Search

Background and Aims The response of subarctic grassland's below-ground to soil warming is key to understanding this ecosystem's adaptation to future climate. Functionally different below-ground plant organs can respond differently to changes in soil temperature (T s). We aimed to understand the below-ground adaptation mechanisms by analysing the dynamics and chemistry of fine roots and rhizomes in relation to plant community composition and soil chemistry, along with the duration and magnitude of soil warming. Methods We investigated the effects of the duration [medium-term warming (MTW; 11 years) and long-term warming (LTW; > 60 years)] and magnitude (0–8.4 °C) of soil warming on below-ground plant biomass (BPB), fine root biomass (FRB) and rhizome biomass (RHB) in geothermally warmed subarctic grasslands. We evaluated the changes in BPB, FRB and RHB and the corresponding carbon (C) and nitrogen (N) pools in the context of ambient, T s < +2 °C and T s > +2 °C scenarios. Key Results BPB decreased exponentially in response to an increase in T s under MTW, whereas FRB declined under both MTW and LTW. The proportion of rhizomes increased and the C–N ratio in rhizomes decreased under LTW. The C and N pools in BPB in highly warmed plots under MTW were 50 % less than in the ambient plots, whereas under LTW, C and N pools in warmed plots were similar to those in non-warmed plots. Approximately 78 % of the variation in FRB, RHB, and C and N concentration and pools in fine roots and rhizomes was explained by the duration and magnitude of soil warming, soil chemistry, plant community functional composition, and above-ground biomass. Plant's below-ground biomass, chemistry and pools were related to a shift in the grassland's plant community composition – the abundance of ferns increased and BPB decreased towards higher T s under MTW, while the recovery of below-ground C and N pools under LTW was related to a higher plant diversity. Conclusion Our results indicate that plant community-level adaptation of below ground to soil warming occurs over long periods. We provide insight into the potential adaptation phases of subarctic grasslands. [ABSTRACT FROM AUTHOR]

Scots pine Pinus sylvestris was originally introduced to Iceland in the beginning of the 20th Century. Extensive plantings started in the late 1940s and, in total, 2–3 million Scots pine seedlings were planted, mainly originating from two counties in northern Norway. Part of this plant material was imported as seedlings.Pine woolly aphid Pineus pini was introduced to Iceland before 1940, most likely on imported seedlings in 1937.High mortality of Scots pine, concurrent with high infestation of the pine woolly aphid, was observed in Iceland during the late 1950s and 1960s and planting was discontinued.Provenance trials with Scots pine were established in Iceland in 2004–2006. They consisted of 15 provenances from Norway, four from Finland, four from Scotland, one from Russia, one from the Austrian Alps and three first generation Scots pine provenances from Iceland, collected from survivors of the epidemic in the 1950s and 1960s. In total, there were 28 provenances.The Icelandic provenances had significantly lower P. pini infestation than all the provenances of non‐Icelandic origin, which indicates that natural selection in Scots pine in Iceland has occurred in favour of individuals less susceptible to P. pini. [ABSTRACT FROM AUTHOR]

Lupinus nootkatensis is an exotic plant species that has been used for large‐scale sowing all around Iceland for land reclamation of eroded surfaces protected from livestock grazing.Until the early 1990s, L. nootkatensis was free from any significant arthropod herbivory in Iceland, whereas, after 1991, many outbreaks of native insect species, primarily Ceramica pisi and Eupithecia satyrata, have been recorded. These outbreaks have caused repeated total defoliation of extensive areas of L. nootkatensis, although the effects on its development are mostly unknown.We studied the effect of: (i) reduced herbivory; (ii) increased herbivory; and (iii) simulated increased herbivory, compared with (iv) unmanipulated herbivory, on defoliation and seed production of L. nootkatensis in a 3‐year field study within two sites at contrasting ages and successional stages.The results obtained showed that: (i) seed production across all treatments was negatively related to defoliation; (ii) reduced herbivory had a positive effect on the number of flowering stems and seed yield; and (iii) these effects depended on age and/or the successional stage because they were only significant in the older L. nootkatensis site.These findings indicate that arthropod herbivory may affect the invasiveness of L. nootkatensis in Iceland by reducing the seed production and the spatial distribution rate of late successional lupin communities. [ABSTRACT FROM AUTHOR]

The broom moth Ceramica pisi, a native species in Iceland, has shown a marked expansion from south Iceland towards the north, concurrent with increasing temperatures.Winter temperatures have increased more than summer temperatures in Iceland and, in the present study, the hypothesis was that the winter warming has facilitated the range shift of C. pisi.Winter survival of pupae in Iceland was studied in the laboratory. Pupae were collected in the autumn of 2012 at five separate locations, then weighed and placed at temperatures from −6 to −18 °C. One month after the sub‐zero treatments, the pupae were placed at room temperature and pupal emergence was recorded.No significant effect of sub‐zero treatments on the survival of C. pisi pupae was found. The primary hypothesis of the present study was therefore rejected.The major factor affecting low temperature survival of C. pisi pupae, however, was their autumn weight. The response was sigmoid and the 5%, 50% and 95% likelihoods for winter survival were at 157, 274 and 393 mg autumn pupal mass, respectively.This finding indicates that factors other than winter temperature, such as summer available thermal budget for larval growth, may be a limiting factor to the spread of C. pisi in Iceland. [ABSTRACT FROM AUTHOR]

The phenology of vegetation, particularly the length of the growing season ( LOS; i.e., the period from greenup to senescence), is highly sensitive to climate change, which could imply potent feedbacks to the climate system, for example, by altering the ecosystem carbon (C) balance. In recent decades, the largest extensions of LOS have been reported at high northern latitudes, but further warming-induced LOS extensions may be constrained by too short photoperiod or unfulfilled chilling requirements. Here, we studied subarctic grasslands, which cover a vast area and contain large C stocks, but for which LOS changes under further warming are highly uncertain. We measured LOS extensions of Icelandic subarctic grasslands along natural geothermal soil warming gradients of different age (short term, where the measurements started after 5 years of warming and long term, i.e., warmed since ≥50 years) using ground-level measurements of normalized difference vegetation index. We found that LOS linearly extended with on average 2.1 days per °C soil warming up to the highest soil warming levels (ca. +10°C) and that LOS had the potential to extend at least 1 month. This indicates that the warming impact on LOS in these subarctic grasslands will likely not saturate in the near future. A similar response to short- and long-term warming indicated a strong physiological control of the phenological response of the subarctic grasslands to warming and suggested that genetic adaptations and community changes were likely of minor importance. We conclude that the warming-driven extension of the LOSs of these subarctic grasslands did not saturate up to +10°C warming, and hence that growing seasons of high-latitude grasslands are likely to continue lengthening with future warming (unless genetic adaptations or species shifts do occur). This persistence of the warming-induced extension of LOS has important implications for the C-sink potential of subarctic grasslands under climate change. [ABSTRACT FROM AUTHOR]

