is incorporated into the underlying soils over time. Although the N and C concentrations of underlying soils increased slightly with biocrust development, nutrient concentrations under the most developed biocrusts remained low compared to the surrounding vegetated tundra. Our results suggest that biocrusts are a persistent feature and play an important role in maintaining the high spatial heterogeneity of the Kangerlussuaq terrestrial landscape. [ABSTRACT FROM AUTHOR]

Seasonal predictability of the minimum sea ice extent (SIE) in the Laptev Sea is investigated using winter coastal divergence as a predictor. From February to May, the new ice forming in wind-driven coastal polynyas grows to a thickness approximately equal to the climatological thickness loss due to summer thermodynamic processes. Estimating the area of sea ice that is preconditioned to melt enables seasonal predictability of the minimum SIE. Wintertime ice motion is quantified by seeding passive tracers along the coastlines and advecting them with the Lagrangian Ice Tracking System (LITS) forced with sea ice drifts from the Polar Pathfinder dataset for years 1992–2016. LITS-derived landfast ice estimates are comparable to those of the Russian Arctic and Antarctic Research Institute ice charts. Time series of the minimum SIE and coastal divergence show trends of −24.2% and +31.3% per decade, respectively. Statistically significant correlation (r = −0.63) between anomalies of coastal divergence and the following September SIE occurs for coastal divergence integrated from February to the beginning of May. Using the coastal divergence anomaly to predict the minimum SIE departure from the trend improves the explained variance by 21% compared to hindcasts based on persistence of the linear trend. Coastal divergence anomalies correlate with the winter mean Arctic Oscillation index (r = 0.69). LITS-derived areas of coastal divergence tend to underestimate the total area covered by thin ice in the CryoSat-2/SMOS (Soil Moisture and Ocean Salinity) thickness dataset, as suggested by a thermodynamic sea ice growth model. [ABSTRACT FROM AUTHOR]

Arctic moisture intrusions have played an important role in warming the Arctic over the past few decades. A prior study found that Coupled Model Intercomparison Project Phase 5 (CMIP5) models exhibit large regional biases in the moisture flux across 70°N. It is shown here that the systematic misrepresentation of the moisture flux is related to the models' overprediction of zonal wavenumber k = 2 contribution and underprediction of k = 1 contribution to the flux. Models with a warmer tropical upper troposphere and El‐Niño‐like tropical surface temperature tend to simulate stronger k = 2 flux, while k = 1 flux is uncorrelated with tropical upper tropospheric temperature and is associated with La‐Niña‐like surface temperature. The models also overpredict the transient eddy moisture flux while underpredicting the stationary eddy flux. Moreover, future projections in Representative Concentration Pathway 8.5 (RCP8.5) simulations show trends in moisture flux that is consistent with biases in historical simulations, suggesting that these CMIP5 projections reflect the same error(s) that cause the model biases. Plain Language Summary: The Arctic is the region where climate change has been most rapid. A growing body of work indicates that moisture intrusions into Arctic have played an important role in warming the Arctic over the past decades. Coupled Model Intercomparison Project Phase 5 (CMIP5) models have served as a critical tool for projecting future climate changes. Therefore, it is imperative to evaluate whether the physical processes governing moisture intrusions are accurately represented by the models. It is shown here that there is a systematic misrepresentation of the moisture flux into the Arctic related to the models' biased representation of tropical temperatures. Moreover, future projections in Representative Concentration Pathway 8.5 (RCP8.5) simulations show moisture flux trends that are consistent with biases in historical simulations, suggesting that these CMIP5 projections reflect the same error(s) that cause the model biases. It is common practice to regard averages across climate model as being the true response to climate forcing. The findings here question this widespread assumption and underscore the need to pay close attention to model biases and their causes. Key Points: Climate models overpredict (underpredict) zonal‐wavenumber k = 2 (k = 1) contribution to moisture flux across Arctic boundaryThe models with warmer tropical upper troposphere tend to simulate stronger k = 2 flux, while no such relation is present for k = 1 fluxClimate models overpredict transient eddy contribution to the flux at the expense of stationary eddy contribution [ABSTRACT FROM AUTHOR]

Satellite‐derived surface soil moisture data are available for the Arctic, but detailed validation is still lacking. Previous studies have shown low correlations between in situ and modeled data. It is hypothesized that soil temperature variations after soil thaw impact MetOp ASCAT satellite‐derived surface soil moisture (SSM) measurements in wet tundra environments, as C band backscatter is sensitive to changes in dielectric properties. We compare in situ measurements of water content within the active layer at four sites across the Arctic in Alaska (Barrow, Sagwon, Toolik) and Siberia (Tiksi), taken in the spring after thawing and in autumn prior to freezing. In addition to the long‐term measurement fields, where sensors are installed deeper in the ground, we designed a monitoring setup for measuring moisture very close to the surface in the Lena River Delta, Siberia. The volumetric water content (VWC) and soil temperature sensors were placed in the moss organic layer in order to account for the limited penetration depth of the radar signal. ASCAT SSM variations are generally very small, in line with the low variability of in situ VWC. Short‐term changes after complete thawing of the upper organic layer, however, seem to be mostly influenced by soil temperature. Correlations between SSM and in situ VWC are generally very low, or even negative. Mean standard deviation matching results in a comparably high root‐mean‐square error (on average 11%) for predictions of VWC. Further investigations and measurement networks are needed to clarify factors causing temporal variation of C band backscatter in tundra regions. Key Points: Organic layer temperature variations explain temporal behavior of soil moisture derived from C band radar in unfrozen wet tundraTemporal variability of volumetric water content from in situ point measurements is representative over distances of several kilometers in tundraNear‐surface volumetric water content predicted from matched ASCAT data has an average RMSE of 11% across five sites in Alaska and Siberia [ABSTRACT FROM AUTHOR]

