Frequent Freeze-thaw Cycles Research Articles

Annual development of mat-forming conjugating green algae Zygnema spp. in hydro-terrestrial habitats in the Arctic

Conjugating green algae of the genus Zygnema (Zygnematophyceae, Streptophyta) are dominant eukaryotic components of hydro-terrestrial microbial mats in the Arctic. Considering the harsh environmental conditions, the aim of this study was to elucidate mechanisms that enable Zygnema spp. to thrive in this habitat. We hypothesized that changes in morphology, physiological performance, and stress tolerance take place during the annual life cycle of the algae. We thus selected four natural populations of Zygnema spp. on Svalbard and investigated them throughout the vegetation season by means of light microscopy and chlorophyll a fluorescence. Additionally, we also investigated one overwintering population. No formation of specialized resting stages (e.g., dormant zygospores) was observed. Markedly, Zygnema spp. survived harsh periods as modified vegetative cells, i.e., pre-akinetes. Pre-akinetes tolerate both desiccation during summer and freezing in winter. These cells are not dormant and therefore recover their physiological activity immediately after transfer to favorable conditions, undergoing rapid growth in the early spring. Nevertheless, once pre-akinetes begin to grow, these newly produced vegetative cells lose stress tolerance. Such rapid dehardening explains their high mortality due to frequent freeze–thaw cycles in the early spring. Arctic Zygnema spp. thus face a phenological trade-off between missing the early growing season and experiencing frost damage.

Gelifluction and Thixotropy of Maritime Antarctic Soils: Small-Scale Measurements with a Rotational Rheometer

Gelifluction, thixotropy and yield stress were measured from < 5 g soil samples taken from Signy, Alectoria, Greenwich, Wiencke and Livingston islands in the maritime Antarctic using a temperature-controlled rotational rheometer. The small sample size that this method permitted is compatible with sampling from sensitive sample locations. An oscillating 10 Pa shear stress was applied to samples at -0.5 kPa water potential. Two freeze-thaw cycles had temperature ramps from 5°C to -10°C over 2 h, followed by -10°C to 5°C over 2 h and finally at 5°C for 1 h. At freezing onset, the shear modulus, G, dropped to 4–50 per cent of thawed G, with no differences between locations. At thawing onset, G dropped to 8–32 per cent of thawed G, with significant differences between locations (P < 0.001). Thixotropy was then measured by applying a 2 kPa oscillating shear stress for 10 min, followed by relaxation at 10 Pa for 2 h. The increased shear stress caused G to drop to less than 8 per cent of the pre-stressed value, with no difference between locations. After 0.1 and 2 h, G was 18–65 per cent and 31–82 per cent of the pre-stressed value, respectively. A shear ramp determined yield stresses ranging from 494–2217 Pa. These findings demonstrate the potential risk of more frequent freeze-thaw cycles or the occurrence of thawed soil to the stability of polar soils. Gelifluction through more frequent freeze-thaw cycles could result in increased slope movement, whereas thixotropy caused by trampling of thawed soils could exacerbate mechanical damage of surface soils. Copyright

A unique ecological niche fosters hybridization of oak-tree and vineyard isolates of Saccharomyces cerevisiae.

Differential adaptation to distinct niches can restrict gene flow and promote population differentiation within a species. However, in some cases the distinction between niches can collapse, forming a hybrid niche with features of both environments. We previously reported that distinctions between vineyards and oak soil present an ecological barrier that restricts gene flow between lineages of Saccharomyces cerevisiae. Vineyard isolates are tolerant to stresses associated with grapes while North American oak strains are particularly tolerant to freeze-thaw cycles. Here, we report the isolation of S. cerevisiae strains from Wisconsin cherry trees, which display features common to vineyards (e.g. high sugar concentrations) and frequent freeze-thaw cycles. Genome sequencing revealed that the isolated strains are highly heterozygous and represent recent hybrids of the oak × vineyard lineages. We found that the hybrid strains are phenotypically similar to vineyard strains for some traits, but are more similar to oak strains for other traits. The cherry strains were exceptionally good at growing in cherry juice, raising the possibility that they have adapted to this niche. We performed transcriptome profiling in cherry, oak and vineyard strains and show that the cherry-tree hybrids display vineyard-like or oak-like expression, depending on the gene sets, and in some cases, the expression patterns linked back to shared stress tolerances. Allele-specific expression in these natural hybrids suggested concerted cis-regulatory evolution at sets of functionally regulated genes. Our results raise the possibility that hybridization of the two lineages provides a genetic solution to the thriving in this unique niche.

