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High elevation and latitude ecosystems are experiencing high levels of anthropogenic atmospheric warming. Climate warming may directly change soil microbial activity and alter ecosystem carbon dynamics and productivity, but increasing evidence suggests these responses may depend on other biotic factors such as plant community composition and abiotic factors such as moisture.We examined how abiotic (warming) and biotic (presence of dominant plant species) factors interact to affect soil microbial processes. Our experiment deployed the independent and combined treatments of experimental warming and dominant plant species removal in a high and low elevation montane meadows. We analysed multiple soil microbial responses to warming and the presence of a dominant plant at three times throughout the growing season, including soil respiration, microbial metabolic functional diversity, microbial biomass carbon and nitrogen, and extracellular enzyme potential activity.Overall, there were few independent microbial responses to either the warming or the removal treatments. There was a significant interaction between warming and the removal of a dominant plant species, where microbial biomass and the activity of some microbial enzymes were lower in warmed plots where the dominant species was removed relative to control plots.The effect of warming on extracellular enzyme activity was typically observed only at the high‐elevation site. In contrast, we found that effects of warming were consistent across the growing season, despite strong temporal variation in microbial properties.Our results emphasize the need to further consider soil microbial responses to warming under multiple environments, including shifts in both biotic and abiotic factors, to aid in predictions of carbon dynamics under future global change. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

Understanding arctic ecosystem function is key to understanding future global carbon (C) and nutrient cycling processes. However, small mammal herbivores can have effects on ecosystems as structure builders and these effects have been underrepresented in the understanding of arctic systems.We examined the impact of small mammal structures (hay piles, runways, latrines) on soils and plants in three arctic tundra regions near Utqiaġvik, Toolik Lake, and Nome, Alaska. Our aims were to (1) examine how vole and lemming structures influence plant and soil nutrient pools and microbial processes, (2) determine if structure effects were similar across tundra system types, and (3) understand how changes in the abundance and cover of these structures during different phases of small mammal multi‐annual population cycles might influence biogeochemical cycling.In general, small mammal structures increased nitrogen (N) availability in soils, although effects varied by study region. Across study regions, hay piles were relatively uncommon (lowest % cover) but increased multiple soil N and P pools, C‐ and N‐acquiring enzyme activities, and leaf phosphorus (P) concentrations, with the specific nutrient variables and size of the effects varying by study region. Latrines had the second highest cover and influenced multiple C, N and P pools, but their effects were mainly observed within a single region. Lastly, runways had the highest % cover of all activity types but increased the fewest number of soil nutrient variables.We conclude that by influencing soil nutrient availability and biogeochemical cycling, small mammal structures can influence bottom‐up regulation of ecosystem function, particularly during the high phase of the small mammal population cycle. Future changes in these population cycles might alter the role of small mammals in the Arctic and have lasting effects on system processes. Read the free Plain Language Summary for this article on the Journal blog [ABSTRACT FROM AUTHOR]

We use a simple model of coupled carbon and nitrogen cycles in terrestrial ecosystems to examine how "explicitly representing grazers" vs. "having grazer effects implicitly aggregated in with other biogeochemical processes in the model" alters predicted responses to elevated carbon dioxide and warming. The aggregated approach can affect model predictions because grazer‐mediated processes can respond differently to changes in climate compared with the processes with which they are typically aggregated. We use small‐mammal grazers in a tundra as an example and find that the typical three‐to‐four‐year cycling frequency is too fast for the effects of cycle peaks and troughs to be fully manifested in the ecosystem biogeochemistry. We conclude that implicitly aggregating the effects of small‐mammal grazers with other processes results in an underestimation of ecosystem response to climate change, relative to estimations in which the grazer effects are explicitly represented. The magnitude of this underestimation increases with grazer density. We therefore recommend that grazing effects be incorporated explicitly when applying models of ecosystem response to global change. [ABSTRACT FROM AUTHOR]

Phosphorus (P) limits or co‐limits plant and microbial life in multiple ecosystems, including the arctic tundra. Although current global carbon (C) models focus on the coupling between soil nitrogen (N) and C, ecosystem P response to climate warming may also influence the global C cycle. Permafrost soils may see enhanced or reduced P availability under climate warming through multiple mechanisms including changing litter inputs through plant community change, changing plant–microbial dynamics, altered rates of mineralization of soil organic P through increased microbial activity, and newly exposed mineral‐bound P via deeper thaw. We investigated the effect of long‐term warming on plant leaf, multiple soil and microbial C, N, and P pools, and microbial extracellular enzyme activities, in Alaskan tundra plots underlain by permafrost. Here, we show that 25 yr of experimental summer warming increases community‐level plant leaf P through changing community composition to favour relatively P‐rich plant species. However, despite associated increases in P‐rich litter inputs, we found only a few responses in the belowground pools of P available for plant and microbial uptake, including a weak positive response for citric acid–extractable PO4 in the surface soil, a decrease in microbial biomass P, and no change in soil P (or C or N) stocks. This weak, neutral, or negative belowground P response to warming despite enhanced litter P inputs is consistent with a growing number of studies in the arctic tundra that find no long‐term response of soil C and N stocks to warming. [ABSTRACT FROM AUTHOR]

