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In northern biomes, growth is nitrogen (N) limited, but bryophytes are abundant. These bryophytes often host N2-fixing microorganisms (diazotrophs) that play a crucial role in the N cycle of these ecosystems. Despite their importance, how the bryophyte-associated N2-fixation varies across species and seasons (summer, autumn, winter, and spring) remains poorly understood. We measured N2-fixation rates for 10 bryophyte species in situ throughout the entire year in the Arctic with additional incubations to verify the method. We measured positive N2-fixation during most of the year, except for the coldest period (February). The species growing in the wettest conditions (Sphagnum spp.) had the highest N2-fixation rates in summer, while bryophytes in drier conditions peaked in N2-fixation rates in spring and autumn. The seasonal variation in N2-fixation activity was pronounced, but similar patterns were found among different species. This study reveals that bryophyte-associated N2-fixation in northern ecosystems is larger than previously assumed, as it occurs over a more extended part of the year than previously inferred. Furthermore, the importance of bryophyte-associated diazotrophs cannot be quantified without considering both the diversity of bryophytes and their variation in N2-fixing seasonal activity patterns. Both future changes in climatic conditions and biodiversity of bryophytes can thus have large implications for the N cycle in arctic regions.

Empirical studies worldwide show that warming has variable effects on plant litter decomposition, leaving the overall impact of climate change on decomposition uncertain. We conducted a meta-analysis of 109 experimental warming studies across seven continents, using natural and standardised plant material, to assess the overarching effect of warming on litter decomposition and identify potential moderating factors. We determined that at least 5.2° of warming is required for a significant increase in decomposition. Overall, warming did not have a significant effect on decomposition at a global scale. However, we found that warming reduced decomposition in warmer, low-moisture areas, while it slightly increased decomposition in colder regions, although this increase was not significant. This is particularly relevant given the past decade's global warming trend at higher latitudes where a large proportion of terrestrial carbon is stored. Future changes in vegetation towards plants with lower litter quality, which we show were likely to be more sensitive to warming, could increase carbon release and reduce the amount of organic matter building up in the soil. Our findings highlight how the interplay between warming, environmental conditions, and litter characteristics improves predictions of warming's impact on ecosystem processes, emphasising the importance of considering context-specific factors.

The future of the northern peatland carbon (C) sink is uncertain as the effects of warming on microbial metabolisms are unclear. While increased microbial CO2 emissions are expected under warming, the response of microbial photosynthesis remains unknown, complicating predictions of net microbial effects on peatland carbon emissions. Here, using a continental-scale experimental study, we show that warming amplifies microbial photosynthesis by 3.4 mgC m−2 h−1 per 1 °C increase. By 2100, this increase translates to a gain of 51.1 Tg of carbon per year from the northern peatland area under the pessimistic SSP 5-8.5 climatic change scenario, offsetting ~14% of projected heterotrophic CO2 emissions in northern peatlands. By linking field and microcosm experiments, we further show that enhanced microbial photosynthesis accelerates peatland CO2 uptake as photosynthetic microbial-C subsidies stimulate nutrient mineralization. These results underscore the importance of photosynthetic microbes for mitigating carbon emissions and supporting long-term carbon storage in peatlands.

Microorganisms play a crucial role in the carbon (C) dynamics of peatlands — a major terrestrial C reservoir. Because of their role in C emissions, heterotrophic microorganisms have attracted much attention over the past decades. CO2-fixing microorganisms (CFMs) remained largely overlooked, while they could attenuate C emissions. Here, we use metabarcoding and digital droplet PCR to survey microorganisms that potentially fix CO2 in different peatlands. We demonstrate that CFMs are abundant and diverse in peatlands, with on average 1021 CFMs contributing up to 40% of the total bacterial abundance. Using a joint-species distribution model, we identified a core and a specific CFM microbiome, the latter being influenced by temperature and nutrients. Our findings highlight that ASV richness and community structure were direct drivers of CFM abundance, while environmental parameters were indirect drivers. These results provide the basis for a better understanding of the role of CFMs in peatland C cycle inputs.

