The decomposition of large stocks of soil organic carbon in thawing permafrost might depend on more than climate change-induced temperature increases: indirect effects of thawing via altered bacterial community structure (BCS) or rooting patterns are largely unexplored. We used a 10-year in situ permafrost thaw experiment and aerobic incubations to investigate alterations in BCS and potential respiration at different depths, and the extent to which they are related with each other and with root density. Active layer and permafrost BCS strongly differed, and the BCS in formerly frozen soils (below the natural thawfront) converged under induced deep thaw to strongly resemble the active layer BCS, possibly as a result of colonization by overlying microorganisms. Overall, respiration rates decreased with depth and soils showed lower potential respiration when subjected to deeper thaw, which we attributed to gradual labile carbon pool depletion. Despite deeper rooting under induced deep thaw, root density measurements did not improve soil chemistry-based models of potential respiration. However, BCS explained an additional unique portion of variation in respiration, particularly when accounting for differences in organic matter content. Our results suggest that by measuring bacterial community composition, we can improve both our understanding and the modeling of the permafrost carbon feedback.

Upward shifts of mountain vegetation lag behind rates of climate warming, partly related to interconnected changes belowground. Here, we unravel above- and belowground linkages by drawing insights from short-term experimental manipulations and elevation gradient studies. Soils will likely gain carbon in early successional ecosystems, while losing carbon as forest expands upward, and the slow, high-elevation soil development will constrain warming-induced vegetation shifts. Current approaches fail to predict the pace of these changes and how much they will be modified by interactions among plants and soil biota. Integrating mountain soils and their biota into monitoring programs, combined with innovative comparative and experimental approaches, will be crucial to overcome the paucity of belowground data and to better understand mountain ecosystem dynamics and their feedbacks to climate.

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In agro-ecosystems, fertilization practices are crucial for sustaining crop productivity. Here, based on a\ud 50-year long-term experiment, we studied the influence of fertilization practices (inorganic and/or\ud organic) and nitrogen (N) application rates on (i) soil physicochemical properties, (ii) microbial and\ud earthworm communities and (iii) crop production. Our results showed that soil organic carbon content\ud was increased by incorporation of crop residues (+2.45%) and farmyard manure application (+6.40%) in\ud comparison to the use of mineral fertilizer alone. In contrast, soil carbon stock was not significantly\ud affected by these fertilization practices. Overall, only farmyard manure application improved soil\ud physicochemical properties compared to mineral fertilization alone. Soil microbial population was\ud enhanced by the application of organic amendments as indicated by microbial biomass and\ud phospholipid-derived fatty acids contents. The fertilization practices and the N application rates\ud affected significantly both the biomass and composition of earthworm populations, especially the epigeic\ud and endogeic species. Finally, farmyard manure application significantly increased crop yield (+3.5%) in\ud comparison to mineral fertilization alone. Crop residue incorporation rendered variable but similar crop\ud yields over the 50-year period. The results of this long-term experiment indicate that the use of organic\ud amendments not only reduces the need for higher amount of mineral N fertilizer but also improves the\ud soil biological properties with direct effects on crop yield

The frequency of freezing events during the early growing season and the vulnerability to freezing of plants in European high-altitude environments could increase under future atmospheric and climate change. We tested early growing season freezing sensitivity in 10 species, from four plant functional types (PFTs) spanning three plant growth forms (PGFs), from a long-term in situ CO 2 enrichment (566 vs. 370 ppm) and 2-year soil warming (1 4 K) experiment at treeline in the Swiss Alps (Stillberg, Davos). By additionally tracking plant phenology, we distinguished indirect phenology-driven CO 2 and warming effects from direct physiology-related effects on freezing sensitivity. The freezing damage threshold (lethal temperature 50) under ambient conditions of the 10 treeline species spanned from À6.7 AE 0.3 1C (Larix decidua) to À9.9 AE 0.6 1C (Vaccinium gaultherioides). PFT, but not PGF, explained a significant amount of this interspecific variation. Long-term exposure to elevated CO 2 led to greater freezing sensitivity in multiple species but did not influence phenology, implying that physiological changes caused by CO 2 enrichment were responsible for the effect. The elevated CO 2 effect on freezing resistance was significant in leaves of Larix, Vaccinium myrtillus, and Gentiana punctata and marginally significant in leaves of Homogyne alpina and Avenella flexuosa. No significant CO 2 effect was found in new shoots of Empetrum hermaphroditum or in leaves of Pinus uncinata, Leontodon helveticus, Melampyrum pratense, and V. gaultherioides. Soil warming led to advanced leaf expansion and reduced freezing resistance in V. myrtillus only, whereas Avenella showed greater freezing resistance when exposed to warming. No effect of soil warming was found in any of the other species. Effects of elevated CO 2 and soil warming on freezing sensitivity were not consistent within PFTs or PGFs, suggesting that any future shifts in plant community composition due to increased damage from freezing events will likely occur at the individual species level.