Herbivores can play an important role in determining arctic ecosystem function with effects determined in part by herbivore identity. We examined the impact of long-term (twenty-two years) small and large mammal herbivore exclusion in two arctic plant communities in northern Alaska: dry heath (DH) and moist acidic tundra (MAT). Our aims were to examine how herbivore exclusion influences (1) plant communities and (2) soil nutrient pools and microbial processes. Though herbivore absence increased moss and decreased evergreen shrub cover in MAT, there were few other significant effects on vegetation in either community. We also observed no influence of exclusion on most soil properties. However, in DH, phosphatase activity was greater in areas where small mammals alone were present, suggesting that they are altering phosphorus (P) availability, perhaps through herbivores' influence on the plant community and subsequently on competition for P with the microbial community. We conclude that herbivore impacts in the Arctic are dependent on both the plant community and herbivore identity (size). We show the importance of understanding the roles of herbivores in the Arctic and contribute to a growing number of herbivore studies in a biome likely to experience future changes in herbivore communities and ecosystem function. [ABSTRACT FROM AUTHOR]

Increasing abundance of tall deciduous shrubs has been shown to affect tundra soil properties, which may further affect carbon and nitrogen cycling in these soils, including through effects on litter decomposition. These tall deciduous shrubs produce a higher quantity of litter, and also a different quality of litter, than tundra species in non-shrub dominated tundra, both of which may affect community-wide decomposition rates. We examined how presence of tall deciduous shrubs, including resulting increases in litter quantity, affects litter decomposition through effects on both the decomposition environment and on litter composition. In a factorial experiment, we modified the decomposition environment by manipulating both the abundance of tall deciduous shrubs (present/absent) and their litter (absent, natural, and doubled). In each environment we decomposed litter of the tall deciduous shrub, Betula glandulosa and two other dominant species in our study area; Salix reticulata , a dwarf deciduous shrub, and Carex consimilis, a graminoid. We report that while tall Betula shrub presence reduced decomposition rates, decomposition was not affected by manipulations of litter quantity. Although litter decomposed slower under deciduous shrubs, B. glandulosa litter decomposed faster than the litter of the other dominant tundra plant species. However, the reduction in decomposition rates due to tall deciduous shrub presence and higher decomposition rates of B. glandulosa litter in comparison to other plant species were of the same magnitude and these opposing effects may result in little to no change of shrub encroachment on litter decomposition rate overall. • Shrub encroachment decreased summer decomposition rates through their physical presence. • Shrubs increase decomposition rates because shrub litter decomposes faster than other plants. • Opposing effects of shrubs of similar magnitude may result in no overall effect on decomposition. [ABSTRACT FROM AUTHOR]

Global change in the Arctic promotes deeper soil thaw and enhanced soil microbial activity, increasing nitrogen (N) and phosphorus (P) availability to plants and microbes in strongly nutrient‐limited ecosystems. This critical, positive climate feedback has been examined through fertilization experiments that describe short‐term (<10 yr) above‐ or belowground responses to combined NP additions, with evidence of enhanced shrub growth, nutrient availability, and soil organic matter decomposition. There has been less opportunity for long‐term comparisons of both above‐ and belowground responses with factorial N and P additions in different systems, despite broad awareness that ecosystem response can shift with time, and the potential for decoupled above‐ vs. belowground or N vs. P responses, currently and with further predicted global change. We examined the response of the plants, soil microbes, and soil nutrients, to factorial N and P additions in the moist acidic tundra (MAT; 26 yr of nutrient additions) and moist non‐acidic tundra (MNT; 16 yr). Aboveground, the MAT plant community continues to change as predicted by earlier studies: Functional groups responded independently to N and P, but NDVI‐biomass, especially of Betula nana, only increased with N addition. Unlike shorter‐term MNT studies, the MNT vegetation, which does not include B. nana, shows few new fertilization responses. Belowground responses were not predicted by aboveground responses in either MAT or MNT. In contrast to the N response aboveground, MAT microbial biomass responded positively and microbial phosphatase activity negatively to P additions, implying possible release from microbial P limitation. Critically, earlier published results of declines in soil total carbon (C) with combined NP addition in the MAT are not present in the long term. We make two conclusions: (1) Arctic ecosystems are not universally N‐limited but also exhibit complex responses to P alone or in combination with N; and (2) the presence or absence of key vegetation species can cascade from aboveground to belowground and restrict the extrapolation of responses of nutrient addition in a single arctic ecosystem to other arctic ecosystems, the short‐term to the long term, or aboveground to belowground. [ABSTRACT FROM AUTHOR]

Deciduous shrub encroachment in tundra ecosystems affects the soil microclimate and, in turn, could affect soil nutrients and microbial processes. Although numerous effects of shrubs on tundra ecosystem properties have been described, there has been little focus on the mechanisms through which they impact tundra functioning. In alpine tundra of northern Canada, we examined 1) the effects of the living shrub canopy and separated them from 2) the effects of increases in litter quantity and quality. In this 4-year study we experimentally manipulated shrub presence (shrub present or removed) and litter quantity (none, natural abundance, and 2x natural abundance) in a fully factorial design. Shrub presence affected soil temperatures by increasing winter and decreased summer soil surface temperatures by about 0.8 °C and 0.6 °C respectively, and also decreased mid-summer soil moisture. Shrub presence also affected both nutrient cycling and microbial processes including decreased summer available nitrogen (N), increased N-immobilization, increased microbial biomass as well as variable effects on potential exoenzyme activity. Increased litter inputs had few significant effects on tundra ecosystems, although additional litter increased both soil C:N ratios and microbial biomass. Our results indicate that shrub encroachment is likely to affect tundra soil properties primarily through mechanisms associated with the presence of the living shrub rather than from the accompanied changing litter quality and quantity. Because of opposing effects of shrubs between seasons and years, we encourage an increased focus on the mechanisms through which shrubs may affect tundra ecosystem functions and year round assessments of these processes. • Shrub canopy presence increases winter but reduces summer soil temperatures. • There were few ecosystem effects of increasing shrub litter quality and quantity. • Shrub presence directly influenced nutrient cycling and microbial processes. [ABSTRACT FROM AUTHOR]