Rainfall manipulation experiments as simulated by terrestrial biosphere models: Where do we stand?

Changes in rainfall amounts and patterns have been observed and are expected to continue in the near future with potentially significant ecological and societal consequences. Modelling vegetation responses to changes in rainfall is thus crucial to project water and carbon cycles in the future. In this study, we present the results of a new model-data intercomparison project, where we tested the ability of 10 terrestrial biosphere models to reproduce the observed sensitivity of ecosystem productivity to rainfall changes at 10 sites across the globe, in nine of which, rainfall exclusion and/or irrigation experiments had been performed. The key results are as follows: (a) Inter-model variation is generally large and model agreement varies with timescales. In severely water-limited sites, models only agree on the interannual variability of evapotranspiration and to a smaller extent on gross primary productivity. In more mesic sites, model agreement for both water and carbon fluxes is typically higher on fine (daily-monthly) timescales and reduces on longer (seasonal-annual) scales. (b) Models on average overestimate the relationship between ecosystem productivity and mean rainfall amounts across sites (in space) and have a low capacity in reproducing the temporal (interannual) sensitivity of vegetation productivity to annual rainfall at a given site, even though observation uncertainty is comparable to inter-model variability. (c) Most models reproduced the sign of the observed patterns in productivity changes in rainfall manipulation experiments but had a low capacity in reproducing the observed magnitude of productivity changes. Models better reproduced the observed productivity responses due to rainfall exclusion than addition. (d) All models attribute ecosystem productivity changes to the intensity of vegetation stress and peak leaf area, whereas the impact of the change in growing season length is negligible. The relative contribution of the peak leaf area and vegetation stress intensity was highly variable among models.

Fast attrition of springtail communities by experimental drought and richness-decomposition relationships across Europe.

Soil fauna play a fundamental role on key ecosystem functions like organic matter decomposition, although how local assemblages are responding to climate change and whether these changes may have consequences to ecosystem functioning is less clear. Previous studies have revealed that a continued environmental stress may result in poorer communities by filtering out the most sensitive species. However, these experiments have rarely been applied to climate change factors combining multiyear and multisite standardized field treatments across climatically contrasting regions, which has limited drawing general conclusions. Moreover, other facets of biodiversity, such as functional and phylogenetic diversity, potentially more closely linked to ecosystem functioning, have been largely neglected. Here, we report that the abundance, species richness, phylogenetic diversity, and functional richness of springtails (Subclass Collembola), a major group of fungivores and detritivores, decreased within 4 years of experimental drought across six European shrublands. The loss of phylogenetic and functional richness was higher than expected by the loss of species richness, leading to communities of phylogenetically similar species sharing evolutionary conserved traits. Additionally, despite the great climatic differences among study sites, we found that taxonomic, phylogenetic, and functional richness of springtail communities alone were able to explain up to 30% of the variation in annual decomposition rates. Altogether, our results suggest that the forecasted reductions in precipitation associated with climate change may erode springtail communities and likely other drought-sensitive soil invertebrates, thereby retarding litter decomposition and nutrient cycling in ecosystems.

Temperature response of soil respiration largely unaltered with experimental warming.

The respiratory release of carbon dioxide (CO2) from soil is a major yet poorly understood flux in the global carbon cycle. Climatic warming is hypothesized to increase rates of soil respiration, potentially fueling further increases in global temperatures. However, despite considerable scientific attention in recent decades, the overall response of soil respiration to anticipated climatic warming remains unclear. We synthesize the largest global dataset to date of soil respiration, moisture, and temperature measurements, totaling >3,800 observations representing 27 temperature manipulation studies, spanning nine biomes and over 2 decades of warming. Our analysis reveals no significant differences in the temperature sensitivity of soil respiration between control and warmed plots in all biomes, with the exception of deserts and boreal forests. Thus, our data provide limited evidence of acclimation of soil respiration to experimental warming in several major biome types, contrary to the results from multiple single-site studies. Moreover, across all nondesert biomes, respiration rates with and without experimental warming follow a Gaussian response, increasing with soil temperature up to a threshold of ∼25 °C, above which respiration rates decrease with further increases in temperature. This consistent decrease in temperature sensitivity at higher temperatures demonstrates that rising global temperatures may result in regionally variable responses in soil respiration, with colder climates being considerably more responsive to increased ambient temperatures compared with warmer regions. Our analysis adds a unique cross-biome perspective on the temperature response of soil respiration, information critical to improving our mechanistic understanding of how soil carbon dynamics change with climatic warming.