We perform a first permafrost higher education curriculum survey in Norden. Permafrost is part of the education within both bio- and geosciences and engineering, and the variation in educational activities reflect this. Five permafrost-specific geoscience and engineering permafrost courses exist, whereas there are 23 bachelor and 25 master courses with a permafrost content ranging from 1% to 50 %. The is large potential and clear needs for closer permafrost teaching collaboration. This could focus on permafrost course development, teaching methods, sharing practical experiences including fieldwork and further developing the educational offer. Such collaboration could establish: 1) An online, joint Nordic specific course on permafrost, sharing the special permafrost competences existing across the universities using digital teaching tools, 2) Nordic collaboration on developing joint, both general but also specific, PhD courses on permafrost, 3) Lifelong education in permafrost, and 4) Internships a part of active permafrost education to better meet the future employers and society’s needs. The Nordic region might also gain largely from establishing an overview-providing interdisciplinary joint Nordic course aiming to characterize the region and its diversity broadly including both natural and social sciences, and naturally covering different topics including permafrost and seasonally frozen ground. The mapping done for this paper will function as a first overall roadmap catalogue providing an overview of all offered courses on permafrost. The overall outcome of our survey shows large potential for increased and deeper inter-university collaboration for further developing joint permafrost higher education both in the form of courses and other educational activities between institutions across Norden, and potentially with ambitions for joint permafrost degrees between several institutions. Based on the presented results and the mapped different future plans for permafrost education across Norden, we discuss the implications of our results, specifically concerning the potential for increased collaboration in Nordic permafrost education. These focus on permafrost course development, teaching methods, sharing practical experiences including fieldwork and further developing the educational offer. In more detail increased collaboration could establish: 1) An online, joint Nordic-specific course on permafrost, sharing the special permafrost competences existing across the universities using digital teaching tools, 2) Nordic collaboration on developing joint PhD courses on permafrost, 3) Lifelong education in permafrost, and 4) Internships as part of active permafrost education to better meet the needs of future employers and society. The Nordic region might also gain largely from establishing an interdisciplinary joint Nordic course, aiming to characterize the region and its diversity broadly and including both natural and social sciences, and naturally covering different topics including permafrost and seasonally frozen ground.

Arctic and alpine tundra ecosystems are large reservoirs of organic carbon^1,2. Climate warming may stimulate ecosystem respiration and release carbon into the atmosphere^3,4. The magnitude and persistency of this stimulation and the environmental mechanisms that drive its variation remain uncertain^5–7. This hampers the accuracy of global land carbon–climate feedback projections^7,8. Here we synthesize 136 datasets from 56 open-top chamber in situ warming experiments located at 28 arctic and alpine tundra sites which have been running for less than 1 year up to 25 years. We show that a mean rise of 1.4 °C [confidence interval (CI) 0.9–2.0 °C] in air and 0.4 °C [CI 0.2–0.7 °C] in soil temperature results in an increase in growing season ecosystem respiration by 30% [CI 22–38%] (n = 136). Our findings indicate that the stimulation of ecosystem respiration was due to increases in both plant-related and microbial respiration (n = 9) and continued for at least 25 years (n = 136). The magnitude of the warming effects on respiration was driven by variation in warming-induced changes in local soil conditions, that is, changes in total nitrogen concentration and pH and by context-dependent spatial variation in these conditions, in particular total nitrogen concentration and the carbon:nitrogen ratio. Tundra sites with stronger nitrogen limitations and sites in which warming had stimulated plant and microbial nutrient turnover seemed particularly sensitive in their respiration response to warming. The results highlight the importance of local soil conditions and warming-induced changes therein for future climatic impacts on respiration.

Observed climate change in northern high latitudes is strongest in winter, but still relatively little is known about the effects of winter climate change on tundra ecosystems. Ongoing changes in winter climate and snow cover will change the intensity, duration and frequency of frost events. Bryophytes form a major component of northern ecosystems but their responses to winter climate changes are largely unknown.

Here, we studied how changes in overall winter climate and snow regime affect frost damage in three common bryophyte taxa that differ in desiccation tolerance in a subarctic tundra ecosystem. We used a snow manipulation experiment where bryophyte cores were transplanted from just above the tree line to similar elevation (i.e. current cold climate) and lower elevation (i.e. near-future warmer climate scenario) in Abisko, Sweden. Here, we measured frost damage in shoots of Ptilidium ciliare, Hylocomium splendens and Sphagnum fuscum with the relative electrolyte leakage (REL) method, during late winter and spring in two consecutive years. We hypothesized that frost damage would be lower in a milder climate (low site) and higher under reduced snow cover and that taxa from moister habitats with assumed low desiccation tolerance would be more sensitive to lower temperature and thinner snow cover than those from drier and more exposed habitats.