Warming-induced microbial decomposition of organic matter in permafrost soils constitutes a climate-change feedback of uncertain magnitude. While physico-chemical constraints on soil functioning are relatively well understood, the constraints attributable to microbial community composition remain unclear. Here we show that biogeochemical processes in permafrost can be impaired by missing functions in the microbial community -functional limitations -likely due to environmental filtering of the microbial community over millennia-long freezing. We inoculated Yedoma permafrost with a functionally diverse exogenous microbial community to test this mechanism by introducing potentially missing microbial functions. This initiated nitrification activity and increased CO2 production by 38% over 161 days. The changes in soil functioning were strongly associated with an altered microbial community composition, rather than with changes in soil chemistry or microbial biomass. The present permafrost microbial community composition thus constrains carbon and nitrogen biogeochemical processes, but microbial colonization, likely to occur upon permafrost thaw in situ, can alleviate such functional limitations. Accounting for functional limitations and their alleviation could strongly increase our estimate of the vulnerability of permafrost soil organic matter to decomposition and the resulting global climate feedback.

Climatic changes resulting from anthropogenic activities over the passed century are repeatedly reported to alter the functioning of pristine ecosystems worldwide, and especially those in cold biomes. Available literature on the process of plant leaf litter decomposition in the temperate Alpine zone is reviewed here, with emphasis on both direct and indirect effects of climate change phenomena on rates of litter decay. Weighing the impact of biotic and abiotic processes governing litter mass loss, it appears that an immediate intensification of decomposition rates due to temperature rise can be retarded by decreased soil moisture, insufficient snow cover insulation, and shrub expansion in the Alpine zone. This tentative conclusion, remains speculative unless empirically tested, but it has profound implications for understanding the biogeochemical cycling in the Alpine vegetation belt, and its potential role as a buffering mechanism to climate change.

Dinitrogen fixation by cyanobacteria is of particular importance for the nutrient economy of cold biomes, constituting the main pathway for new N supplies to tundra ecosystems. It is prevalent in cyanobacterial colonies on bryophytes and in obligate associations within cyanolichens. Recent studies, applying interspecific variation in plant functional traits to upscale species effects on ecosystems, have all but neglected cryptogams and their association with cyanobacteria. Here we looked for species-specific patterns that determine cryptogam-mediated rates of N 2 fixation in the Subarctic. We hypothesised a contrast in N 2 fixation rates (1) between the structurally and physiologically different lichens and bryophytes, and (2) within bryophytes based on their respective plant functional types. Throughout the survey we supplied 15 N-labelled N 2 gas to quantify fixation rates for monospecific moss, liverwort and lichen turfs. We sampled fifteen species in a design that captures spatial and temporal variations during the growing season in Abisko region, Sweden. We measured N 2 fixation potential of each turf in a common environment and in its field sampling site, in order to embrace both comparativeness and realism. Cyanolichens and bryophytes differed significantly in their cyanobacterial N 2 fixation capacity, which was not driven by microhabitat characteristics, but rather by morphology and physiology. Cyanolichens were much more prominent fixers than bryophytes per unit dry weight, but not per unit area due to their low specific thallus weight. Mosses did not exhibit consistent differences in N 2 fixation rates across species and functional types. Liverworts did not fix detectable amounts of N 2 . Despite the very high rates of N 2 fixation associated with cyanolichens, large cover of mosses per unit area at the landscape Plant Soil (2010) 333:507-517