This observational study compares seasonal variations of surface fluxes (turbulent, radiative, and soil heat) and other ancillary atmospheric/surface/permafrost data based on in-situ measurements made at terrestrial research observatories located near the coast of the Arctic Ocean. Hourly-averaged multiyear data sets collected at Eureka (Nunavut, Canada) and Tiksi (East Siberia, Russia) are analyzed in more detail to elucidate similarities and differences in the seasonal cycles at these two Arctic stations, which are situated at significantly different latitudes (80.0°N and 71.6°N, respectively). While significant gross similarities exist in the annual cycles of various meteorological parameters and fluxes, the differences in latitude, local topography, cloud cover, snowfall, and soil characteristics produce noticeable differences in fluxes and in the structures of the atmospheric boundary layer and upper soil temperature profiles. An important factor is that even though higher latitude sites (in this case Eureka) generally receive less annual incoming solar radiation but more total daily incoming solar radiation throughout the summer months than lower latitude sites (in this case Tiksi). This leads to a counter-intuitive state where the average active layer (or thaw line) is deeper and the topsoil temperature in midsummer are higher in Eureka which is located almost 10° north of Tiksi. The study further highlights the differences in the seasonal and latitudinal variations of the incoming shortwave and net radiation as well as the moderating cloudiness effects that lead to temporal and spatial differences in the structure of the atmospheric boundary layer and the uppermost ground layer. Specifically the warm season (Arctic summer) is shorter and mid-summer amplitude of the surface fluxes near solar noon is generally less in Eureka than in Tiksi. During the dark Polar night and cold seasons (Arctic winter) when the ground is covered with snow and air temperatures are sufficiently below freezing, the near-surface environment is generally stably stratified and the hourly averaged turbulent fluxes are quite small and irregular with on average small downward sensible heat fluxes and upward latent heat and carbon dioxide fluxes. The magnitude of the turbulent fluxes increases rapidly when surface snow disappears and the air temperatures rise above freezing during spring melt and eventually reaches a summer maximum. Throughout the summer months strong upward sensible and latent heat fluxes and downward carbon dioxide (uptake by the surface) are typically observed indicating persistent unstable (convective) stratification. Due to the combined effects of day length and solar zenith angle, the convective boundary layer forms in the High Arctic (e.g., in Eureka) and can reach long-lived quasi-stationary states in summer. During late summer and early autumn all turbulent fluxes rapidly decrease in magnitude when the air temperature decreases and falls below freezing. Unlike Eureka, a pronounced zero-curtain effect consisting of a sustained surface temperature hiatus at the freezing point is observed in Tiksi during fall due to wetter and/or water saturated soils. [ABSTRACT FROM AUTHOR]

Knowledge of surface hydrology is essential for many applications, including studies that aim to understand permafrost response to changing climate and the associated feedback mechanisms. Advanced remote sensing techniques make it possible to retrieve a range of land-surface variables, including radar retrieved soil moisture (SSM). It has been pointed out before that soil moisture retrieval from satellite data can be challenging at high latitudes, which correspond to remote areas where ground data are scarce and the applicability of satellite data of this type is essential. This study investigates backscatter variability other than associated with changing soil moisture in order to examine the possible impact on soil moisture retrieval. It focuses on issues specific to SSM retrieval in the Arctic, notably variations related to tundra lakes. ENVISAT Advanced Synthetic Aperture Radar (ASAR) Wide Swath (WS, 120 m) data are used to understand and quantify impacts on Metop (AAdvanced Scatterometer (ASCAT, 25 km) soil moisture retrieval during the snow free period. Sites of interest are chosen according to ASAR WS availability, high or low agreement between output from the land surface model ORCHIDEE and ASCAT derived SSM. Backscatter variations are analyzed with respect to the ASCAT footprint area. It can be shown that the low model agreement is related to water fraction in most cases. No difference could be detected between periods with floating ice (in snow off situation) and ice free periods at the chosen sites. The mean footprint backscatter is however impacted by partial short term surface roughness change. The water fraction correlates with backscatter deviations (relative to a smooth water surface reference image) within the ASCAT footprint areas (R = 0.91)

The organization of ecological communities along local environmental gradients provides important information about how such communities may respond to environmental change. In the Arctic, the importance of gradients in shrub cover and soil moisture for non-marine arthropod communities has been clearly demonstrated. By replicating studies along shrub and moisture gradients at multiple elevations and using space-for-time substitution, it is possible to examine how arthropod communities may respond to future environmental change. We collected and identified 4640 adult specimens of spiders and beetles near Narsarsuaq, South Greenland between 8 July and 25 August, 2014 from 112 pitfall traps. The traps were arranged in eight plots covering local gradients in either soil moisture or tall shrub dominance at both low and high elevation. Multivariate generalized linear models revealed that community composition was significantly related to shrub height and soil moisture, and that this relationship varied between low and high elevation. Among the 46 species we found, more species were unique to the high elevation plots than to the low elevation plots, a finding that was most pronounced for spiders in plots along soil moisture gradients. Indicator species analysis corroborated earlier findings of the indicator value of specific species in Greenland and suggested that beetles may serve as better indicators of specific habitats than spiders. The location of plots along local environmental gradients allowed us to detect fine-scale variation in arthropod communities. Together, our results suggest that Arctic arthropod community responses to environmental change may differ among low and high elevation sites. [ABSTRACT FROM AUTHOR]