Fast Responses of Root Dynamics to Increased Snow Deposition and Summer Air Temperature in an Arctic Wetland.

In wet tundra ecosystems, covering vast areas of the Arctic, the belowground plant biomass exceeds the aboveground, making root dynamics a crucial component of the nutrient cycling and the carbon (C) budget of the Arctic. In response to the projected climatic scenarios for the Arctic, namely increased temperature and changes in precipitation patterns, root dynamics may be altered leading to significant changes in the net ecosystem C budget. Here, we quantify the single and combined effects of 1 year of increased winter snow deposition by snow fences and summer warming by open-top chambers (OTCs) on root dynamics in a wetland at Disko Island (West Greenland). Based on ingrowth bags, snow accumulation decreased root productivity by 42% in the 0-15 cm soil depth compared to ambient conditions. Over the growing season 2014, minirhizotron observations showed that root growth continued until mid-September in all treatments, and it peaked between the end of July and mid-August. During the season, plots exposed to experimental warming showed a significant increase in root number during September (between 39 and 53%) and a 39% increase in root length by the beginning of September. In addition, a significant reduction of root diameter (14%) was observed in plots with increased snow accumulation. Along the soil profile (0-40 cm) summer warming by OTCs significantly increased the total root length (54%), root number (41%) and the root growth in the 20-30 cm soil depth (71%). These results indicate a fast response of this ecosystem to changes in air temperature and precipitation. Hence, on a short-term, summer warming may lead to increased root depth and belowground C allocation, whereas increased winter snow precipitation may reduce root production or favor specific plant species by means of reduced growing season length or increased nutrient cycling. Knowledge on belowground root dynamics is therefore critical to improve the estimation of the C balance of the Arctic.

Mean annual precipitation predicts primary production resistance and resilience to extreme drought.

Extreme drought is increasing in frequency and intensity in many regions globally, with uncertain consequences for the resistance and resilience of ecosystem functions, including primary production. Primary production resistance, the capacity to withstand change during extreme drought, and resilience, the degree to which production recovers, vary among and within ecosystem types, obscuring generalized patterns of ecological stability. Theory and many observations suggest forest production is more resistant but less resilient than grassland production to extreme drought; however, studies of production sensitivity to precipitation variability indicate that the processes controlling resistance and resilience may be influenced more by mean annual precipitation (MAP) than ecosystem type. Here, we conducted a global meta-analysis to investigate primary production resistance and resilience to extreme drought in 64 forests and grasslands across a broad MAP gradient. We found resistance to extreme drought was predicted by MAP; however, grasslands (positive) and forests (negative) exhibited opposing resilience relationships with MAP. Our findings indicate that common plant physiological mechanisms may determine grassland and forest resistance to extreme drought, whereas differences among plant residents in turnover time, plant architecture, and drought adaptive strategies likely underlie divergent resilience patterns. The low resistance and resilience of dry grasslands suggests that these ecosystems are the most vulnerable to extreme drought - a vulnerability that is expected to compound as extreme drought frequency increases in the future.

Global negative vegetation feedback to climate warming responses of leaf litter decomposition rates in cold biomes.

Whether climate change will turn cold biomes from large long-term carbon sinks into sources is hotly debated because of the great potential for ecosystem-mediated feedbacks to global climate. Critical are the direction, magnitude and generality of climate responses of plant litter decomposition. Here, we present the first quantitative analysis of the major climate-change-related drivers of litter decomposition rates in cold northern biomes worldwide. Leaf litters collected from the predominant species in 33 global change manipulation experiments in circum-arctic-alpine ecosystems were incubated simultaneously in two contrasting arctic life zones. We demonstrate that longer-term, large-scale changes to leaf litter decomposition will be driven primarily by both direct warming effects and concomitant shifts in plant growth form composition, with a much smaller role for changes in litter quality within species. Specifically, the ongoing warming-induced expansion of shrubs with recalcitrant leaf litter across cold biomes would constitute a negative feedback to global warming. Depending on the strength of other (previously reported) positive feedbacks of shrub expansion on soil carbon turnover, this may partly counteract direct warming enhancement of litter decomposition.

