The effect of temperature changes on soil communities is an important aspect when estimating the effects of a predicted climate change. The aim of this investigation was to increase knowledge on how freeze-thaw cycles alter the soil microarthropod community in the sub-arctic. The abundance of springtails and mites was investigated after three seasons of prolonged periods of freeze-thaw cycles in the field, and the presence or absence of migration barriers, at two different field sites. Dome shaped transparent plastic greenhouses were successfully used as a novel method to increase freeze-thaw cycle frequencies in the soil. At a fellfield site, freeze-thaw treatment did not lead to significant differences in the five main soil faunal groups, but increased abundance were seen in a number of separate taxa. There was no freeze-thaw treatment effect on soil microbial biomass or soil nutrients, although treatments interacted as inorganic N increased in the separate freeze-thaw and migration barrier treatments. By contrast, at a glade site responses were strong due to more pronounced increases in the number of freeze-thaw cycles. The highest numbers of Collembola after 2 years of treatment were found in the freeze-thaw plots, in combination with migration barriers. The freeze-thaw treatment here also resulted in more Oribatida, microbial biomass C and dissolved organic C. A common hypothesis is that an increased number of freeze-thaw cycles would result in elevated winter mortality in microarthropods due to increased risk of inoculative freezing. However, we observed no increased mortality due to freeze-thaw events. Rather, there was a stimulation of soil microarthropods and microbial biomass, perhaps due to a prolonged period of microbial and faunal activity when the soil is repeatedly frozen and thawed compared to a constantly frozen soil. (c) 2007 Elsevier B.V. All rights reserved.

Extreme climate events are observed and predicted to increase in frequency and duration in high-latitude ecosystems as a result of global climate change. This includes extreme precipitation events, which may directly impact on belowground food webs and ecosystem functioning by their physical impacts and by altering local soil moisture conditions. We assessed responses of the Collembola community in a northern Sphagnum fuscum-dominated ombrotrophic peatland to three years of experimentally increased occurrence of extreme precipitation events. Annual summer precipitation was doubled (an increase of 200mm) by 16 simulated extreme rain events within the three months growing season, where on each occasion 12.5mm of rain was added within a few minutes. Despite this high frequency and intensity of the rain events, no shifts in Collembola density, relative species abundances and community weighted means of three relevant traits (moisture preference, vertical distribution and body size) were observed. This strongly suggests that the peatland Collembola community is unaffected by the physical impacts of extreme precipitation and the short-term variability in moisture conditions. The lack of response is most likely reinforced by the fact that extreme precipitation events do not seem to alter longer-term soil moisture conditions in the peat layers inhabited by soil fauna. This study adds evidence to the observation that the biotic components of northern ombrotrophic peatlands are hardly responsive to an increase in extreme summer precipitation events. Given the importance of these ecosystems for the global C balance, these findings significantly contribute to the current knowledge of the ecological impact of future climate scenarios.

We tested whether bacterial communities of subarctic heath soil are adapted to elevated temperature after experimental warming by open-top greenhouses for 7 or 17 years. The long-term warming by 1–2 °C significantly decreased bacterial community growth, by 28% and 73% after 7 and 17 years, respectively. The decrease was most likely due to decreased availability of labile substrate under warming. However, we found no evidence for temperature adaptation of soil bacterial communities. The optimum temperature for bacterial growth was on average 25 °C, and the apparent minimum temperature for growth between −7.3 and −6.1 °C, and both were unaffected by warming.

Research highlights

▶ Long-term warming of a subarctic heath decreased soil bacterial community growth. ▶ The decrease in bacterial growth was larger after 17 than after 7 years of warming. ▶ Warming did not affect temperature adaptation of the soil bacterial communities.