Summary: Below and aboveground vegetation dynamics are crucial in understanding how climate warming may affect terrestrial ecosystem carbon cycling. In contrast to aboveground biomass, the response of belowground biomass to long‐term warming has been poorly studied.Here, we characterized the impacts of decadal geothermal warming at two levels (on average +3.3°C and +7.9°C) on below and aboveground plant biomass stocks and production in a subarctic grassland.Soil warming did not change standing root biomass and even decreased fine root production and reduced aboveground biomass and production. Decadal soil warming also did not significantly alter the root–shoot ratio. The linear stepwise regression model suggested that following 10 yr of soil warming, temperature was no longer the direct driver of these responses, but losses of soil N were. Soil N losses, due to warming‐induced decreases in organic matter and water retention capacity, were identified as key driver of the decreased above and belowground production. The reduction in fine root production was accompanied by thinner roots with increased specific root area.These results indicate that after a decade of soil warming, plant productivity in the studied subarctic grassland was affected by soil warming mainly by the reduction in soil N. [ABSTRACT FROM AUTHOR]

The growth responses of mature Norway spruce (Picea abies (L.) Karst.) trees exposed to elevated [CO2] (CE; 670–700 ppm) and long-term optimized nutrient availability or elevated air temperature (TE; ±3.9 °C) were studied in situ in northern Sweden in two 3 year field experiments using 12 whole-tree chambers in ca. 40-year-old forest. The first experiment (Exp. I) studied the interactions between CE and nutrient availability and the second (Exp. II) between CE and TE. It should be noted that only air temperature was elevated in Exp. II, while soil temperature was maintained close to ambient. In Exp. I, CE significantly increased the mean annual height increment, stem volume and biomass increment during the treatment period (25, 28, and 22%, respectively) when nutrients were supplied. There was, however, no significant positive CE effect found at the low natural nutrient availability. In Exp. II, which was conducted at the natural site fertility, neither CE nor TE significantly affected height or stem increment. It is concluded that the low nutrient availability (mainly nitrogen) in the boreal forests is likely to restrict their response to the continuous rise in [CO2] and/or TE. [ABSTRACT FROM AUTHOR]

The low availability of nitrogen (N) is believed to be one of the major limiting factors of forest regeneration in Iceland and frequently under Boreal conditions. Lutz spruce (Picea x lutziiLittl.) seedlings were nutrient loaded using four fertilization regimes in the end of nursery rotation in autumn 2008 and planted in the following spring, with or without a single dose of fertilizer, on two treeless sites in N-Iceland with contrasting soil fertility. Measurements were made after one growing season. The highest loading level without additional field fertilization increased new needle mass by 122% and 152%, for the poor and more fertile site, respectively. The highest loading level with field fertilization increased new needle mass equally, by 188% and 189%, for the poor and more fertile site, respectively. Retranslocation of N, from old needles to current needles, increased with more loading. However, it was clear that nutrient loading could not replace field fertilization, as the seedlings generally showed an additive response to field fertilization and nutrient loading; doing both always gave the best results in seedling performance. As the study only covers field establishment during the first year, the long-term effect of nutrient loading of Lutz spruce cannot be predicted. However, it was concluded that loading might provide an additional input for faster plantation establishment during the first growing season after planting. [ABSTRACT FROM AUTHOR]

The aim of present study was to estimate the annual carbon balance of a young Larix sibirica plantation on former heathland pasture in Iceland. The data covered 1 yr of measurements, year 2005. The daily NEE varied from about +7.7 to −22.5 g CO2 m−2 d−1 in October and July, respectively, resulting in annual sequestration of –727 g CO2 m−2 a−1. This annual carbon balance falls within the observed range of published annual NEEs for other plantation forests. Daily net uptake started in middle of April, when thawing of the soil surface began and it increased as the spring progressed. During a cold spell in the middle of May, the air temperature dropped below zero for some days, resulting in a decline of carbon uptake and a slow recovery of the ecosystem. Such freezing incidents are expected to become more frequent at northern latitudes in warmer future climate and their impact should, therefore, be studied further. Moreover, longer time series of measurements on NEE are needed to reveal the effect of weather conditions on long-term carbon balance. Corrections for small errors in winter and night fluxes from open-path gas analysers also need to be developed. [ABSTRACT FROM AUTHOR]

Examines the effects of rising atmospheric carbon dioxide concentration and fertilization on gas exchange of field-grown tree species. Relation between net photosynthesis and foliage nitrogen content; Role of temperature on the increasing effect of carbon dioxide on net photosynthesis; Measures for improving predictions of tree growth responses to carbon dioxide.

Based on our recently developed method to quantify Gunderson and Holling's adaptive cycle, we provide a holistic analysis of a vascular plant community on the volcanic island of Surtsey between 2000 and 2018. We identify one complete adaptive cycle during the study period, which reflects the system's transition from a classic pioneer to a grassland community. Our results support the hypothesis that nutrients brought to the island by breeding gulls are the main driver of this development. The study period includes the beginning of a second cycle, which deviates from the pattern described in the metaphor. Indeed, the cycle's exploitation phase is interrupted by a simultaneous decline of all three systemic variables. We can trace this phenomenon back to a severe drought in 2012. Furthermore, the method allows us to establish the systemic role of individual species during the maturation process. [ABSTRACT FROM AUTHOR]

Climate warming has been suggested to impact high latitude grasslands severely, potentially causing considerable carbon (C) losses from soil. Warming can also stimulate nitrogen (N) turnover, but it is largely unclear whether and how altered N availability impacts belowground C dynamics. Even less is known about the individual and interactive effects of warming and N availability on the fate of recently photosynthesized C in soil. On a 10‐year geothermal warming gradient in Iceland, we studied the effects of soil warming and N addition on CO2 fluxes and the fate of recently photosynthesized C through CO2 flux measurements and a 13CO2 pulse‐labeling experiment. Under warming, ecosystem respiration exceeded maximum gross primary productivity, causing increased net CO2 emissions. N addition treatments revealed that, surprisingly, the plants in the warmed soil were N limited, which constrained primary productivity and decreased recently assimilated C in shoots and roots. In soil, microbes were increasingly C limited under warming and increased microbial uptake of recent C. Soil respiration was increased by warming and was fueled by increased belowground inputs and turnover of recently photosynthesized C. Our findings suggest that a decade of warming seemed to have induced a N limitation in plants and a C limitation by soil microbes. This caused a decrease in net ecosystem CO2 uptake and accelerated the respiratory release of photosynthesized C, which decreased the C sequestration potential of the grassland. Our study highlights the importance of belowground C allocation and C‐N interactions in the C dynamics of subarctic ecosystems in a warmer world. [ABSTRACT FROM AUTHOR]