The aim of this paper is to comprehensively evaluate the abiotic factors that influence changes in the annual growth rates of selected species of tundra plants ( Saxifraga oppositifolia L. and Salix polaris Wahlenb.). The study was conducted in the area of the Fuglebergsletta coastal plain, in the vicinity of the Polish Polar Station (Wedel Jarlsberg Land, SW Spitsbergen). Relationships between the studied phenomenon and basic environmental factors and climate indicators were evaluated. The spatial variation of land surface temperatures (LST) was determined, as were the effects of the physical and chemical properties of soils and the spring melting of snow cover on growth rates. It has been argued that the spatial and seasonal variability of annual growth is determined by the rate at which snow cover disappears and by soil moisture, which determines plants' access to water. Soil moisture depends on soil particle size distribution and weather; it is regulated by the supply of snowmelt water and rainfall as well as by the depth of the top layer of permafrost (thaw depth), which determines the level of groundwater during the growing season. The spatial characteristics of the process of the disappearance of seasonal snow cover are co-determined by the morphology of the substrate and the physical properties of the soil. An important but destructive role is played by thawing episodes, which are increasingly frequent in the winter season, ‘rain-on-snow’ events, and glaze ice. The values of correlation coefficients indicate a positive role for precipitation and negative influence of temperature. The higher the temperature (along with low precipitation), the lesser the extent of plant growth. The observed trend towards warming in polar areas does not inevitably lead to an increase in biomass production. An increase in temperature during the growing season does not necessarily promote plant growth, but rather indicates drought stress caused by the lowering of groundwater levels related to the increase in thaw depth. [ABSTRACT FROM AUTHOR]

The circumpolar expansion of woody deciduous shrubs in arctic tundra alters key ecosystem properties including carbon balance and hydrology. However, landscape-scale patterns and drivers of shrub expansion remain poorly understood, inhibiting accurate incorporation of shrub effects into climate models. Here, we use dendroecology to elucidate the role of soil moisture in modifying the relationship between climate and growth for a dominant deciduous shrub, Salix pulchra, on the North Slope of Alaska, USA. We improve upon previous modeling approaches by using ecological theory to guide model selection for the relationship between climate and shrub growth. Finally, we present novel dendroecology-based estimates of shrub biomass change under a future climate regime, made possible by recently developed shrub allometry models. We find that S. pulchra growth has responded positively to mean June temperature over the past 2.5 decades at both a dry upland tundra site and an adjacent mesic riparian site. For the upland site, including a negative second-order term in the climate-growth model significantly improved explanatory power, matching theoretical predictions of diminishing growth returns to increasing temperature. A first-order linear model fit best at the riparian site, indicating consistent growth increases in response to sustained warming, possibly due to lack of temperature-induced moisture limitation in mesic habitats. These contrasting results indicate that S. pulchra in mesic habitats may respond positively to a wider range of temperature increase than S. pulchra in dry habitats. Lastly, we estimate that a 2°C increase in current mean June temperature will yield a 19% increase in aboveground S. pulchra biomass at the upland site and a 36% increase at the riparian site. Our method of biomass estimation provides an important link toward incorporating dendroecology data into coupled vegetation and climate models. [ABSTRACT FROM AUTHOR]

Impacts of increased winter snowfall and warmer summer air temperatures on nitrous oxide (N 2 O) dynamics in arctic tundra are uncertain. Here we evaluate surface N 2 O dynamics in both wet and dry tundra in West Greenland, subjected to field manipulations with deepened winter snow and summer warming. The potential denitrification activity (PDA) and potential net N 2 O production (N 2 O net) were measured to assess denitrification and N 2 O consumption potential. The surface N 2 O fluxes averaged 0.49 ± 0.42 and 2.6 ± 0.84 μg N 2 O–N m−2 h−1, and total emissions were 212 ± 151 and 114 ± 63 g N 2 O–N scaled to the entire study area of 0.15 km2, at the dry and wet tundra, respectively. The experimental summer warming, and in combination with deepened snow, significantly increased N 2 O emissions at the dry tundra, but not at the wet tundra. The deepened snow increased winter soil temperatures and growing season soil N availability (DON, NH 4 +-N or NO 3 −-N), but no main effect of deepened snow on N 2 O fluxes was found at either tundra ecosystem. The mean PDA was 5- and 121-fold higher than the N 2 O net at the dry and wet tundra, respectively, suggesting that N 2 O might be reduced and emitted as dinitrogen (N 2). Overall, this study reveals modest but evident surface N 2 O fluxes from tundra ecosystems in Western Greenland, and suggests that projected increases in winter precipitation and summer air temperatures may increase N 2 O emissions, particularly at the dry tundra dominating in this region. • We studied climate change effects on N 2 O fluxes in dry and wet tundra West Greenland. • Generally, N 2 O fluxes were relatively modest in both types of vegetation. • Summer warming combined with deepened snow enhanced N 2 O emissions in dry tundra. • Climate change did not affect N 2 O fluxes in wet tundra. [ABSTRACT FROM AUTHOR]

The initial microbiological activity on the forefield of the continuously retreating Werenskiold glacier (West Spitsbergen) along the chronosequence was investigated using both culture-dependent and culture-independent approaches. The prokaryotic cell parameters: total cell number (TCN g −1 d.w.), biomass (μg C g −1 d.w.), average cell volume (ACV, μm 3 ), morphological structure (% cocci, rods, curved cells in TCN) and the contribution of viable cells with intact membrane (% in TCN) were established. The number of opportunistic bacteria (CFU g −1 d.w.) was counted using plate culture methodology. The PCR-DGGE of amplified 16S rRNA gene fragments and sequencing were also used to analyse the total (native soil samples) and culturable (plate wash samples) bacterial community structure. A total of 33 partial 16S rRNA gene sequences were obtained from the excised DGGE bands, with the majority of the sequences closely related to Actinobacteria , Bacteroidetes , and Proteobacteria (β and γ) groups. A high proportion of cultured and uncultured Arthrobacter in the studied glacier foreland soil confirms their role in the initiation of soil formation processes. Changes in the structure of both the native soil samples and wash plate samples communities along the chronosequence indicated their participation in young soil formation. Although initial microbial activity is closely associated with a higher proportion of non-cultivated bacteria, less numerous cultivated bacterial strains also feature significantly in the biodiversity. The main factors positively affecting TCN, BB, CFU, and the contribution of viable cells were water, nitrogen, carbon and organic matter content in the soil. In our study we have shown that a large share of the finest-particle-size fractions in the soil (dust, clay) negatively affects the BB and CFU. The native soil samples number of operational taxonomic units (OTUs) was mostly dependent on the pore water composition, and the C:N ratio in the soil, the wash plate samples number of OTUs was mostly connected with organic matter and soil water content. [ABSTRACT FROM AUTHOR]

