Soil microbial biomass in arctic heaths has been shown to be largely unaffected by treatments simulating climate change with temperature, nutrient and light manipulations. Here, we demonstrate that more than 10 years is needed for development of significant responses, and that changes in microbial biomass are accompanied with strong alterations in microbial community composition. In contrast to slight or nonsignificant responses after 5, 6 and 10 treatment years, 15 years of inorganic NPK fertilizer addition to a subarctic heath had strong effects on the microbial community and, as observed for the first time, warming and shading also led to significant responses, often in opposite direction to the fertilization responses. The effects were clearer in the top 5 cm soil than at the 5-10 cm depth. Fertilization increased microbial biomass C and more than doubled microbial biomass P compared to the non-fertilized plots. However, it only increased microbial biomass N at the 5-10 cm depth. Fertilization increased fungal biomass and the relative abundance of phospholipid fatty acid (PLFA) markers of gram-positive bacteria. Warming and shading decreased the relative abundance of fungal PLFAs, and shading also altered the composition of the bacterial community. The long time lag in responses may be associated with indirect effects of the gradual changes in the plant biomass and community composition. The contrasting responses to warming and fertilization treatments show that results from fertilizer addition may not be similar to the effects of increased nutrient mineralization and availability following climatic warming.

Soil microorganisms, the central drivers of terrestrial Antarctic ecosystems, are being confronted with increasing temperatures as parts of the continent experience considerable warming. Here we determined short-term temperature dependencies of Antarctic soil bacterial community growth rates, using the leucine incorporation technique, in order to predict future changes in temperature sensitivity of resident soil bacterial communities. Soil samples were collected along a climate gradient consisting of locations on the Antarctic Peninsula (Anchorage Island, 67134 0 S, 68108 0 W), Signy Island (60143 0 S, 45138 0 W) and the Falkland Islands (51176 0 S 59103 0 W). At each location, experimental plots were subjected to warming by open top chambers (OTCs) and paired with control plots on vegetated and fell-field habitats. The bacterial communities were adapted to the mean annual temperature of their environment, as shown by a significant correlation between the mean annual soil temperature and the minimum temperature for bacterial growth (T min ). Every 1 1C rise in soil temperature was estimated to increase T min by 0.24-0.38 1C. The optimum temperature for bacterial growth varied less and did not have as clear a relationship with soil temperature. Temperature sensitivity, indicated by Q 10 values, increased with mean annual soil temperature, suggesting that bacterial communities from colder regions were less temperature sensitive than those from the warmer regions. The OTC warming (generally o1 1C temperature increases) over 3 years had no effects on temperature relationship of the soil bacterial community. We estimate that the predicted temperature increase of 2.6 1C for the Antarctic Peninsula would increase T min by 0.6-1 1C and Q 10 (0-10 1C) by 0.5 units.

Atmospheric carbon dioxide enrichment (eCO 2) can enhance plant carbon uptake and growth 1,2,3,4,5 , thereby providing an important negative feedback to climate change by slowing the rate of increase of the atmospheric CO 2 concentration 6. While evidence gathered from young aggrading forests has generally indicated a strong CO 2 fertilization effect on biomass growth 3,4,5 , it is unclear whether mature forests respond to eCO 2 in a similar way. In mature trees and forest stands 7,8,9,10 , photosynthetic uptake has been found to increase under eCO 2 without any apparent accompanying growth response, leaving an open question about the fate of additional carbon fixed under eCO 2 4,5,7,8,9,10,11. Here, using data from the first ecosystemscale Free-Air CO 2 Enrichment (FACE) experiment in a mature forest, we constructed a comprehensive ecosystem carbon budget to track the fate of carbon as the forest responds to four years of eCO 2 exposure. We show that, although the eCO 2 treatment of ambient +150 ppm (+38%) induced a 12% (+247 g C m-2 yr-1) increase in carbon uptake through gross primary production, this additional carbon uptake did not lead to increased carbon sequestration at the ecosystem level. Instead, the majority of the extra carbon was emitted back into the atmosphere via several respiratory fluxes, with increased soil respiration alone accounting for ~50% of the total uptake surplus. Our results call into question the predominant thinking that the capacity of forests to act as carbon sinks will be generally enhanced under eCO 2 , and challenge the efficacy of climate mitigation strategies that rely on ubiquitous CO 2 fertilization as a driver of increased carbon sinks in global forests. Main text Globally, forests act as a large carbon sink, absorbing a significant portion of the anthropogenic CO 2 emissions 1,12 , an ecosystem service that has tremendous social and