Global change is expected to affect soil microbial communities through their responsiveness to temperature. It has been proposed that prolonged exposure to elevated temperatures may lead to progressively larger effects on soil microbial community composition. However, due to the relatively short-term nature of most warming experiments, this idea has been challenging to evaluate. The present study took the advantage of natural geothermal gradients (from +1°C to +19°C above ambient) in two subarctic grasslands to test the hypothesis that long-term exposure (>50 years) intensifies the effect of warming on microbial community composition compared to short-term exposure (5-7 years). Community profiles from amplicon sequencing of bacterial and fungal rRNA genes did not support this hypothesis: significant changes relative to ambient were observed only starting from the warming intensity of +9°C in the long termand +7°C/+3°C in the short term, for bacteria and fungi, respectively. Our results suggest that microbial communities in high-latitude grasslands will not undergo lasting shifts in community composition under the warming predicted for the coming 100 years (+2.2°C to +8.3°C). [ABSTRACT FROM AUTHOR]

Future climate warming in the Arctic will likely increase the vulnerability of soil carbon stocks to microbial decomposition. However, it remains uncertain to what extent decomposition rates will change in a warmer Arctic, because extended soil warming could induce temperature adaptation of bacterial communities. Here we show that experimental warming induces shifts in the temperature–growth relationships of bacterial communities, which is driven by community turnover and is common across a diverse set of 8 (sub) Arctic soils. The optimal growth temperature (Topt) of the soil bacterial communities increased 0.27 ± 0.039 (SE) and 0.07 ± 0.028°C per °C of warming over a 0–30°C gradient, depending on the sampling moment. We identify a potential role for substrate depletion and time‐lag effects as drivers of temperature adaption in soil bacterial communities, which possibly explain discrepancies between earlier incubation and field studies. The changes in Topt were accompanied by species‐level shifts in bacterial community composition, which were mostly soil specific. Despite the clear physiological responses to warming, there was no evidence for a common set of temperature‐responsive bacterial amplicon sequence variants. This implies that community composition data without accompanying physiological measurements may have limited utility for the identification of (potential) temperature adaption of soil bacterial communities in the Arctic. Since bacterial communities in Arctic soils are likely to adapt to increasing soil temperature under future climate change, this adaptation to higher temperature should be implemented in soil organic carbon modeling for accurate predictions of the dynamics of Arctic soil carbon stocks. [ABSTRACT FROM AUTHOR]

Stomatal conductance was quantified with sap flux sensors and whole-tree chambers in mature Norway spruce (Picea abies (L.) Karst.) trees after 3 years of exposure to elevated CO2 concentration ([CO2 ]) in a 13-year nutrient optimization experiment. The long-term nutrient optimization treatment increased tree height by 3.7m(67%) and basal diameter by 8 cm (68%); the short-term elevated [CO2 ] exposure had no effect on tree size or allometry. Nighttime transpiration was estimated as ~7% of daily transpiration in unchambered trees; accounting for the effect of nighttime flux on the processing of sap flux signals increased estimated daily water uptake by ~30%. Crown averaged stomatal conductance (gs) was described by a Jarvis-type model. The addition of a stomatal response time constant (τ) and total capacitance of stored water (Ctot) improved the fit of the model. Model estimates for Ctot scaled with sapwood volume of the bole in fertilized trees. Hydraulic support--defined as a lumped variable of leaf-specific hydraulic conductivity and water potential gradient (KlΔΨ) --was estimated from height, sapwood-to-leaf area ratio (As:Al) and changes in tracheid dimensions. Hydraulic support explained 55% of the variation in gs at reference conditions for trees across nutrient and [CO2 ] treatments. Removal of ~50% of Al from three trees yielded results suggesting that stomatal compensation (i.e., an increase in gs) after pruning scales inversely with KlΔΨ, indicating that the higher the potential hydraulic support after pruning, the less complete the stomatal compensation for the increase in As:Al. [ABSTRACT FROM AUTHOR]

The warming of boreal ecosystems accelerates decomposition and increases nitrogen (N) availability. The impact of increased N on subarctic soil fauna communities, however, remains poorly understood. We investigated the response of soil hexapods to a N addition experiment in a subarctic grassland. We characterized the soil hexapod communities using environmental DNA metabarcoding and analyzed the levels of dissolved organic carbon (DOC), dissolved organic nitrogen (DON), microbial carbon (Cmic), and microbial nitrogen (Nmic). N addition increased DON and Nmic, while DOC and Cmic pools remained unchanged. Furthermore, N addition caused shifts in soil hexapod community compositional diversity between control and N plots in herbivore and microbivore taxa. The levels of DON and Nmic strongly correlated with these shifts, explaining 54% and 45% of the compositional variability, respectively. This study demonstrates a clear link between N availability and shifts in soil hexapod communities, associated to changes in microbial and dissolved N pools in subarctic grasslands. [Display omitted] • Nitrogen (N) addition changed the composition of soil hexapod communities. • N addition strongly increased Dissolved Organic N (DON) and microbial N (Nmic), while DOC and Cmic pools remained unchanged. • The observed shifts in the composition of soil hexapod communities strongly correlated with the increases in DON and Nmic. [ABSTRACT FROM AUTHOR]

A substantial portion of grassland photosynthates is allocated belowground to arbuscular mycorrhizal fungi (AMF), but controversy remains about whether this carbon (C) contributes to soil organic carbon (SOC) under warming. The goal of this study was to investigate how AMF biomass and C sequestered by AMF (C New) are influenced by soil warming. We estimated the AMF biomass and C New , assumed to be mostly AMF necromass, in mycelial ingrowth bags buried for 1, 2, or 3 years in soil under warming (~+0.5–16.4 °C). The AMF biomass had a positive, curvilinear response to warming gradients after one year of burial. About 107 g C m−2 of C New accumulated over the three years and ~12% of this C was from glomalin-related soil protein. Modelling suggested the production rate of AMF biomass was 153 g C m−2 yr−1 with a rapid (36–75 days) turnover while AMF necromass turnover was much slower (1.4 ± 0.2 yr−1). Warming duration (7–9 years vs. > 50 years) did not have significant influence on AMF biomass or C New (P > 0.05). Our results suggest that AMF are more tolerant to increases in temperature than other microbes or fine roots. The dramatic loss of soil C and stable soil aggregates under warming found earlier at this site were not attributed to a decrease in AMF biomass or C New. Despite a low AMF standing biomass, its contribution to SOC may be substantial. Image 1 • AMF biomass (PLFA 16.1ω5) did not decline at warming up to 16 °C above ambient. • About 107 g C m−2 was sequestered by AMF over a period of three years. • Turnover of AMF biomass and necromass was 36–75 days and 1.4 ± 0.2 yr−1 respectively. [ABSTRACT FROM AUTHOR]