Arctic ecosystems are characterized by a wide range of soil moisture conditions and thermal regimes and contribute differently to the net methane ( CH4) budget. Yet, it is unclear how climate change will affect the capacity of those systems to act as a net source or sink of CH4. Here, we present results of in situ CH4 flux measurements made during the growing season 2014 on Disko Island (west Greenland) and quantify the contribution of contrasting soil and landscape types to the net CH4 budget and responses to summer warming. We compared gas flux measurements from a bare soil and a dry heath, at ambient conditions and increased air temperature, using open-top chambers ( OTCs). Throughout the growing season, bare soil consumed 0.22 ± 0.03 g CH4-C m−2 (8.1 ± 1.2 g CO2-eq m−2) at ambient conditions, while the dry heath consumed 0.10 ± 0.02 g CH4-C m−2 (3.9 ± 0.6 g CO2-eq m−2). These uptake rates were subsequently scaled to the entire study area of 0.15 km2, a landscape also consisting of wetlands with a seasonally integrated methane release of 0.10 ± 0.01 g CH4-C m−2 (3.7 ± 1.2 g CO2-eq m−2). The result was a net landscape sink of 12.71 kg CH4-C (0.48 tonne CO2-eq) during the growing season. A nonsignificant trend was noticed in seasonal CH4 uptake rates with experimental warming, corresponding to a 2% reduction at the bare soil, and 33% increase at the dry heath. This was due to the indirect effect of OTCs on soil moisture, which exerted the main control on CH4 fluxes. Overall, the net landscape sink of CH4 tended to increase by 20% with OTCs. Bare and dry tundra ecosystems should be considered in the net CH4 budget of the Arctic due to their potential role in counterbalancing CH4 emissions from wetlands - not the least when taking the future climatic scenarios of the Arctic into account. [ABSTRACT FROM AUTHOR]

The Northern Hemisphere is, to a large extent, underlain by permafrost, which is prone to thawing due to rapid warming in the Arctic during the 21st century. In this context, satellite-derived soil moisture data are valuable for modeling purposes. Assessing the applicability of such data at high latitudes is essential but has, until recently, been given little attention. Recent studies have pointed out that seasonal land cover variations and the presence of small water bodies, which are typical in the Arctic, can cause complications for soil moisture retrieval. Here, it is hypothesized that a bias related to water fraction is caused by variations in the water surface roughness. The impact is quantified for the Metop Advanced Scatterometer by investigations of the higher spatial resolution synthetic aperture radar (SAR) data acquired by ENVISAT Advanced SAR over 11 sites across the Siberian Arctic. The bias calculated as an average over time can be explained by the lake fraction: a water fraction higher than 20% causes a bias of more than 10% relative surface soil moisture. This can, to a great extent, be attributed to wind, based on which a bias correction was developed. The correction was applied and evaluated with in situ soil moisture data, which were available from one of the sites: the Lena Delta. Weak results are obtained because water surfaces correspond mainly to rivers at this site. Variations in discharge, water height, and streams may therefore also affect the water surface roughness. [ABSTRACT FROM PUBLISHER]

Arctic winter precipitation is projected to increase with global warming, but some areas will experience decreases in snow accumulation. Although Arctic CH4 emissions may represent a significant climate forcing feedback, long-term impacts of changes in snow accumulation on CH4 fluxes remain uncertain. We measured ecosystem CH4 fluxes and soil CH4 and CO2 concentrations and 13C composition to investigate the metabolic pathways and transport mechanisms driving moist acidic tundra CH4 flux over the growing season (Jun-Aug) after 18 years of experimental snow depth increases and decreases. Deeper snow increased soil wetness and warming, reducing soil %O2 levels and increasing thaw depth. Soil moisture, through changes in soil %O2 saturation, determined predominance of methanotrophy or methanogenesis, with soil temperature regulating the ecosystem CH4 sink or source strength. Reduced snow (RS) increased the fraction of oxidized CH4 (Fox) by 75-120% compared to Ambient, switching the system from a small source to a net CH4 sink (21 ± 2 and -31 ± 1 mg CH4 m-2 season-1 at Ambient and RS). Deeper snow reduced Fox by 35-40% and 90-100% in medium-(MS) and high-(HS) snow additions relative to Ambient, contributing to increasing the CH4 source strength of moist acidic tundra (464 ± 15 and 3561 ± 97 mg CH4 m-2 season-1 at MS and HS). Decreases in Fox with deeper snow were partly due to increases in plant-mediated CH4 transport associated with the expansion of tall graminoids. Deeper snow enhanced CH4 production within newly thawed soils, responding mainly to soil warming rather than to increases in acetate fermentation expected from thaw-induced increases in SOC availability. Our results suggest that increased winter precipitation will increase the CH4 source strength of Arctic tundra, but the resulting positive feedback on climate change will depend on the balance between areas with more or less snow accumulation than they are currently facing. [ABSTRACT FROM AUTHOR]

Brightness temperatures at 1.4 GHz (L-band) measured by the Soil Moisture and Ocean Salinity (SMOS) Mission have been used to derive the thickness of sea ice. The retrieval method is applicable only for relatively thin ice and not during the melting period. Hitherto, the availability of ground truth sea ice thickness measurements for validation of SMOS sea ice products was mainly limited to relatively thick ice. The situation has improved with an extensive field campaign in the Barents Sea during an anomalous ice edge retreat and subsequent freeze-up event in March 2014. A sea ice forecast system for ship route optimisation has been developed and was tested during this field campaign with the ice-strengthened research vessel RV Lance. The ship cruise was complemented with coordinated measurements from a helicopter and the research aircraft Polar 5. Sea ice thickness was measured using an electromagnetic induction (EM) system from the bow of RV Lance and another EM-system towed below the helicopter. Polar 5 was equipped among others with the L-band radiometer EMIRAD-2. The experiment yielded a comprehensive data set allowing the evaluation of the operational forecast and route optimisation system as well as the SMOS-derived sea ice thickness product that has been used for the initialization of the forecasts. Two different SMOS sea ice thickness products reproduce the main spatial patterns of the ground truth measurements while the main difference being an underestimation of thick deformed ice. Ice thicknesses derived from the surface elevation measured by an airborne laser scanner and from simultaneous EMIRAD-2 brightness temperatures correlate well up to 1.5 m which is more than the previously anticipated maximal SMOS retrieval thickness. [ABSTRACT FROM AUTHOR]

