Changing precipitation patterns and global warming have greatly changed winter snow cover, which can affect litter decomposition process by altering soil microenvironment or microbial biomass and activity. However, it remains unknown how and to what extent snow cover affects litter decomposition during winter and over longer periods of time. Here, we conducted a meta-analysis to synthesize litter decomposition studies under different levels of snow cover. Overall, deepened snow significantly enhanced litter decomposition rate and mass loss by 17% and 3%, respectively. Deepened snow enhanced litter carbon loss by 7% but did not impact the loss of litter nitrogen or phosphorus. Deepened snow increased soil temperature, decreased the frequency of freeze-thaw cycles, and stimulated microbial biomass carbon and bacterial biomass during winter, but had no effect on these parameters in summer. The promoting effect of deepened snow cover on litter decomposition in winter is mainly due to its positive effect on microbial decomposition by increasing soil temperature and reducing freezethaw cycles exceeded its negative effect on physical fragmentation of litter by reducing freeze-thaw cycles. Our findings indicate that the changes in winter snow cover under global change scenarios can greatly impact winter litter decomposition and the associated carbon cycling, which should be taken into consideration when assessing the global carbon budget in modeling.

Litter decomposition represents a major path for atmospheric carbon influx into Arctic soils, thereby controlling below-ground carbon accumulation. Yet, little is known about how tundra litter decomposition varies with microenvironmental conditions, hindering accurate projections of tundra soil carbon dynamics with future climate change. Over 14 months, we measured landscape-scale decomposition of two contrasting standard litter types (Green tea and Rooibos tea) in 90 plots covering gradients of micro-climate and -topography, vegetation cover and traits, and soil characteristics in Western Greenland. We used the tea bag index (TBI) protocol to estimate relative variation in litter mass loss, decomposition rate (k) and stabilisation factor (S) across space, and structural equation modelling (SEM) to identify relationships among environmental factors and decomposition. Contrasting our expectations, microenvironmental factors explained little of the observed variation in both litter mass loss, as well as k and S, suggesting that the variables included in our study were not the major controls of decomposer activity in the soil across the studied tundra landscape. We use these unexpected findings of our study combined with findings from the current literature to discuss future avenues for improving our understanding of the drivers of tundra decomposition and, ultimately, carbon cycling across the warming Arctic.

Tundra soils are one of the world's largest organic carbon stores, yet this carbon is vulnerable to accelerated decomposition as climate warming progresses. The landscape-scale controls of litter decomposition are poorly understood in tundra ecosystems, which hinders our understanding of the global carbon cycle. We examined the extent to which the thermal sum of surface air temperature, soil moisture and permafrost thaw depth influenced litter mass loss and decomposition rates (k), and at which spatial thresholds an environmental variable becomes a reliable predictor of decomposition, using the Tea Bag Index protocol across a heterogeneous tundra landscape on Qikiqtaruk-Herschel Island, Yukon, Canada. We found greater green tea litter mass loss and faster decomposition rates (k) in wetter areas within the landscape, and to a lesser extent in areas with deeper permafrost active layer thickness and higher surface thermal sums. We also found higher decomposition rates (k) on north-facing relative to south-facing aspects at microsites that were wetter rather than warmer. Spatially heterogeneous belowground conditions (soil moisture and active layer depth) explained variation in decomposition metrics at local scales (< 50 m(2)) better than thermal sum. Surprisingly, there was no strong control of elevation or slope on litter decomposition. Our results reveal that there is considerable scale dependency in the environmental controls of tundra litter decomposition, with moisture playing a greater role than the thermal sum at < 50 m(2) scales. Our findings highlight the importance and complexity of microenvironmental controls on litter decomposition in estimates of carbon cycling in a rapidly warming tundra biome.