Contrary to our expectations, frost damage was highest at low elevation, while the effect of snow treatment differed across sites and taxa. At the high site, frost damage was reduced under snow addition in the taxon with the assumed lowest desiccation tolerance, S. fuscum. Surprisingly, frost damage increased with mean temperature in the bryophyte core of the preceding 14 days leading up to REL measurements and decreased with higher frost degree sums, that is, was highest in the milder climate at the low site.

Synthesis Our results imply that climate warming in late winter and spring increases frost damage in bryophytes. Given the high abundance of bryophytes in tundra ecosystems, higher frost damage could alter the appearance and functioning of the tundra landscape, although the short and long-term effects on bryophyte fitness remain to be studied.

Climate change in high latitude regions leads to both higher temperatures and more precipitation but their combined effects on terrestrial ecosystem processes are poorly understood. In nitrogen (N) limited and often moss-dominated tundra and boreal ecosystems, moss-associated N₂ fixation is an important process that provides new N. We tested whether high mean annual precipitation enhanced experimental warming effects on growing season N₂ fixation in three common arctic-boreal moss species adapted to different moisture conditions and evaluated their N contribution to the landscape level. We measured in situ N₂ fixation rates in Hylocomium splendens, Pleurozium schreberi and Sphagnum spp. from June to September in subarctic tundra in Sweden. We exposed mosses occurring along a natural precipitation gradient (mean annual precipitation: 571–1155 mm) to 8 years of experimental summer warming using open-top chambers before our measurements. We modelled species-specific seasonal N input to the ecosystem at the colony and landscape level. Higher mean annual precipitation clearly increased N₂ fixation, especially during peak growing season and in feather mosses. For Sphagnum-associated N₂ fixation, high mean annual precipitation reversed a small negative warming response. By contrast, in the dry-adapted feather moss species higher mean annual precipitation led to negative warming effects. Modelled total growing season N inputs for Sphagnum spp. colonies were two to three times that of feather mosses at an area basis. However, at the landscape level where feather mosses were more abundant, they contributed 50% more N than Sphagnum. The discrepancy between modelled estimates of species-specific N input via N₂ fixation at the moss core versus ecosystem scale, exemplify how moss cover is essential for evaluating impact of altered N₂ fixation. Importantly, combined effects of warming and higher mean annual precipitation may not lead to similar responses across moss species, which could affect moss fitness and their abilities to buffer environmental changes.

Open top chambers (OTCs) were adopted as the recommended warmingmechanism by the International Tundra Experiment network in the early 1990s. Since then, OTCs have been deployed across the globe. Hundreds of papers have reported the impacts of OTCs on the abiotic environment and the biota. Here, we review the impacts of the OTC on the physical environment, with comments on the appropriateness of using OTCs to characterize the response of biota to warming. The purpose of this review is to guide readers to previously published work and to provide recommendations for continued use of OTCs to understand the implications of warming on low stature ecosystems. In short, the OTC is a useful tool to experimentally manipulate temperature; however, the characteristics and magnitude of warming varies greatly in different environments; therefore, it is important to document chamber performance to maximize the interpretation of biotic response.When coupled with long-term monitoring, warming experiments are a valuable means to understand the impacts of climate change on natural ecosystems.

The distribution of roots throughout the soil drives depth-dependent plant–soil interactions and ecosystem processes, particularly in arctic tundra where plant biomass, is predominantly belowground. Vegetation is usually classified from aboveground, but it is unclear whether such classifications are suitable to estimate belowground attributes and their consequences, such as rooting depth distribution and its influence on carbon cycling. We performed a meta-analysis of 55 published arctic rooting depth profiles, testing for differences both between distributions based on aboveground vegetation types (Graminoid, Wetland, Erect-shrub, and Prostrate-shrub tundra) and between ‘Root Profile Types’ for which we defined three representative and contrasting clusters. We further analyzed potential impacts of these different rooting depth distributions on rhizosphere priming-induced carbon losses from tundra soils. Rooting depth distribution hardly differed between aboveground vegetation types but varied between Root Profile Types. Accordingly, modelled priming-induced carbon emissions were similar between aboveground vegetation types when they were applied to the entire tundra, but ranged from 7.2 to 17.6 Pg C cumulative emissions until 2100 between individual Root Profile Types. Variations in rooting depth distribution are important for the circumpolar tundra carbon-climate feedback but can currently not be inferred adequately from aboveground vegetation type classifications.