Impact of diurnal freeze–thaw cycles on the soil nematode Scottnema lindsayae in Taylor Valley, Antarctica

Global climate change scenarios predict not only higher temperatures, but also increased climatic variability. In cold regions, these changes may bring about a shift in the frequency of soil freeze–thaw cycles (FTCs), which represent a significant physiological challenge, especially for small, poikilothermic animals with limited mobility. To assess the impact of FTCs on cold-adapted soil biota, we evaluated freeze–thaw dynamics (i.e., 0 °C crossings) and demographics of the dominant nematode Scottnema lindsayae (proportion of adults, population size) over 20 years in soils at two locations in Taylor Valley, Antarctica. Based on hourly soil temperature data, we demonstrate that FTCs are a frequent feature in Taylor Valley, but with high inter-annual and spatial variability. Valley topography and soil moisture were found to impact FTC frequency, suggesting that basins within Taylor Valley have different susceptibilities to environmental variability. Increased FTC frequency in 1999–2001 coincided with a shift in S. lindsayae populations, with fewer juveniles reaching maturity. In the years following decreased adult proportions, overall S. lindsayae numbers were reduced, implying a strong negative effect of FTCs on in situ recruitment. Our results suggest that increased FTC frequency in the Dry Valleys slows S. lindsayae development, reducing reproductive success, and may take years to impact population size, which demonstrates the importance of long-term research to accurately predict the consequences of climate change on soil biota and biogeochemical cycling in the cold regions.

Forest Gaps Inhibit Foliar Litter Pb and Cd Release in Winter and Inhibit Pb and Cd Accumulation in Growing Season in an Alpine Forest.

AimsThe release of heavy metals (such as Pb and Cd) from foliar litter play an important role in element cycling in alpine forest ecosystems. Although natural forest gaps could play important roles in the release of heavy metals from foliar litter by affecting the snow cover during the winter and solar irradiation during the growing season, few studies have examined these potential roles. The objectives of this study were to document changes in Pb and Cd dynamics during litter decomposition in the center of gaps and under closed canopies and to investigate the factors that controlled these changes during the winter and growing seasons.MethodsSenesced foliar litter from six dominant species, including Kangding willow (Salix paraplesia), Masters larch (Larix mastersiana), Mingjiang fir (Abies faxoniana), Alpine azalea (Rhododendron lapponicum), Red birch (Betula albosinensis) and Mourning cypress (Sabina saltuaria), was placed in litterbags and incubated between the gap center and closed canopy conditions in an alpine forest in the eastern region of the Tibetan Plateau. The litterbags were sampled at the snow formation stage, snow coverage stage, snow melt stage and during the growing season. The Pb and Cd concentrations in the sampled foliar litter were determined by acid digestion (HNO3/HClO4).Important findingsOver one year of decomposition, Pb accumulation and Cd release from the foliar litter occurred, regardless of the foliar litter species. However, Pb and Cd were both released from the foliar litter during the winter and accumulated during the growing season. Compared with the gap center and the canopy gap edge, the extended gap edge and the closed canopy showed higher Pb and Cd release rates in winter and higher Pb and Cd accumulation rates during the growing season, respectively. Statistical analyses indicate that the dynamics of Pb were significantly influenced by frequent freeze–thaw cycles in winter and appropriate hydrothermal conditions during the growing season, the dynamics of Cd were strongly influenced by species and the presence of a forest gap at different decomposition stages. These results show that forest gaps could inhibit Pb and Cd release from foliar litter in the alpine forest of western Sichuan. In addition, a decrease in the snow depth in the winter warming scenario would promote the release of Pb during foliar litter decomposition. There exist some difference that may be influenced by litter quality, microenvironment and microtopography during litter decomposition.

