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Background and aims Nitrogen fixation associated with cryptogams is potentially very important in arctic and subarctic terrestrial ecosystems, as it is a source of new nitrogen (N) into these highly N limited systems. Moss-, lichen-and legume-associated N₂ fixation was studied with high frequency (every second week) during spring, summer, autumn and early winter to uncover the seasonal variation in input of atmospheric N₂ to a subarctic heath with an altered climate. Methods We estimated N₂ fixation from ethylene production by acetylene reduction assay in situ in a field experiment with the treatments: long-vs. short-term summer warming using plastic tents and litter addition (simulating expansion of the birch forest). Results N₂ fixation activity was measured from late April to mid November and 33 % of all N₂ was fixed outside the vascular plant growing season (Jun-Aug). This substantial amount underlines the importance of N₂ fixation in the cold period. Wanning increased N₂ fixation two-to fivefold during late spring. However, longterm summer warming tended to decrease N₂ fixation outside the treatment (tents present) period. Litter alone did not alter N₂ fixation but in combination with warming N₂ fixation increased, probably because N₂ fixation became phosphorus limited under higher temperatures, which was alleviated by the P supply from the litter. Conclusion In subarctic heath, the current N₂ fixation period extends far beyond the vascular plant growing season. Climate warming and indirect effects such as vegetation changes affect the process of N₂ fixation in different directions and thereby complicate predictions of future N cycling.

Purpose The standardized ‘Tea Bag Index’ enables comparisons of litter decomposition rates, a key component of carbon cycling, across ecosystems. However, tea ‘litter’ may leach more than other plant litter, skewing comparisons of decomposition rates between sites with differing moisture conditions. Therefore, some researchers leach tea bags before field incubation. This decreases comparability between studies, and it is unclear if this modification is necessary. Methods We submerged green and rooibos tea bags in water, and measured their leaching losses over time (2 min – 72 h). We also compared leaching of tea to leaf and root litter from other plant species, and finally, compared mass loss of pre-leached and standard tea bags in a fully factorial incubation experiment differing in soil moisture (wet and dry) and soil types (sand and peat). Results Both green and rooibos tea leached strongly, levelling-off at about 40% and 20% mass loss, respectively. Mass loss from leaching was highest in green tea followed by leaves of other plants, then rooibos tea, and finally roots of other plants. When incubated for 4 weeks, both teas showed lower mass loss when they had been pre-leached compared to standard tea bags. However, these differences between standard and pre-leached tea bags were similar in moist vs. dry soils, both in peat and in sand. Conclusions Thus, despite large leaching losses, we conclude that leaching tea bags before field or lab incubation is not necessary to compare decomposition rates between systems, ranging from as much as 5% to 25% soil moisture.

Sphagnum peat bogs store a large fraction of biologically-bound carbon, due to a steady accumulation of plant material over millennia. The resistance of Sphagnum biomass to decay is poorly understood and of high importance for preservation efforts and climate models. Sphagnum peat mostly consists of the polysaccharide-rich cell wall of the moss but the mechanisms by which it resist degradation by microbes remain unclear. Here we show that enzymatic saccharification of peat polysaccharides including cellulose and other glucose-rich polysaccharides is predominantly limited by access to the substrate. The experimental approach involved biotechnological tools including hydrothermal pretreatment to disrupt and relocate cell wall components. This physical change was confirmed by confocal laser scanning microscopy. A cocktail of microbial enzymes (Cellic® CTec3) designed for industrial saccharification of lignocellulose of vascular plants was used to assess enzymatic digestibility of peat polysaccharides. The glucose yield increased from close to zero for untreated peat to 30% and 50% when pretreated at 160 and 180 °C. An overall catalytic rate constant for enzymatic glucose-release from peat-cellulose of 26.98 h -1 was calculated using a kinetic model. This is a similar or higher rate compared to cellulose from vascular plant tissues. With an iron content of 2 g/kg dry peat, oxidative inactivation of enzymes is an important factor to take into account. A high inactivation constant of 125.91 x10 -3 h −1 was found for the used saccharification conditions, but the addition of catalase alleviated the oxidative inactivation and increased the glucose yield with 60% in peat pretreated at 180 °C. These findings show that molecular structures of Sphagnum peat which prevents access for cell wall degrading enzymes can be disrupted by hydrothermal pretreatment. This brings us closer to understanding peat recalcitrance and thus how very large amounts of organic carbon is stored.