• Temperature gradients in geothermal habitats serve as local models of global warming • We studied temperature-dependent traits in springtails from two geothermal habitats • Springtail traits and their plasticity varied within and between forest and grassland • Habitat properties may shape thermal phenotypes to an extent comparable with warming Common-garden experiments with populations sampled along natural thermal gradients help to reveal local adaptation, disentangle environmental and genetic effects, and ultimately predict, by analogy, future biotic responses to climate change. In this regard, geothermal habitats are useful model systems as they exhibit dramatic changes in soil temperature. The springtail Protaphorura pseudovanderdrifti has apparently coped with such local geothermal warming in Iceland, as this species occurs along a more than half-century-old geothermal gradient in a grassland and persists along a newly emerged temperature gradient in a previously non-geothermal planted spruce forest. We measured thermal reaction norms for development and walking speed and acute cold shock tolerance of P. pseudovanderdrifti originating from the grassland and forest geothermal gradients. Temperature-dependent juvenile development showed little variation among subpopulations from the recently warmed forest, probably due to insufficient evolutionary time, but springtails from the warmed grassland plots had significantly steeper reaction norms than their counterparts from the corresponding unwarmed plot. In contrast, cold tolerance and locomotory activity showed no conclusive clinal pattern despite significant within-habitat variation. There appeared to be significant differences between habitats, as springtails from the forest had more temperature-sensitive developmental rate and locomotory activity, walked faster, and exhibited more variable cold tolerance than grassland springtails did. The planting of a forest, therefore, seems to have exerted a stronger effect on the thermal phenotype of P. pseudovanderdrifti than the emergence of a geothermal gradient. Thus, habitat properties may be no less important in shaping thermal reaction norms than the mean temperature. These local-scale findings suggest that, in addition to warming per se, global transformation of communities may drive the evolution of thermal phenotypes to an extent comparable with the effect of rising environmental temperature. [ABSTRACT FROM AUTHOR]

• BrGDGTs were measured in geothermal soils to determine the unique effect of warming. • Here, brGDGT lipids only respond to temperature when the bacterial community changes. • On a global scale, an effect of a community change on the MBT′ 5ME can be inferred. • To weigh this effect in paleoclimate studies, we propose a brGDGT-based lipid proxy. Specific soil bacterial membrane lipids, branched glycerol dialkyl glycerol tetraethers (brGDGTs), are used as an empirical proxy for past continental temperatures. Their response to temperature change is assumed to be linear, caused by physiological plasticity of their, still unknown, source organisms. A well-studied set of geothermally warmed soils (Iceland) shows that the brGDGT fingerprint only changes when the soil mean annual temperature is warmer than 14 °C. This sudden change in brGDGT distribution coincides with an abrupt shift in the bacterial community composition in the same soils. Determining which bacterial OTUs are indicative for each soil cluster shows that Acidobacteria are possible brGDGT producers, together with representatives from the Actinobacteria, Chloroflexi, Gemmationadetes and Proteobacteria. Projecting the lipid fingerprint of the cold and warm bacterial communities onto the global soil calibration dataset creates two distinct soil clusters, in which brGDGTs respond differently to temperature and, especially, soil pH. We show that, on a local scale, a community effect rather than physiological plasticity can be the primary driver of the brGDGT-based paleothermometer along large temperature gradients. This threshold response needs to be taken into account when interpreting brGDGT-based paleo-records. [ABSTRACT FROM AUTHOR]

Global warming is predicted to impact high-latitude and high-altitude forests severely, jeopardizing their overall functioning and carbon storage, both of which depend on the warming response of tree fine root systems. This paper investigates the effect of soil warming on the biomass, morphology and colonizing ectomycorrhizal community of spruce fine and absorptive fine roots. We compare the responses of spruce roots growing at a man-made long-term soil warming (+ 4°C) experiment to results obtained from a geothermal soil temperature gradient (+ 1 to + 14°C) extending to the forest die-off edge, to shed light on the generalizability of the warming response and reveal any thresholds in acclimation ability. Trees in warmer soils formed longer and less-branched absorptive roots with higher specific root length and area, and lower root tissue density in both spruce stands, irrespective of warming method and location. Soil warming at the experimental warming site also supported the occurrence of a more varied EcM community and an increase in the abundance of Tomentella spp., indicating a shift in nutrient foraging. Fine and absorptive fine root biomass decreased toward warmer soil, with a sharp reduction occurring between + 4 and + 6°C from the ambient and leading to the collapse of the fine root system at the geothermal gradient. At the experimental warming site, the applied + 4°C warming had no effect on fine and absorptive fine root biomass. The similar fine root responses at the two warming sites suggest that the observations possibly reflect general acclimation patterns in spruce forests to global warming. [ABSTRACT FROM AUTHOR]