Snowed in for survival: Quantifying the risk of winter damage to overwintering field crops in northern temperate latitudes

Autumn-sown field crops have important agronomic advantages (e.g., reduction of soil erosion and nutrient leaching, maximizing the use of spring moisture) and have the potential to be highly productive even though adverse winter conditions can negatively affect crop viability and yield. In the face of the unpredictable weather patterns and the expected shifts in climate in the near future, there is an imperative to develop methods to quantify both the risk of winter damage and how it is affected by altered climatic conditions and crop variety. We propose a set of indices to characterize synthetically the risk of crop damage stemming from cold spells, extended periods at low temperature, frequent occurrence of freeze-thaw cycles, and prolonged snow cover. An existing model of crop hardening and dehardening is further developed to account in full for the variability of lethal threshold temperature among individual plants. This model is coupled to a simple yet realistic description of crop-sensed temperature, so that required inputs are limited to crop-specific responses to low temperature and standard meteorogical data (average daily temperature and snow depth). This framework is applied to winter wheat under the current climatic conditions for central and southern Sweden. The roles of variety-specific hardening ability, temperature, and snow are assessed separately, thus obtaining indications of the potential impacts of variety selection and future predicted changes in temperature and snow cover in the region. Variety-specific hardening ability and response to exposure to low temperature may drastically alter the extent of winter damage. The most prevalent damaging mechanism depends on the climatic regime, with crops in colder areas benefiting from extended snow cover. A tradeoff between temperature (and hence latitude) and snow emerges, with locations at intermediate latitudes subjected to the highest risk of crop damage from exposure to low temperature and frequent freeze–thaw cycles. The same locations are also characterized by the highest inter-annual variability in the extent of winter damage – a fact that has potential implications for yield reliability.

Microbial ecology and biogeochemistry of continental Antarctic soils.

The Antarctica Dry Valleys are regarded as the coldest hyperarid desert system on Earth. While a wide variety of environmental stressors including very low minimum temperatures, frequent freeze-thaw cycles and low water availability impose severe limitations to life, suitable niches for abundant microbial colonization exist. Antarctic desert soils contain much higher levels of microbial diversity than previously thought. Edaphic niches, including cryptic and refuge habitats, microbial mats and permafrost soils all harbor microbial communities which drive key biogeochemical cycling processes. For example, lithobionts (hypoliths and endoliths) possess a genetic capacity for nitrogen and carbon cycling, polymer degradation, and other system processes. Nitrogen fixation rates of hypoliths, as assessed through acetylene reduction assays, suggest that these communities are a significant input source for nitrogen into these oligotrophic soils. Here we review aspects of microbial diversity in Antarctic soils with an emphasis on functionality and capacity. We assess current knowledge regarding adaptations to Antarctic soil environments and highlight the current threats to Antarctic desert soil communities.

Effects of freeze–thaw cycles on phosphorus adsorption and desorption in the black soil of northeastern China

Frequent freeze–thaw cycles (FTCs) can profoundly affect the chemical properties of soils. We conducted laboratory experiments to explore the effects of freeze–thaw treatments on phosphorus (P) adsorption (Pads) and desorption (Pdes) and to determine the capacity of P retention (Pret) via Pads and Pdes in the fertile black soil of northeastern China. The effects of 0, 1, 3, 6, and 10 FTCs on soil P adsorption and desorption were determined. Each cycle consisted of freezing for 12 h at −10 °C and thawing for 12 h at +7 °C, mimicking a diurnal pattern, at initial soil-moisture contents of 20%, 30%, 40%, and 50% and at amounts of added P of 10, 20, 40, 60, and 80 mg P L−1. Pads increased significantly with increasing amounts of added P. The freeze–thaws of black soils significantly decreased Pads and buffering capacities, which would promote the release of adsorbed P and increase the risk of reducing soil P. High moisture content also affected the behavior of soil P adsorption by reducing adsorbed P as the number of FTCs increased. The P-isotherm data for all soils at equilibrium-P concentrations fit the Langmuir equation well (R2 = 0.93 or higher). Pads had exponential and linear relationships with Pdes capacity and Pret capacity, respectively.