Basidiomycete fungi are the main decomposers of dead wood with an impact on the global carbon cycle. Their degradative mechanisms have been well-studied under aerobic conditions. Here, we study their activity in oxygen-depleted environments. We use metaproteomics in a field study to identify active wood-decomposing fungi and their enzymes at different depths from the wood surface, including in oxygen-depleted conditions. In vitro, we observe that the brown-rot fungus Fomitopsis pinicola can grow on wood in complete anoxia. Using 13 C solid-state NMR, we demonstrate the degradation of plant cell-wall polysaccharides and fungal growth in the absence of oxygen. Proteomic analyses reveal that F. pinicola switches from a Fenton chemistry-based process under aerobic conditions to the secretion of plant cell wall-active enzymes in anoxia. Our finding that wood decay fungi can thrive in complete anoxia provides a deeper understanding of lignocellulose degradation mechanisms in nature and raises opportunities for the development of bio-inspired anaerobic processes. The degradation of dead wood by basidiomycete fungi relies on Fenton chemistry under aerobic conditions. Here, Röllig et al. show that these fungi can also thrive and degrade wood in anoxia, switching from a Fenton chemistry-based process to the secretion of plant cell wall-active enzymes.

With accelerating climate change, higher summer rainfall and warmer soils are expected for High Arctic ecosystems . Yet, how increased rainfall and soil nutrient availability will affect plant composition and ecosystem carbon (C) storage in these arid, low-productivity ecosystems remains unclear. We utilised a long-term experiment in dry shrub heath tundra in Zackenberg, NE Greenland, in which nitrogen and phosphorus availability was increased and precipitation doubled experimentally every summer for 25 years. We determined soil and vegetation C pools, plant cover and leaf chemistry, and ecosystem CO 2 fluxes in peak growing season. Watering increased the cover of graminoids and all plants by 78% and 18%, respectively, which likely drove a moderate 6% increase in upper soil C stocks. Soil respiration was consistently stimulated in watered plots, confirming high sensitivity of soil microbes to moisture in dry tundra, but also stimulation of microbial activity by increased plant inputs. We suggest that belowground processes linked to root growth, root exudation, and/or microbial turnover of organic matter are important in driving the C pool changes. Our results show that increased summer rainfall can lead to greening and enhanced soil C pool in High Arctic dry heaths, potentially providing moderate negative feedback to climate change.

Bryophytes are abundant in tundra ecosystems, where they affect carbon and nitrogen cycling through primary production and associations with N2-fixing bacteria. Bryophyte responses to climate warming are inconclusive, likely because species-specific responses are poorly understood. Here we investigated how warming affects the growth and nitrogenase activity of 10 tundra bryophyte species in two tundra landscapes. Collected bryophyte samples were grown in temperature-controlled growth chambers for 12 weeks at five temperatures from 3 to 18 °C. We measured growth, N concentration, δ15N, and δ13C after 3 months and nitrogenase activity after 5 and 12 weeks. Bryophyte growth and associated nitrogenase activity generally increased with temperature, but species differed in their optima. Bryophyte N concentration and δ15N indicated that, for some species, increased N2-fixation could not compensate for growth-induced N limitation. High landscape coverage and large positive warming effects on feather mosses and Sphagnum species highlight their competitive advantages, confirm earlier field observations, and contribute to the mechanistic understanding of differential bryophyte growth in response to warming. We suggest that indirect effects of climate change, such as surface drying and shrub expansion, are likely main threats to slow-growing bryophytes across the Arctic, with consequences for biodiversity and C balance.

