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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]

Understanding global change impacts on the globally important carbon storage in alpine, Arctic and sub-Arctic soils requires knowledge of the mechanisms underlying the balance between plant primary productivity and decomposition. Given that nitrogen availability limits both processes, understanding the response of the soil nitrogen cycle to shifts in temperature and other global change factors is crucial for predicting the fate of cold biome carbon stores. Measurements of soil enzyme activities at different positions of the nitrogen cycling network are an important tool for this purpose. We review a selection of studies that provide data on potential enzyme activities across natural, seasonal and experimental gradients in cold biomes. Responses of enzyme activities to increased nitrogen availability and temperature are diverse and seasonal dynamics are often larger than differences due to experimental treatments, suggesting that enzyme expression is regulated by a combination of interacting factors reflecting both nutrient supply and demand. The extrapolation from potential enzyme activities to prediction of elemental nitrogen fluxes under field conditions remains challenging. Progress in molecular ‘-omics’ approaches may eventually facilitate deeper understanding of the links between soil microbial community structure and biogeochemical fluxes. In the meantime, accounting for effects of the soil spatial structure and in situ variations in pH and temperature, better mapping of the network of enzymatic processes and the identification of rate-limiting steps under different conditions should advance our ability to predict nitrogen fluxes. [ABSTRACT FROM AUTHOR]

Climate change in arctic tundra is projected to increase soil fertility, which may alter plant community composition and ecosystem processes by shifting niche space to favour particular species' life-history strategies. The rate and magnitude of change in soil fertility may be critical to determining plant community responses, and so effects of slow increases in nutrient availability due to climate warming may differ substantially from those of chronic high-level fertilizer additions., We investigated above- and below-ground plant biomass responses to experimental summer warming and above-ground responses to nutrient additions (low-level N and factorial N and P) in a mesic birch hummock tundra community in the central Canadian Low Arctic after eight years of experimental treatment., Plant community biomass responses to experimental warming were fundamentally different from those of high-level N and/or P additions, mainly due to opposing effects on the evergreen shrubs. Evergreen shrub above-ground biomass increased 66% with greenhouse warming, but decreased on average 70% with high-level N and/or P additions, driven by the strong responses of Rhododendron subarcticum. Because of this evergreen response, greenhouse-warming increased total above-ground biomass by 32% and total below-ground biomass by 70%, but did not significantly change the total above-ground/below-ground biomass ratio. However, warming increased the shoot/root ratio of Betula glandulosa threefold., Increased soil fertility created interactions between N and P availability, whereby increased P availability led to a substantial increase in inorganic N availability. Meanwhile, the growth of several species that span a range of different functional groups was stimulated by the separate N and P additions. These factorial fertilization results highlight the importance of understanding climate warming impacts on availability of both of these nutrients in order to predict plant community responses., Synthesis. Our results strongly suggest that the trajectory of mesic tundra vegetation change with warming depends critically on the rate of increase in soil fertility. The relatively large greenhouse-induced biomass increase in evergreen compared to deciduous shrubs suggests that carbon balance and albedo feedbacks to warming will be restricted in mesic tundra ecosystems, at least in their early responses to climate change. [ABSTRACT FROM AUTHOR]

1. Changing temperature regimes and precipitation patterns in the Subarctic will impact on vegetation composition and diversity including those of bryophyte and lichen communities, which are major drivers of high-latitude carbon and nutrient cycling and hydrology. 2. We investigated the relative importance of such impacts at different temporal, spatial and plant functional scales in subarctic Sphagnum fuscum-dominated peatlands, comprising both an in situ warming experiment and natural climatic and topographic gradients in northern Sweden and Norway. We applied multivariate analyses to investigate the relationships among cryptogam and vascular plant species composition and abiotic (temperature, moisture) and biotic ( Sphagnum growth) regimes at various scales. 3. At the short-term temporal scale (4-year warming experiment), increased temperature yielded no clear effect on cryptogam or vascular plant species composition. Spatially, direct effects of temperature were decisive for overall species composition across regions (macro-scale) rather than within one region (meso-scale). Moisture and Sphagnum growth were drivers of species composition at all spatial scales, and Sphagnum growth itself depended on its position on the microtopographic gradient and on temperature. 4. Grouping of bryophytes and lichens at increasing scales of functional aggregation from species, growth form to the major higher taxon level ( Sphagnum, other mosses, liverworts, lichens) revealed mostly increasing correlation with climate regimes and Sphagnum growth. Excluding liverworts from the analysis tended to reduce the correlation. 5. Abundances of lichens, liverworts, non- Sphagnum mosses and (to a lesser degree) vascular plants were negatively related to Sphagnum abundance. Few cryptogam and vascular plant species showed a positive relationship with Sphagnum abundance. Correspondingly, cryptogam species richness and Shannon Index on peatlands strongly declined as Sphagnum abundance increased, while indices for vascular plants showed no significant relationship. 6. Synthesis. Scale, be it spatial or functional, strongly determined which environmental drivers showed the clearest relationships with vegetation composition and diversity. Our findings will help to optimize predictions about long-term effects of climate on peatland vegetation composition, and subsequently its feedbacks to carbon and water cycles, at the regional scale. [ABSTRACT FROM AUTHOR]