Freeze–thaw cycles and soil water content effects on infiltration rate of three Saskatchewan soils

Fouli, Y., Cade-Menun, B. J. and Cutforth, H. W. 2013. Freeze–thaw cycles and soil water content effects on infiltration rate of three Saskatchewan soils. Can. J. Soil Sci. 93: 485–496. Many soils at high latitudes or elevations freeze and thaw seasonally. More frequent freeze–thaw cycles (FTCs) may affect ecosystem diversity and productivity because freeze–thaw cycles cause changes in soil physical properties and affect water movement in the landscape. This study examined the effects of FTCs (0, 1, 5, and 10) and antecedent soil water content [at soil water potentials (SWP) −1.5, −0.033 and −0.02 MPa] on the infiltration rate of three Saskatchewan soils (a clay, a loam, and a loamy sand). A tension infiltrometer was used at tensions [water potentials of the tension infiltrometer (WPT)] −5, −10 and −15 cm. Infiltration rates increased with increasing SWPs for the loam and clay soils due to higher infiltrability into drier soils. Infiltration rates decreased with increasing SWPs for the loamy sand, probably the result of less surface tension, unimodal porosity, and increased gravitational potential. Infiltration rates either decreased or did not change with increasing FTCs, and this may be due to increased water viscosity as temperatures approach freezing. Also, ice may have formed in soil pores after frequent FTCs, causing lower infiltration rates. Infiltration rates for clay at −1.5 MPa were higher than for loam or loamy sand, probably the result of clay mineralogy and potential shrinking and cracking. Soil texture and initial water content had a significant effect on infiltration rates, and FTCs either maintained or lowered infiltration rates.

Temporal dynamics of abiotic and biotic factors on leaf litter of three plant species in relation to decomposition rate along a subalpine elevation gradient.

Relationships between abiotic (soil temperature and number of freeze-thaw cycles) or biotic factors (chemical elements, microbial biomass, extracellular enzymes, and decomposer communities in litter) and litter decomposition rates were investigated over two years in subalpine forests close to the Qinghai-Tibet Plateau in China. Litterbags with senescent birch, fir, and spruce leaves were placed on the forest floor at 2,704 m, 3,023 m, 3,298 m, and 3,582 m elevation. Results showed that the decomposition rate positively correlated with soil mean temperature during the plant growing season, and with the number of soil freeze-thaw cycles during the winter. Concentrations of soluble nitrogen (N), phosphorus (P) and potassium (K) had positive effects but C:N and lignin:N ratios had negative effects on the decomposition rate (k), especially during the winter. Meanwhile, microbial biomass carbon (MBC), N (MBN), and P (MBP) were positively correlated with k values during the first growing season. These biotic factors accounted for 60.0% and 56.4% of the variation in decomposition rate during the winter and the growing season in the first year, respectively. Specifically, litter chemistry (C, N, P, K, lignin, C:N and lignin:N ratio) independently explained 29.6% and 13.3%, and the microbe-related factors (MBC, MBN, MBP, bacterial and fungal biomass, sucrase and ACP activity) explained 22.9% and 34.9% during the first winter and the first growing season, respectively. We conclude that frequent freeze-thaw cycles and litter chemical properties determine the winter decomposition while microbe-related factors play more important roles in determining decomposition in the subsequent growing season.

Impact of early and late winter icing events on sub‐arctic dwarf shrubs

Polar regions are predicted to undergo large increases in winter temperature and an increased frequency of freeze-thaw cycles, which can cause ice layers in the snow pack and ice encasement of vegetation. Early or late winter timing of ice encasement could, however, modify the extent of damage caused to plants. To determine impacts of the date of ice encasement, a novel field experiment was established in sub-arctic Sweden, with icing events simulated in January and March 2008 and 2009. In the subsequent summers, reproduction, phenology, growth and mortality, as well as physiological indicators of leaf damage were measured in the three dominant dwarf shrubs: Vaccinium uliginosum, Vaccinium vitis-idaea and Empetrum nigrum. It was hypothesised that January icing would be more damaging compared to March icing due to the longer duration of ice encasement. Following 2 years of icing, E. nigrum berry production was 83% lower in January-iced plots compared to controls, and V. vitis-idaea electrolyte leakage was increased by 69%. Conversely, electrolyte leakage of E. nigrum was 25% lower and leaf emergence of V. vitis-idaea commenced 11 days earlier in March-iced plots compared to control plots in 2009. There was no effect of icing on any of the other parameters measured, indicating that overall these study species have moderate to high tolerance to ice encasement. Even much longer exposure under the January icing treatment does not clearly increase damage.