Abstract Soil warming can increase soil organic carbon (SOC) mineralization, triggering a positive climate‑carbon cycle feedback loop. Globally, many soil warming experiments have examined losses of bulk SOC, but few have assessed changes in quality. Accurate knowledge of the latter is required for an in-depth understanding and improved prediction of SOC feedback to climate change. In this study, we used Rock-Eval thermal analysis (RE6) to characterize shifts in SOC thermal stability and bulk chemistry after six years of geothermal warming by 0.6 °C, 1.8 °C, 3.9 °C, 9.9 °C, 16.3 °C, 40 °C, and 80 °C in an Icelandic grassland topsoil (0–10 cm). We also used the strong warming-induced depletion of SOC (up to 92% in the 80 °C soil) in comparisons of chemical oxidation-resistant and biogeochemically resistant SOC, which are generally assumed to be similar in nature. Sodium hypochlorite (NaOCl) and hydrogen peroxide (H 2 O 2) were used for oxidation. Warming-resistant SOC was strongly depleted in hydrocarbons and enriched in oxygen, confirming that SOC oxidation state, and thus energy content, is an important driver for biogeochemical stability. This was supported by findings that thermal stability, i.e., the amount of energy (temperature) necessary to pyrolyze or oxidize SOC, strongly increased with warming intensity. Of the 31 RE6 parameters tested, the most warming-sensitive were hydrogen index (HI, ρ = −0.84), oxygen index (OI RE6 , ρ = 0.83), proportion of total pyrolyzed carbon released as hydrocarbons at 200–650 °C (S2/PC, ρ = −0.86), and the temperature at which a certain proportion of CO 2 evolved during pyrolysis (ρ > 0.8). Chemical oxidation of unwarmed soil caused average relative SOC losses of 61% (NaOCl) and 91% (H 2 O 2) and shifts in RE6 properties that differed strongly from warming-induced shifts at comparable SOC losses. Chemical oxidation-resistant SOC was more enriched in oxygen, but slightly enriched in hydrocarbons, and less thermostable than comparable naturally depleted SOC at the same time. A certain overlap, especially for NaOCl-treated soils, is likely, while H 2 O 2 -oxidized soils showed very distinct RE6 properties. We concluded that i) soil warming leads to strong shifts in SOC bulk chemistry and thermal stability and ii) H 2 O 2 should be avoided in isolation of a slow SOC kinetic pool. Highlights • Naturally warmed soils were enriched in oxygen and depleted in hydrogen. • Thermal stability, especially during pyrolysis, increased with warming intensity. • This indicates a close link of thermal and biogeochemical stability. • Chemically oxidized SOC, especially with H 2 O 2 , had distinct Rock-Eval properties. • This questions the use of chemical oxidation to mimic biological decay. [ABSTRACT FROM AUTHOR]

Warming in subarctic ecosystems will be two-fold higher compared to lower latitudes under current climate change projections. While the effects of warming in northern ecosystems on plants and microorganisms have been extensively studied, the responses of soil fauna have received much less attention, despite their important role in regulating key soil processes. We analyzed the response of soil hexapod communities in a subarctic grassland exposed to a natural geothermal gradient in Iceland with increases of +3 and + 6 °C above ambient temperature. We characterized hexapod communities using environmental DNA (eDNA) metabarcoding. We analyzed the amounts of microbial carbon (Cmic), microbial N (Nmic), dissolved organic C (DOC) and dissolved organic N (DON) and then assessed whether these variables could help to account for the compositional dissimilarity of ground hexapod communities across temperatures. The increases in soil temperature did lead to changes in the composition of hexapod communities. The compositional differences caused by +6 °C plots were correlated with a decrease in Cmic and Nmic, soil DOC and DON. Our results highlight the response of soil hexapods to warming, and their interaction with microbial biomass ultimately correlated with changes in the availabilities of soil C and N. [Display omitted] [ABSTRACT FROM AUTHOR]

Climatic warming has been hypothesized to accelerate organic matter decomposition by soil microorganisms and thereby enhance carbon (C) release to the atmosphere. However, the long-term consequences of soil warming on belowground biota interactions are poorly understood. Here we investigate how geothermal warming by 6 °C for more than 50 years affects soil microbiota. Using metatranscriptomics we obtained comprehensive profiles of the prokaryotic, eukaryotic and viral players of the soil microbial food web. When compared to ambient soil temperature conditions, we found pronounced differences in taxa abundances within and between trophic modules of the soil food web. Specifically, we observed a 'trophic downgrading' at elevated temperature, with soil fauna decreasing in abundance, while predatory bacteria and viruses became relatively more abundant. We propose that the drivers for this shift are previously observed decreases in microbial biomass and soil organic carbon, and the increase in soil bulk density (decrease in soil porosity) at elevated temperature. We conclude that a trophic downgrading may have important implications for soil carbon sequestration and nutrient dynamics in a warming world. • Long-term (>50 yrs) soil warming by 6 °C affects (micro-)biota composition. • A significant shift in taxa abundance of trophic modules of the soil food web. • A 'trophic downgrading'; de- and increase in fauna and predatory bacteria/viruses. [ABSTRACT FROM AUTHOR]

Abstract: Human activities have caused global changes of atmospheric chemistry resulting in increased temperature especially in the colder regions of the northern hemisphere. Since warming of the environment can have drastic effects on terrestrial ecosystems it is important to experimentally evaluate the extent of such effects in long‐term field‐based experiments. In this study we make use of both recent (short‐term) and long‐term geothermal warming of Icelandic soils to examine the responses of Collembola, an ecologically important group of soil invertebrates, to warming. On the basis of metabolic scaling theory, we hypothesized that species of small size would be more successful in warmed soils than species of larger size. Further we expected that top‐soil‐dwelling species would benefit more from warming than deep‐soil‐dwelling species. In order to test these hypotheses we sampled Collembola along replicated gradients of increasing temperature in areas that had been heated for about 6 years and more than 50 years respectively. Collembola were identified to species level, counted and the community‐weighted mean trait scores for six functional and ecological traits were calculated. Results show that both short‐term and long‐term soil warming caused a shift towards a higher relative abundance of species with small body size. Furthermore, abundance of top‐soil‐dwelling Collembola tended to increase after short‐term warming, but the opposite was observed after long‐term warming. Using trait‐based diversity indices (FRic and RaoQ), we show that functional richness and diversity of Collembola communities was significantly reduced (almost halved) as a result of short‐term soil warming to about 10°C above normal, but this effect was not detected in plots equally warmed for more than 50 years. This indicates that the functional diversity of Collembola communities have high resilience towards soil warming in a long‐term perspective. A plain language summary is available for this article. [ABSTRACT FROM AUTHOR]

Understanding how and why soil microbial communities respond to temperature changes is important for understanding the drivers of microbial distribution and abundance. Studying soil microbe responses to warming is often made difficult by concurrent warming effects on soil and vegetation and by a limited number of warming levels preventing the detection of non-linear effects. A unique area in Iceland, where soil temperatures have recently increased due to geothermic activity, created a stable warming gradient in both grassland (dominated by Agrostis capillaris) and forest (Picea sitchensis) vegetation. By sampling soils which had been subjected to four years of temperature elevation (ambient (MAT 5.2 °C) to +40 °C), we investigated the shape of the response of soil bacterial communities to warming, and their associated community temperature adaptation. We used 16S rRNA amplicon sequencing to profile bacterial communities, and bacterial growth-based assays (3H-Leu incorporation) to characterize community adaptation using a temperature sensitivity index (SI, log (growth at 40 °C/4 °C)). Despite highly dissimilar bacterial community composition between the grassland and forest, they adapted similarly to warming. SI was 0.6 (equivalent to a minimum temperature for growth of between −6 and −7 °C) in both control plots. Both diversity and community composition, as well as SI, showed similar threshold dynamics along the soil temperature gradient. There were no significant changes up to soil warming of 6–9 °C above ambient, beyond which all indices shifted in parallel, with SI increasing from 0.6 to 1.5. The consistency of these responses provide evidence for an important role for temperature as a direct driver of bacterial community shifts along soil temperature gradients. • A geothermic gradient allowed evaluation of direct temperature effects on bacteria. • The 4-years old gradient spanned temperatures from ambient to +40 °C. • Under both forest and grassland, effects were observed in soils warmed >6–9 °C. • Soil bacterial community composition and temperature adaptation changed similarly. • This correlation suggests an important role for direct temperature effects. [ABSTRACT FROM AUTHOR]