Shrubland primary production and soil respiration diverge along European climate gradient.

Above- and belowground carbon (C) stores of terrestrial ecosystems are vulnerable to environmental change. Ecosystem C balances in response to environmental changes have been quantified at individual sites, but the magnitudes and directions of these responses along environmental gradients remain uncertain. Here we show the responses of ecosystem C to 8-12 years of experimental drought and night-time warming across an aridity gradient spanning seven European shrublands using indices of C assimilation (aboveground net primary production: aNPP) and soil C efflux (soil respiration: Rs). The changes of aNPP and Rs in response to drought indicated that wet systems had an overall risk of increased loss of C but drier systems did not. Warming had no consistent effect on aNPP across the climate gradient, but suppressed Rs more at the drier sites. Our findings suggest that above- and belowground C fluxes can decouple, and provide no evidence of acclimation to environmental change at a decadal timescale. aNPP and Rs especially differed in their sensitivity to drought and warming, with belowground processes being more sensitive to environmental change.

Long-term and realistic global change manipulations had low impact on diversity of soil biota in temperate heathland.

In a dry heathland ecosystem we manipulated temperature (warming), precipitation (drought) and atmospheric concentration of CO2 in a full-factorial experiment in order to investigate changes in below-ground biodiversity as a result of future climate change. We investigated the responses in community diversity of nematodes, enchytraeids, collembolans and oribatid mites at two and eight years of manipulations. We used a structural equation modelling (SEM) approach analyzing the three manipulations, soil moisture and temperature, and seven soil biological and chemical variables. The analysis revealed a persistent and positive effect of elevated CO2 on litter C:N ratio. After two years of treatment, the fungi to bacteria ratio was increased by warming, and the diversities within oribatid mites, collembolans and nematode groups were all affected by elevated CO2 mediated through increased litter C:N ratio. After eight years of treatment, however, the CO2-increased litter C:N ratio did not influence the diversity in any of the four fauna groups. The number of significant correlations between treatments, food source quality, and soil biota diversities was reduced from six to three after two and eight years, respectively. These results suggest a remarkable resilience within the soil biota against global climate change treatments in the long term.

Differential responses of grass and a dwarf shrub to long-term changes in soil microbial biomass C, N and P following factorial addition of NPK fertilizer, fungicide and labile carbon to a heath.

Microbial immobilization may decrease the inorganic nutrient concentrations of the soil to the extent of affecting plant nutrient uptake and growth. We have hypothesized that graminoids with opportunistic nutrient-acquisition strategies are strongly influenced by nutrient limitation imposed by microbes, whereas growth forms such as dwarf shrubs are less affected by the mobilization-immobilization cycles in microbes. By adding NPK fertilizer, labile C (sugar) and fungicide (benomyl) over a 5 yr period in a fully factorial design, we aimed to manipulate the sink-source potential for nutrients in a non-acidic heath tundra soil. After 2 yr, N and P accumulated in the microbial biomass after fertilization with no change in microbial C, which suggests that nutrients did not limit microbial biomass growth. After 5 yr, microbial C was enhanced by 60% in plots with addition of labile C, which points to C-limitation of the microbial biomass. Microbial biomass N and P tended to increase following addition of labile C, by 10 and 25%, respectively. This caused decreased availability of NH4 + and P, showing close microbial control of nutrient availability. The most common graminoid, Festuca ovina, responded to fertilizer addition with a strong increase, and to labile C addition with a strong decrease in cover, providing the first direct field evidence that nutrient limitation imposed by immobilizing microbes can affect the growth of tundra plants. Also in support of our hypothesis, following addition of labile C the concentrations of N and K in leaves and that of N in roots of F. ovina decreased, whilst the demand of roots for P increased. In contrast, the most common dwarf shrub, Vaccinium uliginosum, was only slightly sensitive to changes in resource availability, showing no cover change after 4 yr addition of labile C and fertilizer, and little change in leaf nutrient concentrations. We suggest that the differential responses of the two growth forms are due to differences in storage and nutrient uptake pathways, with the dwarf shrub having large nutrient storage capacity and access to organic forms of N through its mycorrhizal association. While the fungicide had no effect on ericoid mycorrhizal colonization of roots or symbiotic function inferred from plant 15 N natural abundance, it decreased microbial biomass C and N after 2 yr. Throughout the fifth season, the availability of soil NO3 - and inorganic P was decreased with no change in microbial biomass C, N or P, suggesting a negative impact of benomyl on N and P mineralization.