Summary• Biogenic volatile organic compound (BVOC) emissions from arctic ecosystems are important in view of their role in global atmospheric chemistry and unknown feedbacks to global warming. These cold ecosystems are hotspots of climate warming, which will be more severe here than averaged over the globe. We assess the effects of climatic warming on non-methane BVOC emissions from a subarctic heath.• We performed ecosystem-based chamber measurements and gas chromatographymass spectrometry (GC-MS) analyses of the BVOCs collected on adsorbent over two growing seasons at a wet subarctic tundra heath hosting a long-term warming and mountain birch (Betula pubescens ssp. czerepanovii) litter addition experiment.• The relatively low emissions of monoterpenes and sesquiterpenes were doubled in response to an air temperature increment of only 1.9-2.5°C, while litter addition had a minor influence. BVOC emissions were seasonal, and warming combined with litter addition triggered emissions of specific compounds.• The unexpectedly high rate of release of BVOCs measured in this conservative warming scenario is far above the estimates produced by the current models, which underlines the importance of a focus on BVOC emissions during climate change. The observed changes have implications for ecological interactions and feedback effects on climate change via impacts on aerosol formation and indirect greenhouse effects.

Little is known about the contribution of bacteria and fungi to decomposition of different carbon compounds in arctic soils, which are an important carbon store and possibly vulnerable to climate warming. Soil samples from a subarctic tundra heath were incubated with 13 C-labeled glucose, acetic acid, glycine, starch, and vanillin, and the incorporation of 13 C into different phospholipid fatty acids (PLFA; indicative of growth) and neutral lipid fatty acids (NLFA; indicative of fungal storage) was measured after 1 and 7 days. The use of 13 C-labeled substrates allowed the addition of substrates at concentrations low enough not to affect the total amount of PLFA. The label of glucose and acetic acid was rapidly incorporated into the PLFA in a pattern largely corresponding to the fatty acid concentration profile, while glycine and especially starch were mainly taken up by bacteria and not fungi, showing that different groups of the microbial community were responsible for substrate utilization. The 13 C-incorporation from the complex substrates (starch and vanillin) increased over time. There was significant allocation of 13 C into the fungal NLFA, except for starch. For glucose, acetic acid, and glycine, the allocation decreased over time, indicating use of the storage products, whereas for vanillin incorporation into fungal NLFA increased during the incubation. In addition to providing information on functioning of the microbial communities in an arctic soil, our study showed that the combination of PLFA and NLFA analyses yields additional information on the dynamics of substrate degradation.

Summary• Emissions of isoprene, a reactive hydrocarbon, from Subarctic vegetation are not well documented. However, the Arctic is likely to experience the most pronounced effects of climatic warming, which may increase temperature-dependent isoprene emission.• Here, we assessed isoprene emission from a Subarctic heath subjected to a 3-4°C increase in air temperature and mountain birch (Betula pubescens ssp. czerepanovii) litter addition for 7-8 yr, simulating climatic warming and the subsequent expansion of deciduous shrub species and migration of the treeline. The measurements were performed using the dynamic chamber method on a wet heath with a mixture of shrubs, herbs and graminoids.• Isoprene emissions averaged across the treatments were 36 ± 5 µg m • These results show that isoprene emissions from Subarctic heaths are comparable to those from Subarctic peatlands. Climatic warming will increase the emissions, and the amount of carbon lost as isoprene, from Subarctic heath ecosystems.

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