The article presents the results of ground temperature measurements taken at 1 cm to 100 cm below ground level on the Kaffiøyra Plain (NW Spitsbergen) from 1 September 2012 to 31 August 2014. Observations of thermal conditions were carried out at three sites located in characteristic polar ecotopes: on a beach, in tundra and on a moraine. The results of ground temperature measurements for the Kaffiøyra Plain were compared to the observations of weather conditions at the nearby meteorological station in Ny-Ålesund. The variability of ground temperature was analysed in annual, seasonal and diurnal courses. These reflected the prevailing meteorological conditions at the time. Substantial differences in ground thermal conditions were found between the ecotopes, which was due to the morphological diversity of the ground, its moisture content, vegetation and snow cover, as well as the depth of the permafrost. [ABSTRACT FROM AUTHOR]

Permafrost thaw can alter the soil environment through changes in soil moisture, frequently resulting in soil saturation, a shift to anaerobic decomposition, and changes in the plant community. These changes, along with thawing of previously frozen organic material, can alter the form and magnitude of greenhouse gas production from permafrost ecosystems. We synthesized existing methane (CH4) and carbon dioxide (CO2) production measurements from anaerobic incubations of boreal and tundra soils from the geographic permafrost region to evaluate large-scale controls of anaerobic CO2 and CH4 production and compare the relative importance of landscape-level factors (e.g., vegetation type and landscape position), soil properties (e.g., pH, depth, and soil type), and soil environmental conditions (e.g., temperature and relative water table position). We found fivefold higher maximum CH4 production per gram soil carbon from organic soils than mineral soils. Maximum CH4 production from soils in the active layer (ground that thaws and refreezes annually) was nearly four times that of permafrost per gram soil carbon, and CH4 production per gram soil carbon was two times greater from sites without permafrost than sites with permafrost. Maximum CH4 and median anaerobic CO2 production decreased with depth, while CO2:CH4 production increased with depth. Maximum CH4 production was highest in soils with herbaceous vegetation and soils that were either consistently or periodically inundated. This synthesis identifies the need to consider biome, landscape position, and vascular/moss vegetation types when modeling CH4 production in permafrost ecosystems and suggests the need for longer-term anaerobic incubations to fully capture CH4 dynamics. Our results demonstrate that as climate warms in arctic and boreal regions, rates of anaerobic CO2 and CH4 production will increase, not only as a result of increased temperature, but also from shifts in vegetation and increased ground saturation that will accompany permafrost thaw. [ABSTRACT FROM AUTHOR]

Nitrogen (N) mineralization, nutrient availability, and plant growth in the Arctic are often restricted by low temperatures. Predicted increases of cold-season temperatures may be important for plant nutrient availability and growth, given that N mineralization is also taking place during the cold season. Changing nutrient availability may be reflected in plant N and chlorophyll content and lead to increased photosynthetic capacity, plant growth, and ultimately carbon (C) assimilation by plants. In this study, we increased snow depth and thereby cold-season soil temperatures in high Arctic Svalbard in two vegetation types spanning three moisture regimes. We measured growing-season availability of ammonium (NH), nitrate (NO), total dissolved organic carbon (DOC) and nitrogen (TON) in soil; C, N, δN and chlorophyll content in Salix polaris leaves; and leaf sizes of Salix, Bistorta vivipara, and Luzula arcuata at peak season. Nutrient availability was significantly higher with increased snow depth in the two mesic meadow vegetation types, but not in the drier heath vegetation. Nitrogen concentrations and δN values of Salix leaves were significantly higher in all vegetation types, but the leaf sizes were unchanged. Leaves of Bistorta and Luzula were significantly larger but only significantly so in one moist vegetation type. Increased N and chlorophyll concentrations in leaves indicate a potential for increased growth (C uptake), supported by large leaf sizes for some species. Responses to cold-season soil warming are vegetation type- and species-specific, with potentially stronger responses in moister vegetation types. This study therefore highlights the contrasting effect of snow in a tundra landscape and has important implications for projections of whole tundra responses to climate change. [ABSTRACT FROM AUTHOR]

The contents of chemical elements in soils and plants collected from the tundra ecosystems of Western Taimyr (Russia) have been studied to determine the natural factors affecting the concentrations of chemical elements in plants and soils. Naturally high contents of Be, Cu, Mn, Nb, Ni, Pb, Sr, V, and Zn have been found in the soils, most likely due to polymetallic ore deposits in the region. The increased accumulation of Fe, Mn, Ni, Pb, and Cd by plants has also been observed, which could be attributed to low soil pH and excessive soil moistening. [ABSTRACT FROM AUTHOR]