Rising temperatures in the Arctic and the expansion of plants to higher latitudes will significantly alter belowground microbial communities and their activity. Given that microbial communities are major producers of greenhouse gases, understanding the magnitude of microbial responses to warming and increased carbon input to Arctic soils is necessary to improve global climate change models. In this study, active layer and permafrost soils from northern Greenland (81 degrees N) were subjected to increased carbon input, in the form of plant litter, and temperature increase, using a combined field and laboratory approach. In the field experiment, unamended or litter-amended soils were transplanted from the permafrost layer to the top soil layer and incubated for one year, whereas in the laboratory experiment active layer and permafrost soils with or without litter amendment were incubated at 4 degrees C or 15 degrees C for six weeks. Soil microbial communities were evaluated using bacterial 16S and fungal ITS amplicon sequencing and respiration was used as a measure of microbial activity. Litter amendment resulted in similar changes in microbial abundances, diversities and structure of microbial communities, in the field and lab experiments. These changes in microbial communities were likely due to a strong increase in fast-growing bacterial copiotrophic taxa and basidiomycete yeasts. Furthermore, respiration was significantly higher with litter input for both active layer and permafrost soil and with both approaches. Temperature alone had only a small effect on microbial communities, with the exception of the field-incubated permafrost soils, where we observed a shift towards oligotrophic taxa, specifically for bacteria. These results demonstrate that alterations in High Arctic mineral soils may result in predictable shifts in the soil microbiome.

Aim Litter humification is vital for carbon sequestration in terrestrial ecosystems. Probing the litter humification of treeline ecotone will be helpful to understand soil carbon afflux in alpine regions under climate change. Methods Foliar litter of six plant functional groups was chosen in an alpine treeline ecotone of the eastern Tibetan Plateau, and a field litterbag decomposition experiment (669 days) was conducted in an alpine shrubland (AS) and a coniferous forest (CF). Environmental factors, litter quality, humus concentrations (total humus, Huc; humic acid, HAc; and fulvic acid, FAc) and hue coefficient (Delta logK and E4/E6) were measured to explore litter humification processes. Results Litter humification was controlled by both litter stoichiometric traits and local-environment conditions, while stoichiometric traits played a more obvious regulatory role. Significant discrepancies in litter humus were detected among six plant functional groups; more precisely, litter of evergreen conifer and shrubs showed a net accumulation of Huc and FAc during winter, whereas others experienced more mineralization than accumulation. Huc, HAc, and hue coefficient were mainly controlled by cellulose/N, cellulose/P, C/N, lignin/P, lignin/N, etc., yet FAc was more susceptible to local-environment conditions. Meanwhile, Huc, HAc and FAc, as well as humification degree and E4/E6 differed between AS and CF, with faster humification in AS. Conclusion We suggest that litter stoichiometric traits are more responsible for regulating litter humification than environmental conditions in elevational gradients. Furthermore, potential upward shifts by plants may accelerate litter humification in alpine ecosystems.

Tundra regions are projected to warm rapidly during the coming decades. The tundra biome holds the largest terrestrial carbon pool, largely contained in frozen permafrost soils. With warming, these permafrost soils may thaw and become available for microbial decomposition, potentially providing a positive feedback to global warming. Warming may directly stimulate microbial metabolism but may also indirectly stimulate organic matter turnover through increased plant productivity by soil priming from root exudates and accelerated litter turnover rates. Here, we assess the impacts of experimental warming on turnover rates of leaf litter, active layer soil and thawed permafrost sediment in two high-arctic tundra heath sites in NE-Greenland, either dominated by evergreen or deciduous shrubs. We incubated shrub leaf litter on the surface of control and warmed plots for 1 and 2 years. Active layer soil was collected from the plots to assess the effects of 8 years of field warming on soil carbon stocks. Finally, we incubated open cores filled with newly thawed permafrost soil for 2 years in the active layer of the same plots. After field incubation, we measured basal respiration rates of recovered thawed permafrost cores in the lab. Warming significantly reduced litter mass loss by 26% after 1 year incubation, but differences in litter mass loss among treatments disappeared after 2 years incubation. Warming also reduced litter nitrogen mineralization and decreased the litter carbon to nitrogen ratio. Active layer soil carbon stocks were reduced 15% by warming, while soil dissolved nitrogen was reduced by half in warmed plots. Warming had a positive legacy effect on carbon turnover rates in thawed permafrost cores, with 10% higher respiration rates measured in cores from warmed plots. These results demonstrate that warming may have contrasting effects on above- and belowground tundra carbon turnover, possibly governed by microbial resource availability.