Effects of soil frost on growth, composition and respiration of the soil microbial decomposer community

Most climate change scenarios predict that the variability of weather conditions will increase in coming decades. Hence, the frequency and intensity of freeze-thaw cycles in high-latitude regions are likely to increase, with concomitant effect on soil carbon biogeochemistry and associated microbial processes. To address this issue we sampled riparian soil from a Swedish boreal forest and applied treatments with variations in four factors related to soil freezing (temperature, treatment duration, soil water content and frequency of freeze-thaw cycles), at three levels in a laboratory experiment, using a Central Composite Face-centred (CCF) experimental design. We then measured bacterial (leucine incorporation) and fungal (acetate in ergosterol incorporation) growth, basal respiration, soil microbial phospholipid fatty acid (PLFA) composition, and concentration of dissolved organic carbon (DOC). Fungal growth was higher in soil exposed to freeze-thawing perturbations and freezing temperatures of −6 °C and −12 °C, than under more constant conditions (steady 0 °C). The opposite pattern was found for bacteria, resulting in an increasing fungal-to-bacterial growth ratio following more intensive winter conditions. Soil respiration increased with water content, decreased with treatment duration and appeared to mainly be driven by treatment-induced changes in the DOC concentration. There was a clear shift in the PLFA composition at 0 °C, compared with the two lower temperatures, with PLFA markers associated with fungi as well as a number of unsaturated PLFAs being relatively more common at 0 °C. Shifts in the PLFA pattern were consistent with those expected for phenotypic plasticity of the cell membrane to low temperatures. There were small declines in PLFA concentrations after freeze-thawing and with longer durations. However, the number of freeze-thaw events had no effect on the microbiological variables. The findings suggest that the higher frequency of freeze-thaw events predicted to follow the global warming will likely have a limited impact on soil microorganisms.

Lignocellulose Aerogel from Wood-Ionic Liquid Solution (1-Allyl-3-methylimidazolium Chloride) under Freezing and Thawing Conditions

This article reports a facile preparation of a lignocellulose aerogel from a solution of wood in an ionic liquid by cyclic freeze-thaw (FT) process. Trema orientalis wood flour was dissolved in 1-allyl-3-methylimidazolium chloride (AMImCl), an effective ionic liquid, and then repeatedly frozen at -20 °C and thawed at 20 °C for several times, and then finally regenerated in water. The hydrogel obtained was solvent-exchanged to acetone, washed with liquid carbon dioxide, and finally dried by releasing the carbon dioxide at critical temperature to obtain the lignocellulose aerogel. The aerogel had an open 3D fibrillar network and could be transformed from nanofibrillar to sheet-like skeletons with hierarchical micro- and nanoscale morphology and porosity by adjusting the FT treatment cycles. The frequency of FT cycles influenced the intensity, specific surface, crystallinity, and thermostability of the aerogel. This research highlights new opportunities for the development of porous and flexible aerogel scaffolds.

Frequent freeze-thaw cycles yield diminished yet resistant and responsive microbial communities in two temperate soils: a laboratory experiment

Few studies have been conducted on adaptations of microbial communities to low and fluctuating temperatures using environmentally relevant conditions. In this study, six Himalayan and two temperate soils were selected as candidates for low-temperature/freeze-thaw (FT)-adapted and susceptible soils, respectively. Redundancy analysis with forward selection was used to create a model of environmental parameters explaining variability in the initial microbial abundance and 4 °C activities. The best predictor was soil carbon, explaining more than 74% of data variability (P=0.002), despite significant differences in the soil characteristics and environmental history. We tested the hypothesis that the reproduced Himalayan FT fluctuations select physiologically similar communities in distinct soils. Microcosms were experimentally subjected to two separate 50 and 60 FT cycle (FTC) experiments. A significant decrease in abundance, 4 °C basal respiration and drastic rearrangements in community-level physiological profiles (CLPP) were observed in microcosms with temperate soils until 40 FTC. CLPP remained distinct from those of the Himalayan soils. Minor changes were observed in the Himalayan soils, confirming that microbial populations with physiological traits consistent with the noncontinuous permafrost conditions reside in the Himalayan soils, whereas the surviving temperate soil microorganisms actively adjusted to novel environmental conditions.