Environmental control and intersite variations of phenolics in Betula nana in tundra ecosystems.

• Secondary metabolites make leaves unpalatable for herbivores and influence decomposition. Site-specific differences are presented in phenolics and nitrogen in Betula nana leaves from dwarf shrub tundra at Abisko, northern Sweden, and from tussock tundra at Toolik Lake, Alaska, subjected to a decade of warming, fertilization and shading. • Nitrogen and a number of phenolics, including condensed and hydrolysable tannins, flavonoids, phenolic glucosides and chlorogenic acids, were analysed in B. nana leaves. • Phenolic concentrations showed marked between-site differences (e.g. condensed tannins were 50% higher at Abisko than at Toolik); responses to the environmental manipulations were more pronounced at Toolik compared with Abisko. Warming increased condensed tannins and decreased hydrolysable tannins at Toolik, but had no effect at Abisko, whereas fertilization and shading generally decreased concentrations. • Betula invests less carbon in phenolics at Toolik than at Abisko and shows a greater response to environmental changes by investing more carbon in growth and less to phenolic production. Hence, the Toolik population has a lower herbivore-defense level, which declines further if nutrient availability increases. By contrast, under warmer conditions, the increase in bulk phenolics and decrease in leaf palatability are greater at Toolik than at Abisko.

Effects on plant production after addition of labile carbon to arctic/alpine soils.

Biomass production was analysed in Festuca vivipara, grown for 3 months in pots with non-sterilized or sterilized soil after factorial addition of three levels of labile carbon combined with high and low levels of N and P. The soil was a nutrient-poor subarctic heath soil. In the non-sterilized soil plant biomass production increased strongly only in the treatment with high levels of both N and P, which suggests that both nutrients limited plant growth. In the sterilized soil addition of a high level of N without P addition gave almost the same growth response as in the combined NP treatment. This was because of a more than 30-fold increase of inorganic phosphorus in the soil as P was released from the killed microbial biomass after sterilization. Sugar addition reduced plant growth in all treatments. The reduction in plant growth was dose dependent within the range of 0-450 µg C g-1 soil added to the non-sterilized soil, but the response levelled off at 233 µg C g-1 soil in the soil that had been sterilized at the start of the experiment. The plant response, together with observed depletion of soil inorganic N and P, indicated that the microbial biomass immobilized nutrients efficiently and reduced plant growth when extra labile carbon was added. The inhibition of growth was lower, however, in the soil which had been sterilized, probably because of a slow recovery of the microbial populations in it. Two of the nutrient-carbon solutions closely matched the N, P and C concentrations in a solution containing leaf extracts of Cassiope tetragona and Betula tortuosa that had been used previously to test for possible allelopathic effects of compounds in the leaf extracts. These extracts also reduced plant growth. The growth reduction was equally large or larger after nutrient-sugar addition than after addition of leaf extracts in three out of the four possible combinations of species and sterilized or non-sterilized soil. In the fourth case (Betula extract added to sterilized soil), the effect was larger when leaf extract was added than after addition of the nutrient-carbon solution. This could be due to a low rate of microbial degradation of phytotoxic substances in this soil because of a slow recovery of the microbial populations after sterilization. The generally stronger or equal effect of the nutrient-sugar addition compared to the leaf extract addition leads to the conclusion that microbial nutrient immobilization and microbial competition for nutrients increased as a function of labile carbon addition with the extract. Hence, it appears that enhanced microbial activity and microbial nutrient immobilization rather than phytotoxic effects was the primary reasons for the reduced biomass production in F. vivipara even after addition of the leaf extracts.

Effects of labile soil carbon on nutrient partitioning between an arctic graminoid and microbes.