Knowledge of surface hydrology is essential for many applications, including studies that aim to understand permafrost response to changing climate and the associated feedback mechanisms. Advanced remote sensing techniques make it possible to retrieve a range of land-surface variables, including radar retrieved soil moisture (SSM). It has been pointed out before that soil moisture retrieval from satellite data can be challenging at high latitudes, which correspond to remote areas where ground data are scarce and the applicability of satellite data of this type is essential. This study investigates backscatter variability other than associated with changing soil moisture in order to examine the possible impact on soil moisture retrieval. It focuses on issues specific to SSM retrieval in the Arctic, notably variations related to tundra lakes. ENVISAT Advanced Synthetic Aperture Radar (ASAR) Wide Swath (WS, 120 m) data are used to understand and quantify impacts on Metop (AAdvanced Scatterometer (ASCAT, 25 km) soil moisture retrieval during the snow free period. Sites of interest are chosen according to ASAR WS availability, high or low agreement between output from the land surface model ORCHIDEE and ASCAT derived SSM. Backscatter variations are analyzed with respect to the ASCAT footprint area. It can be shown that the low model agreement is related to water fraction in most cases. No difference could be detected between periods with floating ice (in snow off situation) and ice free periods at the chosen sites. The mean footprint backscatter is however impacted by partial short term surface roughness change. The water fraction correlates with backscatter deviations (relative to a smooth water surface reference image) within the ASCAT footprint areas (R = 0.91??0.97). Backscatter deviations of up to 5 dB can occur in areas with less than 50% water fraction and an assumed soil moisture related range (sensitivity) of 7 dB in the ASCAT data. The sensitivity is also positively correlated with water fraction in regions with low land-surface model agreement (R = 0.68). A precise quantification of the impact on soil moisture retrieval would, however, need to consider actual soil moisture changes and sensor differences. The study demonstrates that the usage of higher spatial resolution data than currently available for SSM is required in lowland permafrost environments. [ABSTRACT FROM AUTHOR]

Surface and groundwater in large pan-Arctic river basins are changing rapidly. High-quality estimates of these changes are challenging because of the limits on the data quality and time span of satellite observations. Here, the term pan-Arctic river refers to the rivers flowing to the Arctic Ocean basin. In this study, we provide a new evaluation of groundwater storage (GWS) changes in the Lena, Ob, Yenisei, Mackenzie and Yukon River basins from the GRACE total water storage anomaly product, in situ runoff, soil moisture form models and a snow water equivalent product that has been significantly improved. Seasonal Trend decomposition using Loess was utilized to obtain trends in GWS. Changes in surface water (SW) between 1984 and 2019 in these basins were also examined based on the Joint Research Centre Global Surface Water Transition data. Results suggested that there were great GWS losses in the North American river basins, totaling approximately −219 km3, and GWS gains in the Siberian river basins, totaling ~340 km3, during 2002–2017. New seasonal and permanent SWs are the primary contributors to the SW transition, accounting for more than 50% of the area of the changed SW in each basin. Changes in the Arctic hydrological system will be more significant and various in the case of rapid and continuous changes in permafrost. [ABSTRACT FROM AUTHOR]

At high latitudes, changes in soil moisture could alter soil temperatures independently of air temperature changes by interacting with the snow thermal rectifier. The authors investigated this mechanism with model experiments in the Community Land Model 4 (CLM4) with prescribed atmospheric forcing and vegetation state. Under equilibrium historical conditions, increasing CO2 concentrations experienced by plants from 285 to 857 ppm caused local increases in soil water-filled pore space of 0.1-0.2 in some regions throughout the globe. In permafrost regions that experienced this moistening, vertical- and annual- mean soil temperatures increased by up to 3°C (0.27°C averaged over all permafrost areas). A similar pattern of moistening and consequent warming occurred in simulations with prescribed June-September (JJAS) rainfall increases of 25% over historical values, a level of increase commensurate with projected future rainfall increases. There was a strong sensitivity of the moistening responses to the baseline hydrological state. Experiments with perturbed physics confirmed that the simulated warming in permafrost soils was caused by increases in the soil latent heat of fusion per unit volume and in the soil thermal conductivity due to the increased moisture. In transient Representative Concentration Pathway 8.5 (RCP8.5) scenario experiments, soil warming due to increased CO2 or JJAS rainfall was smaller in magnitude and spatial extent than in the equilibrium experiments. Active-layer deepening associated with soil moisture changes occurred over less than 8% of the current permafrost area because increased heat of fusion and soil thermal conductivity had compensating effects on active-layer depth. Ongoing modeling challenges make these results tentative. [ABSTRACT FROM AUTHOR]

Snow cover plays a major role in the climate, hydrological and ecological systems of the Arctic and other regions through its influence on the surface energy balance (e.g. reflectivity), water balance (e.g. water storage and release), thermal regimes (e.g. insulation), vegetation and trace gas fluxes. Feedbacks to the climate system have global consequences. The livelihoods and well-being of Arctic residents and many services for the wider population depend on snow conditions so changes have important consequences. Already, changing snow conditions, particularly reduced summer soil moisture, winter thaw events and rain-on-snow conditions have negatively affected commercial forestry, reindeer herding, some wild animal populations and vegetation. Reductions in snow cover are also adversely impacting indigenous peoples' access to traditional foods with negative impacts on human health and well-being. However, there are likely to be some benefits from a changing Arctic snow regime such as more even run-off from melting snow that favours hydropower operations. [ABSTRACT FROM AUTHOR]

Ammonium nitrate (NH4NO3) is a common water pollutant associated with many industrial and municipal activities. One solution to reduce exposure of sensitive aquatic systems to nitrogenous compounds is to atomize (atmospherically disperse in fine particles) contaminated water over the Arctic tundra, which will reduce nitrogen loading to surface water. The toxicity of ammonium nitrate to Arctic soils, however, is poorly understood. In the present study, we characterized the biogeochemical toxicity and phytotoxicity of ammonium nitrate solutions in four different Arctic soils and in a temperate soil. Soil was exposed to a range of ammonium nitrate concentrations over a 90-d period. Dose responses of carbon mineralization, nitrification, and phytotoxicity endpoints were estimated. In addition to direct toxicity, the effect of ammonium nitrate on ecosystem resilience was investigated by dosing nitrogen-impacted soils with boric acid. Ammonium nitrate had no effect on carbon mineralization activity and only affected nitrification in one soil, a polar desert soil from Cornwallis Island, Northwest Territories, Canada. In contrast, ammonium nitrate applications (43 mmol N/L soil water) significantly impaired seedling emergence, root length, and shoot length of northern wheatgrass (Elymus lanceolatus). Concentrations of ammonium nitrate in soil water that inhibited plant parameters by 20% varied between 43 and 280 mmol N/L soil water, which corresponds to 2,100 to 15,801 mg/L of ammonium nitrate in the application water. Arctic soils were more resistant to ammonium nitrate toxicity compared with the temperate soil under these study conditions. It is not clear, however, if this represents a general trend for all polar soils, and because nitrogen is an essential macronutrient, nitrogenous toxicity likely should be considered as a special case for soil toxicity. [ABSTRACT FROM AUTHOR]