Characteristics of soil animal community in the subalpine/alpine forests of western Sichuan during onset of freezing

Seasonal freeze–thaw cycle is a common phenomenon in the subalpine/alpine forest region, and may have a significant influence on the structure and function of soil animal community. To understand the characteristics and dynamics of soil animal community as well as its response to repeated freeze–thaw events in this region during onset of freezing, a field experiment was conducted to investigate the composition, abundance, and diversity of soil fauna in primary fir ( Abies faxoniana) forest, fir and birch ( Betula albosinensis) mixed forest and secondary fir ( A. faxoniana) forest, which were three representative forests in the subalpine and alpine forest region in western Sichuan. Soil samples were collected from November 3 to November 27, 2008, which was defined as onset of freezing based on the simultaneous monitoring of soil temperature. Soil macrofauna were picked up by hand in the fields. Mesofauna were separated and collected from the soil samples by Baermann and Tullgren methods, respectively. By preliminary identification, 40,942 individuals were collected, which belonged to 7 phyla, 15 classes and 25 orders in the three sampling forests. 16,557, 14,669 and 9716 individuals were found in primary forest, mixed forest and secondary forest, respectively. In comparison with the mineral soil layer, the soil organic layer had higher density and groups of soil fauna. Furthermore, following the repeated freeze–thaw events, density and groups of soil fauna experienced a distinct decrease in both soil organic layer and mineral soil layer, and this trend in soil organic layer was more significant in the primary forest, owing to higher intense and more frequent freeze–thaw cycles in soil organic layer of the primary forest. The results revealed that soil animals were sensitive bio-indicators to environmental changes, such as repeated freeze–thaw events and dry–wet cycle. Meanwhile, the results also implied that the dynamics of the structure and function of soil animal community during the onset of freezing was of ecological significance to understand the wintertime ecological process in soils.

Increased mid-twentieth century riverbank erosion rates related to the demise of mill dams, South River, Virginia

A recent hypothesis suggests that fluvial processes in areas of the eastern United States are strongly influenced by the demise of colonial mill dams, rather than reflecting a quasi-equilibrium adjustment to the current hydrologic and sediment regime. We evaluated the control of colonial mill dams on twentieth and twenty-first century bank erosion rates on the South River, Virginia, through studies of historical aerial photographs, historical documents, hydrologic and climatic records, and hydraulic modeling. Historical sources and aerial photographs document eight colonial mill dams along the study reach in the early twentieth century; all but one of these dams disappeared in the 1950s, and the last was breached by 1976. From initially low values between 1937 and 1957, mean bank erosion rates increased by more than a factor of 2 after 1957, remaining high through 2005. Accelerated bank erosion rates cannot be explained by changes in storm intensity, the frequency of freeze-thaw cycles, or by changes in the density of riparian trees. Hydraulic modeling suggests that mill dams reduced velocities of the 5 yr flood through ~80% of our study reach. By considering the timing of mill dam loss, the spatial extent of backwater influence, and the locations of our study sites, we find that the loss of mill dams explains the observed trends in bank erosion rates at 9 (and possibly 10) of our 14 monitoring sites. These results support the hypothesis that the demise of mill dams has been an important influence on fluvial processes in the region.

Soil formation in Phobjikha Valley, Central Bhutan with special regard to the redistribution of loessic sediments

Soil development and landscape evolution were studied in the basin-shaped Phobjikha Valley at 2900–3200 m a.s.l., to the west of the Black Mountain Range, West Central Bhutan. The local environmental setting with strong along-valley winds, frequent freeze–thaw cycles, extensive dry periods and sparse vegetation cover seems to encourage the generation and short-distance transport of silt-sized particles. The effects of this process are evidenced in the smooth valley morphology and in the nature of the examined pedons. Their involvement in continuing redistribution of local sediments is reflected by a homogeneous silty-clayey and stone-free texture, varying profile depths, buried topsoils and weakly developed recent A horizons. In protected locations, in situ weathering of metamorphic parent materials results in alu-andic features with bulk densities <0.9 g cm −3, (Al o + ½Fe o) > 2%, and phosphate retention >95%. Dominance by Al-hydroxy interlayered clay minerals and large amounts of well-crystallised iron oxides indicate an advanced stage of weathering. In areas of preferred eolian deposition, argic and ferralic features emerge, with clay contents of up to 60% and surface areas of >50 m 2 g −1. Under forest, umbric horizons can develop. CEC eff is below 10 cmol c kg −1 at all sites. Cluster and factor analyses of soil chemical and physical parameters confirm the redistribution of local sediments as a dominant factor behind the measured variables. No clear indication of glacial activities in the area was found, whereas the massive silty sediments in the lower parts of most profiles, the presence of debris slopes, and the asymmetric cross sections of the side valleys suggest periglacial conditions. Buried topsoils dated at about 2000 conventional 14C years BP indicate a weakening or absence of sediment influx under wetter conditions towards the end of the Holocene climate optimum. Charcoal on top of paleosols suggests that human activities of deforestation, grazing and arable agriculture may have contributed to the reactivation of local sediment redistribution until today.