Climatic warming leads to the expansion of deciduous shrubs and trees in the Arctic. This leads to higher leaf litter inputs, which together with warming may alter the rate of carbon and nutrient cycling in the arctic ecosystems. We assessed effects of factorial warming and additional litter on the soil ecosystem of a subarctic heath in a 7-year-long field experiment. Fine root biomass, dissolved organic carbon (DOC) and total C concentration increased in response to warming, which probably was a result of the increased vegetation cover. Litter addition increased the concentration of inorganic P in the uppermost 5cm soil, while decreasing the pool of total P per unit area of the organic profile and having no significant effects on N concentrations or pools. Microbial biomass C and N were unaffected by the treatments, while the microbial biomass P increased significantly with litter addition. Soil ergosterol concentration was also slightly increased by the added litter in the uppermost soil, although not statistically significantly. According to a principal component analysis of the phospholipid fatty acid profiles, litter addition differed from the other treatments by increasing the relative proportion of biomarkers for Gram-positive bacteria. The combined warming plus litter addition treatment decreased the soil water content in the uppermost 5cm soil, which was a likely reason for many interactions between the effects of warming and litter addition. The soil organic matter quality of the combined treatment was also clearly different from the control based on a near-infrared reflectance (NIR) spectroscopic analysis, implying that the treatment altered the composition of soil organic matter. However, it appears that the biological processes and the microbial community composition responded more to the soil and litter moisture conditions than to the change in the quality of the organic matter.

It is predicted that Arctic regions may experience an increase in mean temperature in the future. This will affect the frequency of severe climatic events such as summer droughts and freeze-thaw cycles. In order to undemand the impact of recurring freezing and thawing on soil organisms and their environment, intact plant-soil samples from the sub-Arctic were subjected to a series of such events. Springtail and mite species composition and abundance were monitored at intervals throughout the experiment. Furthermore, nutrient content and mobilisation in the soil and soil microbial biomass and nutrient content were examined. There was no conclusive evidence that recurring freeze-thaw events had a negative effect on the investigated soil faunal groups, and the treatment even seemed to stimulate the abundance of Acaridida. Respiration of soil subjected to 16 freeze-thaw cycles was low when kept at -2 degreesC and high when kept at +2 degreesC, indicating rapid response of microbial activity even after long exposure to low and fluctuating temperatures. Oribatida and Gamasida displayed a higher abundance in controls kept at -2 degreesC for up to 80 days, compared to controls at +2 degreesC and the freeze-thaw treatment. The Collembola were unaffected by the temperature treatments, but increased in abundance over time. The microbial C:N ratio increased after 40 days at -2 degreesC. indicating a higher degree of fungal dominance and lower tolerance of bacteria to constant freezing, but not to freeze-thaw. decline in inorganic and microbial P during the experiment, and the proportionally stronger decrease of inorganic and microbial P than N in frozen soil compared to +2 degreesC soil, suggests that P is affected more than N mineralisation by freezing. (C) 2004 Elsevier B.V. All rights reserved.

Environmental changes feedback to climate through their impact on soil functions such as carbon (C) and nutrient sequestration. Abiotic conditions and the interactions between above- and belowground biota drive soil responses to environmental change but these (a)biotic interactions are challenging to study. Nonetheless, better understanding of these interactions would improve predictions of future soil functioning and the soil-climate feedback and, in this context, permafrost soils are of particular interest due to their vast soil C-stores. We need new tools to isolate abiotic (microclimate, chemistry) and biotic (roots, fauna, microorganisms) components and to identify their respective roles in soil processes. We developed a new experimental setup, in which we mimic thermokarst (permafrost thaw-induced soil subsidence) by fitting thawed permafrost and vegetated active layer sods side by side into mesocosms deployed in a subarctic tundra over two growing seasons. In each mesocosm, the two sods were separated from each other by barriers with different mesh sizes to allow varying degrees of physical connection and, consequently, (a)biotic exchange between active layer and permafrost. We demonstrate that our mesh-approach succeeded in controlling 1) lateral exchange of solutes between the two soil types, 2) colonization of permafrost by microbes but not by soil fauna, and 3) ingrowth of roots into permafrost. In particular, experimental thermokarst induced a similar to 60% decline in permafrost nitrogen (N) content, a shift in soil bacteria and a rapid buildup of root biomass (+33.2 g roots m(-2) soil). This indicates that cascading plant-soil-microbe linkages are at the heart of biogeochemical cycling in thermokarst events. We propose that this novel setup can be used to explore the effects of (a)biotic ecosystem components on focal biogeochemical processes in permafrost soils and beyond.