Accompanying the seasonal soil freeze-thaw cycle, microbial decomposition of litter exhibited different dynamic response to various snow thicknesses. In this study, we used real-time qPCR to investigate the abundance of bacteria, archaea, ammonia-oxidizing archaea (AOA) and bacteria (AOB), and the amoA gene transcripts, during the decomposition of dwarf bamboo (Fargesia nitida) litter under different snow patches at various snow-cover stages in an alpine forest on the eastern Tibetan Plateau in China. The effects of snow thickness were significant, with thicker snow patches resulting in higher microbial abundance and the amoA gene transcripts, while the degree of the effects were different. Compared with AOB, AOA were more abundant on the majority of sampling dates during the freeze-thaw period, and as well as their amoA gene transcripts. AOA are more persistent and abundant than AOB, and the higher AOA/AOB ratios were observed clearly in shrub litter and continued to decrease as the snow thickness increased, meanwhile gradually increased under uniform snow thickness over time. Our results suggested that the reduced seasonal snow cover and shortened freeze-thaw cycle periods caused by winter warming would significantly affect the ammonia oxidizers particularly tied to the ammonia oxidation process, and then could contribute to N cycle as related to litter in alpine forest ecosystems.

Decomposition is a key process in carbon and nutrient cycling. However, little is known about its response to altered winter soil temperature regimes in boreal forests. Here, the impact of soil frost on cellulose decomposition over 1 yr and soil biotic activity (bait-lamina sticks) over winter, in spring, and in summer was investigated using a long-term (9-yr) snow-cover manipulation experiment in a boreal Picea abies forest. The experiment consisted of the treatments: snow removal, increased insulation, and ambient control. The snow removal treatment caused longer and deeper soil frost (minimum temperature - 8.6 degrees C versus - 1.4 degrees C) at 10 cm soil depth in comparison with control, while the increased insulation treatment resulted in nearly no soil frost during winter. Annual cellulose decomposition rates were reduced by 46% in the snow removal manipulation in comparison with control conditions. Increased insulation had no significant effect on decomposition. The decomposition was mainly driven by microorganisms, as no significant difference was observed for containers enclosed with a 44-mu m and a 1-mm mesh. Soil biotic activity was slightly increased by both the snow removal and the increased insulation treatment in comparison with control conditions over winter. However, this effect disappeared over spring and summer. We conclude that soil frost can have strong effects on decomposition in boreal ecosystems. Further studies should investigate to which degree the observed reduction in decomposition due to reduced snow cover in winter slows or even offsets the expected increase in decomposition rates with global warming.

Permafrost thaw can affect decomposition rates by changing environmental conditions and litter quality. As permafrost thaws, soils warm and thermokarst (ground subsidence) features form, causing some areas to become wetter while other areas become drier. We used a common substrate to measure how permafrost thaw affects decomposition rates in the surface soil in a natural permafrost thaw gradient and a warming experiment in Healy, Alaska. Permafrost thaw also changes plant community composition. We decomposed 12 plant litters in a common garden to test how changing plant litter inputs would affect decomposition. We combined species' tissue-specific decomposition rates with species and tissue-level estimates of aboveground net primary productivity to calculate community-weighted decomposition constants at both the thaw gradient and warming experiment. Moisture, specifically growing season precipitation and water table depth, was the most significant driver of decomposition. At the gradient, an increase in growing season precipitation from 200 to 300mm increased mass loss of the common substrate by 100%. At the warming experiment, a decrease in the depth to the water table from 30 to 15cm increased mass loss by 100%. At the gradient, community-weighted decomposition was 21% faster in extensive than in minimal thaw, but was similar when moss production was included. Overall, the effect of climate change and permafrost thaw on surface soil decomposition are driven more by precipitation and soil environment than by changes to plant communities. Increasing soil moisture is thereby another mechanism by which permafrost thaw can become a positive feedback to climate change.

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.