Alpine and Arctic treeline expansion depends on establishment of tree seedlings beyond the current treeline, which is expected to occur with climate warming. However, treelines often fail to respond to higher temperatures, and it is therefore likely that other environmental factors are important for seedling establishment. We aimed to analyse our current understanding of how temperature and a range of other environmental drivers affect tree seedling establishment at the alpine and Arctic treelines world‐wide and to assess the relative importance of temperature compared with other factors and how they interact. We collected 366 observations from 76 experimental and observational papers for a qualitative analysis of the role of a wide range of environmental factors on tree seed germination, tree seedling growth, survival and natural occurrence. For a subset of these studies, where the experimental design allowed, we conducted formal meta‐analyses to reveal if there were global drivers for different seedling life traits. The analyses showed that a wide range of abiotic and biotic factors affected tree seedling establishment besides from temperature, including water, snow, nutrients, light and surrounding vegetation. The meta‐analyses showed that different seedling life stages do not respond similarly to environmental factors. For example, temperature had positive effects on growth, while tree seedling survival and germination showed mixed responses to warming. Further, warming was as often as not the strongest factor controlling tree seedling establishment, when compared to with one of five other environmental factors. Moreover, warming effects often depended on other factors such as moisture or the presence of surrounding vegetation. Our results suggest that population dynamics of trees at the alpine and Arctic treeline is responsive to environmental changes and show that there is a clear need for multifactorial studies if we want to fully understand and predict the interplay between warming and other environmental factors and their effect on tree seedling establishment across current treelines. A plain language summary is available for this article. Plain Language Summary

Introduction: Survival and growth of tree seedlings are key processes of regeneration in forest ecosystems. However, little is known about how climate warming modulates seedling performance either directly or in interaction with understory vegetation and post-fire successional stages.Methods: We measured survival (over 3 years) and growth of seedlings of three tree species (Betula pubescens, Pinus sylvestris, and Picea abies) in a full-factorial field experiment with passive warming and removal of two plant functional groups (feather moss and/or ericaceous shrubs) along a post-fire chronosequence in an unmanaged boreal forest.Results: Warming had no effect on seedling survival over time or on relative biomass growth. Meanwhile, moss removal greatly increased seedling survival overall, while shrub removal canceled this effect for B. pubescens seedlings. In addition, B. pubescens and P. sylvestris survival benefitted most from moss removal in old forests (>260 years since last fire disturbance). In contrast to survival, seedling growth was promoted by shrub removal for two out of three species, i.e., P. sylvestris and P. abies, meaning that seedling survival and growth are governed by different understory functional groups affecting seedling performance through different mechanism and modes of action.Discussion: Our findings highlight that understory vegetation and to a lesser extent post-fire successional stage are important drivers of seedling performance while the direct effect of climate warming is not. This suggests that tree regeneration in future forests may be more responsive to changes in understory vegetation or fire regime, e.g., indirectly caused by warming, than to direct or interactive effects of rising temperatures.

Moss-associated nitrogen (N2) fixation is one of the main inputs of new N in pristine ecosystems that are characterized by low N availability. Previous studies have shown that N2 fixation is inhibited by inorganic N (IN) inputs, but if N2 fixation in mosses is similarly affected by organic N (ON) remains unknown. Here, we assessed N2 fixation in two dominant mosses in boreal forests (Pleurozium schreberi and Sphagnum capillifolium) in response to different levels of N, simulating realistic (up to 4 kg N ha−1 year−1) and extreme N addition rates in pristine ecosystems (up to 20 kg N ha−1 year−1) of IN (ammonium nitrate) and ON (alanine and urea). We also assessed if N2 fixation can recover from the N additions. In the realistic scenario, N2 fixation was inhibited by increasing NH4NO3 additions in P. schreberi but not in S. capillifolium, and alanine and urea stimulated N2 fixation in both moss species. In contrast, in the extreme N additions, increasing N inputs inhibited N2 fixation in both moss species and all N forms. Nitrogen fixation was more sensitive to N inputs in P. schreberi than in S. capillifolium and was higher in the recovery phase after the realistic compared to the extreme N additions. These results demonstrate that N2 fixation in mosses is less sensitive to organic than inorganic N inputs and highlight the importance of considering different N forms and species-specific responses when estimating the impact of N inputs on ecosystem functions such as moss-associated N2 fixation.