We measured partitioning of N and P uptake between soil microorganisms and potted Festuca vivipara in soil from a subarctic heath in response to factorial addition of three levels of labile carbon (glucose) combined with two levels of inorganic N and P. The glucose was added to either non-sterilized or sterilized (autoclaved) soils in quantities which were within the range of reported, naturally occurring amounts of C released periodically from the plant canopy. The aims were, firstly, to examine whether the glucose stimulated microbial nutrient uptake to the extent of reducing plant nutrient uptake. This is expected in nutrient-deficient soils if microbes and plants compete for the same nutrients. Secondly, we wanted to test our earlier␣interpretation that growth reduction observed in graminoids after addition of leaf extracts could be caused directly by labile carbon addition, rather than by phytotoxins in the extracts. Addition of high amounts of N did not affect the microbial N pool, whereas high amounts of added P significantly increased the microbial P pool, indicating a luxury P uptake in the microbes. Both plant N and in particular P uptake increased strongly in response to soil sterilization and to addition of extra N or P. The increased␣uptake led to enhanced plant growth when both elements were applied in high amounts, but only led to increased tissue concentrations without growth responses when the nutrients were added separately. Glucose had strong and contrasting effects on plant and microbial N and P uptake. Microbial N and P uptake increased, soil inorganic N and P concentrations were reduced and plant N and P uptake declined when glucose was added. The responses were dose-dependent within the range of 0-450 µg C g-1 soil added to the non-sterilized soil. The opposite responses of plants and microbes showed that plant acquisition of limiting nutrients is dependent on release of nutrients from the soil microbes, which is under strong regulation by the availability and microbial uptake of labile C. Hence, we conclude, firstly, that the microbial populations can compete efficiently with plants for nutrients to an extent of affecting plant growth when the microbial access to labile carbon is high in nutrient deficient soils. We also conclude that reduced growth of plants after addition of leaf extracts to soil can be caused by carbon-induced shifts in nutrient partitioning between plants and microbes, and not necessarily by phytotoxins added with the extracts as suggested by some experiments.

Leaf 15N abundance of subarctic plants provides field evidence that ericoid, ectomycorrhizal and non-and arbuscular mycorrhizal species access different sources of soil nitrogen.

The natural abundance of the nitrogen isotope 15, δ15N, was analysed in leaves of 23 subarctic vascular plant species and two lichens from a tree-line heath at 450 m altitude and a fellfield at 1150 m altitude close to Abisko in N. Sweden, as well as in soil, rain and snow. The aim was to reveal if plant species with different types of mycorrhizal fungi also differ in their use of the various soil N sources. The dwarf shrubs and the shrubs, which in combination formed more than 65% of the total above-ground biomass at both sites, were colonized by ericoid or ectomycorrhizal fungi. Their leaf δ15N was between-8.8 and-5.5‰ at the heath and between-6.1 and -3.3‰ at the fellfield. The leaf δ15N of non- or arbuscular mycorrhizal species was markedly different, ranging from -4.1 to -0.4‰ at the heath, and from -3.4 to+2.2‰ at the fellfield. We conclude that ericoid and ectomycorrhizal dwarf shrubs and shrubs utilize a distinct N source, most likely a fraction of the organic N in fresh litter, and not complexed N in recalcitrant organic matter. The latter is the largest component of soil total N, which had a δ15N of -0.7‰ at the heath and +0.5‰ at the fellfield. Our field-based data thus support earlier controlled-environment studies and studies on the N uptake of excised roots, which have demonstrated protease activity and amino acid uptake by ericoid and ectomycorrhizal tundra species. The leaves of ectomycorrhizal plants had slightly higher δ15N (fellfield) and N concentration than leaves of the ericoids, and Betula nana, Dryas octopetala and Salix spp. also showed NO inf3sup- reductase activity. These species may depend more on soil inorganic N than the ericoids. The δ15N of non- or arbuscular mycorrhizal species indicates that the δ15N of inorganic N available to these plants was higher than that of average fresh litter, probably due to high microbial immobilization of inorganic N. The δ15N of NH inf4sup+ -N was +12.3‰ in winter snow and +1.9‰ in summer rain. Precipitation N might be a major contributer in species with poorly developed root systems, e.g. Lycopodium selago. Our results show that coexisting plant species under severe nutrient limitation may tap several different N sources: NH inf4sup+ , NO inf3sup- and organic N from the soil, atmospheric N2, and N in precipitation. Ericoid and ectomycorrhizal fungi are of major importance for plant N uptake in tundra ecosystems, and mycorrhizal fungi probably exert a major control on plant δ15N in organic soils.

Inhibition of growth, and effects on nutrient uptake of arctic graminoids by leaf extracts - allelopathy or resource competition between plants and microbes?