Responses of Antarctic soil microbial communities and associated functions to temperature and freeze–thaw cycle frequency

Climatic changes will not only result in higher overall temperature, but also in greater variability in weather conditions. Antarctic soils are subjected to extremely variable conditions in the form of frequent freeze-thaw cycles (FTCs), but the importance of alteration in FTC frequency, compared with increases in average temperature and indirect vegetation-mediated effects on soil microorganisms, is still unknown. We therefore designed two complementary microcosm experiments using undisturbed soil cores from Signy Island (60 degrees 43'S, 45 degrees 38'W) in the maritime Antarctic. The experiments consisted of soil core incubations with or without the overlying vegetation at four different temperatures and six different FTC regimes. We assessed bacterial and fungal density and community structure, as well as the density of several key genes in microbial nutrient cycles using a combination of RNA- and DNA-based molecular fingerprinting and quantitative PCR approaches in addition to enzymatic activity assays. Results showed that bacteria were more affected by warming than by changes in FTC frequency. In contrast, fungal community structure and abundance were mostly influenced by FTC frequency, as well as the presence of vegetation cover. The relative densities of several bacterial gene families involved in key steps of the N-cycle were affected by FTCs, while warming had little or no effect. The FTCs and incubation temperature also strongly influenced laccase enzymatic activity in soil. In total, our results suggest that, in addition to climatic warming, increased climatic variability may also have a profound impact on Antarctic microbial communities. Although these effects are difficult to detect with assays of total bacterial community structure, they do become manifest in the analysis of key functional gene densities.

Significance of cold‐season respiration and photosynthesis in a subarctic heath ecosystem in Northern Sweden

AbstractWhile substantial cold‐season respiration has been documented in most arctic and alpine ecosystems in recent years, the significance of cold‐season photosynthesis in these biomes is still believed to be small. In a mesic, subartic heath during both the cold and warm season, we measured in situ ecosystem respiration and photosynthesis with a chamber technique at ambient conditions and at artificially increased frequency of freeze–thaw (FT) cycles during fall and spring. We fitted the measured ecosystem exchange rates to respiration and photosynthesis models with R2‐values ranging from 0.81 to 0.85. As expected, estimated cold‐season (October, November, April and May) respiration was significant and accounted for at least 22% of the annual respiratory CO2 flux. More surprisingly, estimated photosynthesis during this period accounted for up to 19% of the annual gross CO2 uptake, suggesting that cold‐season photosynthesis partly balanced the cold‐season respiratory carbon losses and can be significant for the annual cycle of carbon. Still, during the full year the ecosystem was a significant net source of 120 ± 12 g C m−2 to the atmosphere. Neither respiration nor photosynthetic rates were much affected by the extra FT cycles, although the mean rate of net ecosystem loss decreased slightly, but significantly, in May. The results suggest only a small response of net carbon fluxes to increased frequency of FT cycles in this ecosystem.

Freeze thaw cycles in Toronto, Canada in a changing climate

Freeze thaw cycles are examined in Toronto Canada. Using data from 1960 to 1989 for three Toronto area weather stations, trends in freeze thaw activity, the relationship to mean monthly temperature and projections of freeze thaw activity are examined. For downtown Toronto the annual frequency of freeze thaw cycles is decreasing significantly, most notably in the shoulder months of October and April. At the Pearson International Airport and the Toronto Island Airport similar annual trends were not found, however there was evidence of decreased freeze thaw activity in April and October. Polynomial curve fitting provided functional relationships between mean monthly temperature and freeze thaw activity. These relationships enabled the assessment of freeze thaw activity under synthetic warming conditions. The results of this analysis show that the warming of the magnitude typically projected for the rest of this century will not likely generate a significant change in the freeze thaw activity although there are indications that the freeze thaw season will contract.