Nitrous oxide (N 2 O) and methane (CH 4 ) emissions were measured along three natural geothermal soil temperature (T s ) gradients in freely drained upland soils in a volcanic area in Iceland. Two of the T s gradients (underneath a grassland (GN) and a forest site (FN), respectively) were recently formed (in May 2008) and thus subjected to relatively short-term warming. The third T s gradient, underneath another grassland site (GO), had been subjected to long-term soil warming (over at least 45 years). The N 2 O and CH 4 emissions were measured using the static chamber method. In addition, subsurface soil gas concentrations (5–20 cm) were studied. N 2 O emissions from GN were slightly higher than those from GO in the temperature elevation range up to +5 ° C, while CH 4 uptake rates were similar. Under moderate soil warming (<+5 ° C) there were no significant increases in gas flux rates within any of the sites, but when soil warming exceeded +20 ° C, both N 2 O and CH 4 emissions increased significantly at all sites. While net uptake of CH 4 (up to −0.15 mg CH 4 m −2 h −1 ) and occasional N 2 O uptake (up to −12 μg N 2 O m −2 h −1 ) were measured in the unwarmed plots at all sites, net emissions were only measured from the warmest plots (up to 2600 μg N 2 O m −2 h −1 and up to 1.3 mg CH 4 m −2 h −1 ). The subsurface soil N 2 O concentrations increased with soil warming, indicating enhanced N-turnover. Subsurface soil CH 4 concentrations initially decreased under moderate soil warming (up to +5 ° C), but above that threshold they also increased significantly. A portion of the N 2 O and CH 4 emitted from the warmest plots may, however, be geothermally derived, this should be further confirmed with isotope studies. In conclusion, our research suggests that moderate increases in soil temperature (up to +5 ° C) may not significantly increase N 2 O and CH 4 emissions at these upland soils, both in the short and longer term. However, warming trends exceeding +5 ° C as predicted for 2100 in pessimistic scenarios may cause increased trace gas emissions and thus significant positive feedbacks to climate change. [ABSTRACT FROM AUTHOR]

Plant invasion is an emerging driver of global change worldwide. We aimed to disentangle its impacts on plant-soil nutrient concentrations. We conducted a meta-analysis of 215 peer-reviewed articles and 1233 observations. Invasive plant species had globally higher N and P concentrations in photosynthetic tissues but not in foliar litter, in comparison with their native competitors. Invasive plants were also associated with higher soil C and N stocks and N, P, and K availabilities. The differences in N and P concentrations in photosynthetic tissues and in soil total C and N, soil N, P, and K availabilities between invasive and native species decreased when the environment was richer in nutrient resources. The results thus suggested higher nutrient resorption efficiencies in invasive than in native species in nutrient-poor environments. There were differences in soil total N concentrations but not in total P concentrations, indicating that the differences associated to invasive plants were related with biological processes, not with geochemical processes. The results suggest that invasiveness is not only a driver of changes in ecosystem species composition but that it is also associated with significant changes in plant-soil elemental composition and stoichiometry. [ABSTRACT FROM AUTHOR]

Terrestrial carbon cycle feedbacks to global warming are major uncertainties in climate models. For in-depth understanding of changes in soil organic carbon ( SOC) after soil warming, long-term responses of SOC stabilization mechanisms such as aggregation, organo-mineral interactions and chemical recalcitrance need to be addressed. This study investigated the effect of 6 years of geothermal soil warming on different SOC fractions in an unmanaged grassland in Iceland. Along an extreme warming gradient of +0 to ~+40 °C, we isolated five fractions of SOC that varied conceptually in turnover rate from active to passive in the following order: particulate organic matter ( POM), dissolved organic carbon ( DOC), SOC in sand and stable aggregates ( SA), SOC in silt and clay ( SC- rSOC) and resistant SOC ( rSOC). Soil warming of 0.6 °C increased bulk SOC by 22 ± 43% (0-10 cm soil layer) and 27 ± 54% (20-30 cm), while further warming led to exponential SOC depletion of up to 79 ± 14% (0-10 cm) and 74 ± 8% (20-30) in the most warmed plots (~+40 °C). Only the SA fraction was more sensitive than the bulk soil, with 93 ± 6% (0-10 cm) and 86 ± 13% (20-30 cm) SOC losses and the highest relative enrichment in 13C as an indicator for the degree of decomposition (+1.6 ± 1.5‰ in 0-10 cm and +1.3 ± 0.8‰ in 20-30 cm). The SA fraction mass also declined along the warming gradient, while the SC fraction mass increased. This was explained by deactivation of aggregate-binding mechanisms. There was no difference between the responses of SC- rSOC (slow-cycling) and rSOC (passive) to warming, and 13C enrichment in rSOC was equal to that in bulk soil. We concluded that the sensitivity of SOC to warming was not a function of age or chemical recalcitrance, but triggered by changes in biophysical stabilization mechanisms, such as aggregation. [ABSTRACT FROM AUTHOR]

The rising levels of atmospheric carbon dioxide concentration ([ CO2]) and temperature have the potential to substantially affect the terrestrial water and energy balance by altering the stomatal conductance and transpiration of trees., Many models assume decreases in stomatal conductance and plant water use under rising [ CO2], which has been used as a plausible explanation for the positive global trend in river run-off over the past century. Plant water use is, however, also affected by changes in temperature, precipitation and land use, and there is yet no consensus about the contribution of different drivers to temporal trends of evapotranspiration ( ET) and river run-off., In this study, we assessed water-use responses to climate change by using both long-term monitoring and experimental data in Swedish boreal forests. Historical trends and patterns in ET of large-scale boreal landscapes were determined using climate and run-off data from the past 50 years, while explicit tree water-use responses to elevated [ CO2] and/or air temperature were examined in a whole-tree chamber experiment using mature Norway spruce ( Picea abies (L.) Karst.) trees., The results demonstrated that ET estimated from water budgets at the catchment scale increased by 18% over the past 50 years while run-off did not significantly change. The increase in ET was related to increasing precipitation and a steady increase in forest standing biomass over time. The whole-tree chamber experiment showed that Norway spruce trees did not save water under elevated [ CO2] and that experimentally elevated air temperature did not increase transpiration as decreased stomatal conductance cancelled the effect of higher vapour pressure deficit in warmed air., Our findings have important implications for projections of future water use of European boreal coniferous forests, indicating that changes in precipitation and standing biomass are more important than the effects of elevated [ CO2] or temperature on transpiration rates. [ABSTRACT FROM AUTHOR]