Ice-wedge polygons and drained thaw lake basins cover large portions of the Barrow Peninsula of Alaska. These landforms lead to variations in topography in the relatively flat coastal plain. These variations in topography, while small, lead to large variations in soil moisture, which have a substantial impact on carbon, water, and energy fluxes in these systems. In this study, we hypothesize that the spatial patterns of soil moisture control variations in vegetation production, and hence remotely sensed greenness. We investigate the relationship between soil moisture and a remotely sensed measure of vegetation greenness, the normalized difference vegetation index (NDVI). Soil moisture was measured in the summer of 2000 in the fetch of an eddy flux tower (0.5 km2 footprint), and NDVI data were collected using an aerial digital multispectral camera system. Results indicate that the relationship between soil moisture and NDVI was significant in areas dominated by high-centered polygons and troughs, and where the general slope of the terrain promoted the redistribution of soil moisture. However, in areas with little to no relief, the correlation between soil moisture and NDVI was not significant. [ABSTRACT FROM AUTHOR]

Abstract: Bioremediation is often considered to be a feasible clean-up method for hydrocarbon contaminated sites. Several factors such as low temperatures and limited nutrient and moisture availability, are supposed to reduce hydrocarbon biodegradation in contaminated Arctic soils. In previous studies, fertilizing Arctic soils has led to both increased and decreased hydrocarbon mineralization. Identifying limiting factors is therefore considered to be of importance for increased bioremediation efficiency. In this laboratory study, the effects of increased salinity (ionic strength) and varying concentrations of nutrient and soil moisture on hexadecane mineralization in a hydrocarbon contaminated and nutrient deficient high Arctic soil were assessed. Ammonium chloride (NH4Cl) was added to give nitrogen concentrations ranging from 0 to 1000 mg NH4–N/kg soil, corresponding to molar cation concentrations of NH4 + from 0 to 71 mmol/kg soil. Parallel experiments with equivalent cation concentrations of Na+ were obtained by adding sodium chloride (NaCl). Soil samples with combinations of NH4 + and Na+ were also included. Soil moisture content varied from 10 to 20%. Fertilizing with NH4–N increased the total hexadecane mineralization compared to unfertilized soil at all concentrations investigated. Increased salinity in soil caused by addition of NH4 + and Na+ extended the acclimation time but did not have an inhibitory effect on the total hexadecane mineralization at the end of the experiment. After 36 days incubation the total hexadecane mineralization was about 12% of initial concentration in all soil samples added NH4 +. In soil samples added Na+ the mineralization was similar to unfertilized control, about 3.5%. In soils with highest moisture content (20%) the total hexadecane mineralization was reduced compared to soils with 10, 12 and 15% moisture, presumably as a consequence of oxygen shortage. Highest mineralization rates were found in soil samples added 50–200 mg NH4–N/kg at 10% moisture, where 50–58 mg hexadecane/kg/day was mineralized. [Copyright &y& Elsevier]

1 The response of peat-rich permafrost soils to human-induced climate change may be especially important in modifying the global C-flux. We examined the Holocene developmental record of a High Arctic peat-forming wetland to investigate its sensitivity to past climate change and aid understanding of the likely effects of future climate warming on high-latitude ecosystems. 2 The microhabitat of mosses was quantified in the present-day polygon-complex at Bylot Island (73° N, 80° W) and used to interpret the radiocarbon-dated macrofossil record of three cores, comprising c. 3500 years of wetland development. Recurrent wet and dry phases in the reconstructed palaeohydrological record indicated pronounced temporal variability. Wet and dry phases were compared between cores and with palaeoclimatic proxy values, measured as percentage melt and δ18O in nearby ice cores. 3 Periodic wet and dry phases appear unrelated to past climate over c. 50% of the combined stratigraphic records, and are attributable instead to geomorphological mechanisms. At other times, association of wet and dry phases with significantly lower and higher values of percentage melt and δ18O indicate a possible effect of past climate change on polygon hydrology and vegetation, although inconsistencies between cores suggest that local geomorphological processes continued to modify a regional climatic effect. However, during a period incorporating the Little Ice Age ( c. 305–530 cal. yearsbp), reconstructed moisture and vegetation change is pronounced and consistent among all three cores. 4 The results provide strong evidence for the sensitivity of a High Arctic terrestrial ecosystem to past climate change during the Holocene. The estimated magnitude of changes in soil moisture between wet and dry phases is sufficient to imply recurrent shifts in wetland function, periodically impacted upon by pronounced climatic variability, although controlled principally by autogenic processes. The structure and function of such wetlands may therefore be susceptible to predicted, human-induced climate warming. [ABSTRACT FROM AUTHOR]

The Arctic coastal plain of Alaska is characterized by marked heterogeneity in microtopography and above ground vegetation productivity at a variety of scales. This heterogeneity may be expected to lead to large variations in near surface soil moisture and have a substantial impact on measured and modeled fluxes of carbon and water. In this study, we hypothesized that microtopography was the primary control over the spatial patterns of near surface soil moisture. Near surface soil moisture measurements were collected in the summers of 2000, 2001, 2002 and 2003 in the fetch of an eddy flux tower (0.5 km²). Results confirmed the expected relationship between intra- and inter-seasonal variations in near surface soil moisture and variations in precipitation. However, over two time periods, near surface soil moisture increased without corresponding measured precipitation inputs and this was attributed to fog and dew, which are difficult to measure, and/or the melting of the active layer. Spatial variations in near surface soil moisture are largely controlled by microtopography in areas characterized by high centered polygons and troughs. In areas without large variations in microtopography, macrotopography, in the form of drained thaw lakes, has a substantial control over near surface soil moisture. [ABSTRACT FROM AUTHOR]

