The circumpolar Arctic is currently facing multiple global changes that have the potential to alter the capacity of tundra soils to store carbon. Yet, predicting changes in soil carbon is hindered by the fact that multiple factors simultaneously control processes sustaining carbon storage and we do not understand how they act in concert. Here, we investigated the effects of warmer temperatures, enhanced soil nitrogen availability, and the combination of these on tundra carbon stocks at three different grazing regimes: on areas with over 50‐yr history of either light or heavy reindeer grazing and in 5‐yr‐old exlosures in the heavily grazed area. In line with earlier reports, warming generally decreased soil carbon stocks. However, our results suggest that the mechanisms by which warming decreases carbon storage depend on grazing intensity: under long‐term light grazing soil carbon losses were linked to higher shrub abundance and higher enzymatic activities, whereas under long‐term heavy grazing, carbon losses were linked to drier soils and higher enzymatic activities. Importantly, under enhanced soil nitrogen availability, warming did not induce soil carbon losses under either of the long‐term grazing regimes, whereas inside exclosures in the heavily grazed area, also the combination of warming and enhanced nutrient availability induced soil carbon loss. Grazing on its own did not influence the soil carbon stocks. These results reveal that accounting for the effect of warming or grazing alone is not sufficient to reliably predict future soil carbon storage in the tundra. Instead, the joint effects of multiple global changes need to be accounted for, with a special focus given to abrupt changes in grazing currently taking place in several parts of the Arctic.

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Composition and functioning of arctic soil fungal communities may alter rapidly due to the ongoing trends of warmer temperatures, shifts in nutrient availability, and shrub encroachment. In addition, the communities may also be intrinsically shaped by heavy grazing, which may locally induce an ecosystem change that couples with increased soil temperature and nutrients and where shrub encroachment is less likely to occur than in lightly grazed conditions.We tested how 4 yr of experimental warming and fertilization affected organic soil fungal communities in sites with decadal history of either heavy or light reindeer grazing using highthroughput sequencing of the internal transcribed spacer 2 ribosomal DNA region.Grazing history largely overrode the impacts of short-term warming and fertilization in determining the composition of fungal communities. The less diverse fungal communities under light grazing showed more pronounced responses to experimental treatments when compared with the communities under heavy grazing. Yet, ordination approaches revealed distinct treatment responses under both grazing intensities.If grazing shifts the fungal communities in Arctic ecosystems to a different and more diverse state, this shift may dictate ecosystem responses to further abiotic changes. This indicates that the intensity of grazing cannot be left out when predicting future changes in fungi-driven processes in the tundra.

Plant meristems were previously thought to be sterile. Today, meristem-associated shoot endophytes are mainly reported as contaminants from plant tissue cultures, the number of observed species being very low. However, the few strains characterized have the capacity for infecting host cells and affecting plant growth and development. Here we studied the communities of endophytic bacteria in the buds of mountain birch (Betula pubescens ssp. czerepanovii (N. I. Orlova) Hämet-Ahti) exposed to winter moth (Operophtera brumata L.) herbivory, to identify differences between sprouts and branches of mature birch trees. Mountain birch of the high subarctic is cyclically exposed to winter moth and produces sprouts to generate new trees as a survival mechanism. The majority (54%) of operational taxonomic units belonged to Xanthomonadaceae and Pseudomonales of Proteobacteria. Most of the observed species were classified as Xanthomonas (28%). Sprout buds had the highest diversity, containing approximately three times more species, and significantly more (43%) Pseudomonas species than the mature trees (14%). Our results demonstrate that endophytic communities of buds are richer than previously thought. We suggest that the meristem-associated endophytes should be studied further for a possible role in sprouting and aiding regeneration of trees.