Understanding and predicting how global warming affects the structure and functioning of natural ecosystems is a key challenge of the 21st century. Isolated laboratory and field experiments testing global change hypotheses have been criticized for being too small-scale and overly simplistic, whereas surveys are inferential and often confound temperature with other drivers. Research that utilizes natural thermal gradients offers a more promising approach and geothermal ecosystems in particular, which span a range of temperatures within a single biogeographic area, allow us to take the laboratory into nature rather than vice versa. By isolating temperature from other drivers, its ecological effects can be quantified without any loss of realism, and transient and equilibrial responses can be measured in the same system across scales that are not feasible using other empirical methods. Embedding manipulative experiments within geothermal gradients is an especially powerful approach, informing us to what extent small-scale experiments can predict the future behaviour of real ecosystems. Geothermal areas also act as sentinel systems by tracking responses of ecological networks to warming and helping to maintain ecosystem functioning in a changing landscape by providing sources of organisms that are preadapted to different climatic conditions. Here, we highlight the emerging use of geothermal systems in climate change research, identify novel research avenues, and assess their roles for catalysing our understanding of ecological and evolutionary responses to global warming. [ABSTRACT FROM AUTHOR]

• Negative priming was observed in long-term warmed soils when amended with glucose. • Warming-induced SOM modifications, and not of the microbial community, reduced priming. • Microbial nitrogen mining did not lead to increased priming. • Incubation warming increased microbial respiration, but not the priming effect. Priming is the change of microbial soil organic matter (SOM) decomposition induced by a labile carbon (C) source. It is recognised as an important mechanism influencing soil C dynamics and C storage in terrestrial ecosystems. Microbial nitrogen (N) mining in SOM and preferential substrate utilisation, i.e., a shift in microbial carbon use from SOM to more labile energy sources, are possible, counteracting, mechanisms driving the priming effect. Climate warming and increased N availability might affect these mechanisms, and thus determine the direction and magnitude of the priming effect. Hence, these abiotic factors can indirectly affect soil C stocks, which makes their understanding crucial for predicting the soil C feedback in a warming world. We conducted a short-term incubation experiment (6 days) with soils from a subarctic grassland that had been subjected to long-term geothermal warming (> 55 years) by 2-4°C above unwarmed soil. Soil samples were amended with 13C-labelled glucose and 15N-labelled NH 4 NO 3. We found a significantly negative relationship between in situ warming and cumulative primed C, with negative priming in the warmed soils. The negative priming suggests that preferential substrate utilisation was a key mechanism in our experiment. Our results indicate that changes in SOM characteristics associated with the in situ warming gradient can play a major role in determining the rate and direction of the priming effect. Additionally, we found that neither microbial N limitation nor N addition affected the priming effect, providing evidence that in our experiment, N mining did not lead to positive priming. [ABSTRACT FROM AUTHOR]

Riparian forests (RF) growing along streams, rivers and lakes comprise more than 2% of the forest area in the Nordic countries (considering a 10 m wide zone from the water body). They have special ecological functions in the landscape. They receive water and nutrients from the upslope areas, are important habitats for biodiversity, have large soil carbon stores, but may emit more greenhouse gases (GHG) than the uplands. In this article, we present a review of the environmental services related to water protection, terrestrial biodiversity, carbon storage and greenhouse gas dynamics provided by RF in the Nordic countries. We discuss the benefits and trade-offs when leaving the RF as a buffer against the impacts from upland forest management, in particular the impacts of clear cutting. Forest buffers are effective in protecting water quality and aquatic life, and have positive effects on terrestrial biodiversity, particularly when broader than 40 m, whereas the effect on the greenhouse gas exchange is unclear. [ABSTRACT FROM AUTHOR]

The objective of the study was to investigate the interactive effects of elevated atmospheric carbon dioxide concentration, [CO2], and temperature on the wood properties of mature field-grown Norway spruce ( Picea abies (L.) Karst.) trees. Material for the study was obtained from an experiment in Flakaliden, northern Sweden, where trees were grown for 3 years in whole-tree chambers at ambient (365 μmol mol−1) or elevated [CO2] (700 μmol mol−1) and ambient or elevated air temperature (ambient +5.6 °C in winter and ambient +2.8 °C in summer). Elevated temperature affected both wood chemical composition and structure, but had no effect on stem radial growth. Elevated temperature decreased the concentrations of acetone-soluble extractives and soluble sugars, while mean and earlywood (EW) cell wall thickness and wood density were increased. Elevated [CO2] had no effect on stem wood chemistry or radial growth. In wood structure, elevated [CO2] decreased EW cell wall thickness and increased tracheid radial diameter in latewood (LW). Some significant interactions between elevated [CO2] and temperature were found in the anatomical and physical properties of stem wood (e.g. microfibril angle, and LW cell wall thickness and density). Our results show that the wood material properties of mature Norway spruce were altered under exposure to elevated [CO2] and temperature, although stem radial growth was not affected by the treatments. [ABSTRACT FROM AUTHOR]

Data on net CO2 exchange from eight forests in Denmark, Sweden, Finland and Iceland were used to analyse which factors were controlling photosynthesis and respiration. The forests consisted of different species ranging in climatic condition from temperate to subarctic. Only well mixed conditions were analysed ( u* > 0.3 m s−1). The parameters of a light response function showed strong seasonal variations with similar behaviour for all stands except for a beech forest where the development of a vigorous ground vegetation in spring affected the photosynthesis parameters differently as compared to the other forests. The beech forest also showed the highest respiration rates in the earlier part of the growing season in contrast to the other forests that showed maximum values in late part of July. The mean half-monthly nighttime respiration rates were well explained by an equation with one fitting parameter, the respiration rate at 10 °C, with an r2= 0.864 for all stands together. The difference between the stands concerning both photosynthesis and respiration parameters were largely explained by the differences in LAI. After normalizing for LAI, the only remaining correlation was between respiration and stand age. These results are promising for application of remote sensing for estimation of respiration as well as gross primary productivity from forests. [ABSTRACT FROM AUTHOR]