We report on the carbon and oxygen stable isotope composition of fossil tree material collected at the White Mountain locality of the Buchanan Lake Formation on Axel Heiberg Island in the High Arctic of Canada. The fossils are Middle Eocene in age and have been permineralized with carbonate. Microscopic examination of fossils revealed them to be the remains of Metasequoia stems, composed of secondary carbonate (calcite) and original wood intermingled at the cellular level. Because the specimens show little compression, crushing, or tissue degradation, we believe that carbonate permineralization occurred soon after burial, and therefore provides insight into Eocene carbon cycling at the locality. The carbon isotope signature of the carbonate suggests that methanogenesis resulted in a 13C-enriched CO2 pool that equilibrated with soil water and gave rise to unusually 13C-enriched CaCO3. Tree fossil carbonate exhibited strikingly high δ13C values (+4.0 to +7.4‰) compared to published Phanerozoic pedogenic carbonate δ13C values. These δ13C values, in conjunction with fractionation factors (α) previously determined for carbonate precipitation and methanogenic pathways, indicate an Eocene soil CO2 pool containing 80–95% CO2 produced as a by-product of acetate-fermentation methanogenesis. Because methane in the atmosphere is a powerful greenhouse gas, we suggest that methane emissions from Axel Heiberg soils contributed to the relatively warm Arctic climate during the Middle Eocene. [Copyright &y& Elsevier]

Carbon fluxes between natural ecosystems and the atmosphere have received increased attention in recent years due to the impact they have on climate. In order to investigate independently how soil moisture and temperature control carbon fluxes into and out of a dry subarctic dwarf shrub dominated heath, monoliths containing soil and plants were incubated at three different moisture levels and subjected to four different temperature levels between 7 and 20 °C. Ecosystem CO2 exchange was monitored continuously day and night during the 16 to 18 days that each of three experiments lasted. Additionally, the carbon allocation pattern of the plants was investigated by labelling monoliths with 14CO2 followed by harvest of above and below ground plant parts. The results revealed that the three different soil moisture levels caused distinctly differing levels of CO2 fluxes. Also, both carbon fixation calculated as gross ecosystem production (GEP) and carbon release measured as ecosystem respiration (ER) increased with increasing temperatures, with ER increasing faster than GEP. Hence, short term carbon loss from the ecosystem accelerated with raised temperatures. Temperature sensitivity of the ecosystem was dependent on the soil moisture level, shown by differing Q10 values of both GEP and ER at different soil moisture levels. It is therefore highly important to take soil moisture levels into consideration when evaluating responses of ecosystem carbon balance to changes in temperature. The greatest C fixation took place via the two most dominant species of the ecosystem, Vaccinium uliginosum and Empetrum hermaphroditum, with the former being responsible for the different size of C fixation at the three moisture levels. [ABSTRACT FROM AUTHOR]

Ellenberg's bio-indication system for soil moisture (F), soil nitrogen (N)and soil reaction (R)was examined, based on 559 vegetation samples and environmental characteristics (vegetation cover, soil depth, soil moisture, chemical soil properties) from four Faroe islands. The original indicator values from central Europe were used for the calculation of weighted community indicator values of F, N and R. These were regressed with respect to environmental data, applying standard curvilinear regression and generalized linear modelling (GLM) and new predicted values of community indicator values were obtained from the best model. Faroe species optima values of 162 taxa for one or more of the three Ellenberg scales were derived from fitting Huisman-Olff-Fresco (HOF) models of species abundance with respect to predicted community indicator values and are proposed as new Ellenberg species indicator values to be used in the Faroe Islands. F was best correlated with a GLM model containing soil moisture, organic soil fraction, soil depth and total vegetation cover, R with a GLM model containing pH and calcium in %organic soil fraction, N with total phosphorus in % organic soil fraction. The calibrated species indicator scales are much truncated, as compared with the original values, resulting in significantly different overall distributions of the original and new species indicator values. The recalculated community indicator values are much better correlated to environmental measurements. Several species do not have clear optima, but linear or monotone relationships to the examined indicator scales. This probably indicates that the occurrence of some species in the Faroe Islands are either determined by factors other than moisture, pH or soil nutrient status or, given the young age and environmental instability of the islands, are governed by stochastic mechanisms. Extension of Ellenberg indicator values outside central Europe should always be carefully ... [ABSTRACT FROM AUTHOR]

Differential interferometry of synthetic aperture radar (DInSAR) can be used to generate high-precision surface displacement maps in continuous permafrost environments, capturing isotropic surface subsidence and uplift associated with the seasonal freeze and thaw cycle. We generated seasonal displacement maps using DInSAR with ultrafine-beam Radarsat-2 data for the summers of 2013, 2015, and 2019 at Cape Bounty, Melville Island, and examined them in combination with a land-cover classification, meteorological data, topographic data, optical satellite imagery, and in situ measures of soil moisture, soil temperature, and depth to the frost table. Over the three years studied, displacement magnitudes (estimated uncertainty ± 1 cm) of up to 10 cm per 48-day DInSAR stack were detected. However, generally, the displacement was far smaller (up to 4 cm). Surface displacement was found to be most extensive and of the greatest magnitude in low-lying, wet, and steeply sloping areas. The few areas where large vertical displacements (>2.5 cm) were detected in multiple years were clustered in wet, low lying areas, on steep slopes or ridges, or close to the coast. DInSAR also captured the expansion of two medium-sized retrogressive thaw slumps (RTS), exhibiting widespread negative surface change in the slump floor. [ABSTRACT FROM AUTHOR]