Previous research has shown that plant extracts, e.g. from boreal dwarf shrubs and trees, can cause reduced growth of neighbouring plants: an effect known as allelopathy. To examine whether arctic and subarctic plants could also be affected by leaching of phytochemicals, we added extracts from the commonly occurring arctic dwarf shrubs Cassiope tetragona and Empetrum hermaphroditum, and from mountain birch, Betula pubescens ssp. tortuosa to three graminoid species, Carex bigelowii, Festuca vivipara and Luzula arcuata, grown in previously sterilized or non-sterilized arctic soils. The graminoids in non-sterilized soil grew more slowly than those in sterilized soil. Excised roots of the plants in non-sterilized soil had higher uptake rate of labelled P than those in sterilized soil, demonstrating larger nutrient deficiency. The difference in growth rate was probably caused by higher nutrient availability for plants in soils in which the microbial biomass was killed after soil sterilization. The dwarf shrub extracts contained low amounts of inorganic N and P and medium high amounts of carbohydrates. Betula extracts contained somewhat higher levels of N and much higher levels of P and carbohydrates. Addition of leaf extracts to the strongly nutrient limited graminoids in non-sterilized soil tended to reduce growth, whereas in the less nutrient limited sterilized soil it caused strong growth decline. Furthermore, the N and P uptake by excised roots of plants grown in both types of soil was high if extracts from the dwarf shrubs (with low P and N concentrations) had been added, whereas the P uptake declined but the N uptake increased after addition of the P-rich Betula extract. In contrast to the adverse extract effects on plants, soil microbial respiration and soil fungal biomass (ergosterol) was generally stimulated, most strongly after addition of the Betula extract. Although we cannot exclude the possibility that the reduced plant growth and the concomitant stimulation of microbial activity were caused by phytochemicals, we believe that this was more likely due to labile carbon in the extracts which stimulated microbial biomass and activity. As a result microbial uptake increased, thereby depleting the plant available pool of N and P, or, for the P-rich Betula extract, depleting soil inorganic N alone, to the extent of reducing plant growth. This chain of events is supported by the negative correlation between plant growth and sugar content in the three added extracts, and the positive correlation between microbial activity, fungal biomass production and sugar content, and are known reactions when labile carbon is added to nutrient deficient soils.

Microbial biomass C, N and P in two arctic soils and responses to addition of NPK fertilizer and sugar: implications for plant nutrient uptake.

The soil microbial carbon (C), nitrogen (N) and phosphorus (P) pools were quantified in the organic horizon of soils from an arctic/alpine low-altitude heath and a high-altitude fellfield by the fumigation-extraction method before and after factorial addition of sugar, NPK fertilizer and benomyl, a fungicide. In unamended soil, microbial C, N and P made up 3.3-3.6%, 6.1-7.3% and 34.7% of the total soil C, N and P content, respectively. The inorganic extractable N pool was below 0.1% and the inorganic extractable P content slightly less than 1% of the total soil pool sizes. Benomyl addition in spring and summer did not affect microbial C or nutrient content analysed in the autumn. Sugar amendments increased microbial C by 15 and 37% in the two soils, respectively, but did not affect the microbial nutrient content, whereas inorganic N and P either declined significantly or tended to decline. The increased microbial C indicates that the microbial biomass also increased but without a proportional enhancement of N and P uptake. NPK addition did not affect the amount of microbial C but almost doubled the microbial N pool and more than doubled the P pool. A separate study has shown that CO2 evolution increased by more than 50% after sugar amendment and by about 30% after NPK and NK additions to one of the soils. Hence, the microbial biomass did not increase in response to NPK addition, but the microbes immobilized large amounts of the added nutrients and, judging by the increased CO2 evolution, their activity increased. We conclude: (1) that microbial biomass production in these soils is stimulated by labile carbon and that the microbial activity is stimulated by both labile C and by nutrients (N); (2) that the microbial biomass is a strong sink for nutrients and that the microbial community probably can withdraw substantial amounts of nutrients from the inorganic, plant-available pool, at least periodically; (3) that temporary declines in microbial populations are likely to release a flush of inorganic nutrients to the soil, particularly P of which the microbial biomass contained more than one third of the total soil pool; and (4) that the mobilization-immobilization cycles of nutrients coupled to the population dynamics of soil organisms can be a significant regulating factor for the nutrient supply to the primary producers, which are usually strongly nutrient-limited in arctic ecosystems.