Contents Temperate and boreal forest ecosystems contain a large part of the carbon stored on land, in the form of both biomass and soil organic matter. Increasing atmospheric [CO2], increasing temperature, elevated nitrogen deposition and intensified management will change this C store. Well documented single-factor responses of net primary production are: higher photosynthetic rate (the main [CO2] response); increasing length of growing season (the main temperature response); and higher leaf-area index (the main N deposition and partly [CO2] response). Soil organic matter will increase with increasing litter input, although priming may decrease the soil C stock initially, but litter quality effects should be minimal (response to [CO2], N deposition, and temperature); will decrease because of increasing temperature; and will increase because of retardation of decomposition with N deposition, although the rate of decomposition of high-quality litter can be increased and that of low-quality litter decreased. Single-factor responses can be misleading because of interactions between factors, in particular those between N and other factors, and indirect effects such as increased N availability from temperature-induced decomposition. In the long term the strength of feedbacks, for example the increasing demand for N from increased growth, will dominate over short-term responses to single factors. However, management has considerable potential for controlling the C store. [ABSTRACT FROM AUTHOR]

This article is a response to Korell et al., 26, 325–327; See also the Commentary on this article by Muller et al., 26, e4–e5; See also the response to this Letter to the Editor by Korell et al., 26, 328–329. [ABSTRACT FROM AUTHOR]

Abstract: Forest carbon exchange contributes significantly to the global carbon balance and is therefore being monitored around the world, most notably using eddy covariance technology. In order to extrapolate from these measurements, we need to understand why carbon balance (or net ecosystem production, NEP) differs among forests. Here, we use a detailed model of forest carbon exchange applied to three coniferous European forests with differing NEP to pinpoint reasons for the differences among these sites. The model was parameterised using extensive ecophysiological data obtained at each site. These data gave evidence of major differences among sites in climate, leaf physiology, respiring biomass, leaf area index, and soil and biomass respiration rates. The model was compared with eddy covariance data and found to satisfactorily simulate carbon exchange by each forest. Simulations were then run which interchanged canopy structure, physiology and meteorology among sites, allowing us to quantify the contribution of each factor to the inter-site differences in gross primary productivity (GPP), ecosystem respiration (RE) and NEP. The most important factor was the difference in respiration rates, particularly soil respiration rates, among sites. Climate was also very important, with differences in incident photosynthetically active radiation (PAR) affecting GPP and differences in temperature affecting both GPP and RE. Effects of leaf area index, respiring biomass and leaf physiology on NEP were secondary, but still substantial. The work provides detailed quantitative evidence of the major factors causing differences in NEP among coniferous forests. [Copyright &y& Elsevier]

The objective of the present study was to investigate the interactive effects of elevated [CO2] and soil nutrient availability on secondary xylem structure and chemical composition of 41-year-old Norway spruce (Picea abies (L.) Karst.) trees. The nonfertilized and irrigated-fertilized trees were, for 3 years, continuously exposed to elevated [CO2] in whole-tree chambers. Elevated [CO2] decreased concentrations of soluble sugars, acid-soluble lignin and nitrogen in stem wood, but the effects were not consistent between sampling height and/or fertilization. The effect of 2∗ambient [CO2] on wood structure depended on the exposure year and/or fertilization. Radial lumen diameter decreased and annual ring width increased in the second year of exposure (1999) in elevated [CO2]. In the latter, the CO2 effect was significant only in the nonfertilized trees. Stem wood chemistry and structure were significantly affected by fertilization. Fertilization increased the concentrations of nitrogen and gravimetric lignin, annual ring width, and radial lumen diameter. Fertilization decreased C/N ratio, mean ring density, earlywood density, latewood density, cell wall thickness, cell wall index, and latewood percentage. We conclude that elevated [CO2] had only minor effects on wood properties while fertilization had more marked effects and thus may affect ecosystem processes and suitability of wood for different end-use purposes. [ABSTRACT FROM AUTHOR]

The effect of warming is stronger in arctic and sub-arctic latitudes than in temperate and tropical zones. We studied soil metabolomes along two soil-warming gradients (0 to +15 °C). One temperature gradient has been present for at least 50 years and possibly even centuries (long-term treatment), while the second gradient was created after a shallow crustal earthquake in 2008 (short-term treatment). Soil metabolomes at the two sites responded differently to warming. At the short-term warmed site, warming of ≤+3 °C already shifted soil metabolomic profiles relative to the controls, whereas at the long-term warmed site the soil metabolome only shifted at temperatures ≥+5 °C. Saccharides and amino acids, primary metabolites involved in protective mechanisms against heat, were the main compounds accumulated at the highest soil warming levels. Some secondary metabolites associated with a broad spectrum of stressors, like phenolic acids and terpenes, were also up-regulated. Across the IPCC scenario's, most climate models predict a substantial rise in mean annual temperature of up to 8 °C in the Arctic region by the end of the 21st century. Our results suggest that temperature increases of ≥+5 °C would permanently alter soil metabolomic profile, whereas smaller temperature increases of (≤+3 °C) would affect soil metabolome profile transiently, not permanently. • The time and intensity of warming exposure affect differently soil function. • Soil function could acclimatize to smaller temperature increases of ≤+3 °C. • Temperature increases of ≥+5 °C could permanently alter soil function. • Under long-term warming: ≥+5 °C, soil contained more metabolites against heat stress. [ABSTRACT FROM AUTHOR]

Although ongoing research has revealed some of the main drivers behind global spatial patterns of microbial communities, spatio-temporal dynamics of these communities still remain largely unexplored. Here, we investigate spatio-temporal variability of both bacterial and eukaryotic soil microbial communities at local and intercontinental scales. We compare how temporal variation in community composition scales with spatial variation in community composition, and explore the extent to which bacteria, protists, fungi and metazoa have similar patterns of temporal community dynamics. All soil microbial groups displayed a strong correlation between spatial distance and community dissimilarity, which was related to the ratio of organism to sample size. Temporal changes were variable, ranging from equal to local between-sample variation, to as large as that between communities several thousand kilometers apart. Moreover, significant correlations were found between bacterial and protist communities, as well as between protist and fungal communities, indicating that these microbial groups change in tandem, potentially driven by interactions between them. We conclude that temporal variation can be considerable in soil microbial communities, and that future studies need to consider temporal variation in order to reliably capture all drivers of soil microbiome changes. [ABSTRACT FROM AUTHOR]