Comparison of vertical and horizontal flow planted and unplanted subsurface flow wetlands treating municipal wastewater.

In the search for design criteria for constructed wetlands (CWs) in Nepal a semi-scale experimental setup including horizontal flow (HF) and vertical flow (VF) CWs was developed. This paper compares the performance of HF and VF wetlands, and planted with unplanted beds. The experimental setup consists of two units of HF and VF beds of size 6 m × 2 m × 0.6 m and 6 m × 2 m × 0.8 m (length × width × depth) respectively. For both HF and VF systems, one unit was planted with Phragmites karka (local reed) and one was not planted. The systems were fed with wastewater drawn from the grit chamber of a municipal wastewater treatment plant. The media consisted of river gravel. In the first phase of the experiment the hydraulic loading rate (HLR) was varied in steps; 0.2, 0.08, 0.04 m(3)/m(2)/d and the percent removal increase with decrease in HLR for all beds and parameters except for total phosphorus. In the second phase the loading rate of 0.04 m(3)/m(2)/d was run for 7 months. In both parts of the experiment the planted beds performed better than the unplanted beds and the VF better than the HF beds. To meet Nepalese discharge standards HF beds are sufficient, but to meet stricter requirements a combination of HF and VF beds are recommended.

Two decades of experimental manipulations of heaths and forest understory in the subarctic.

Current atmospheric warming due to increase of greenhouse gases will have severe consequences for the structure and functioning of arctic ecosystems with changes that, in turn, may feed back on the global-scale composition of the atmosphere. During more than two decades, environmental controls on biological and biogeochemical processes and possible atmospheric feedbacks have been intensely investigated at Abisko, Sweden, by long-term ecosystem manipulations. The research has addressed questions like environmental regulation of plant and microbial community structure and biomass, carbon and nutrient pools and element cycling, including exchange of greenhouse gases and volatile organic compounds, with focus on fundamental processes in the interface between plants, soil and root-associated and free-living soil microorganisms. The ultimate goal has been to infer from these multi-decadal experiments how subarctic and arctic ecosystems will respond to likely environmental changes in the future. Here we give an overview of some of the experiments and main results.

Site-dependent N uptake from N-form mixtures by arctic plants, soil microbes and ectomycorrhizal fungi.

Soil microbes constitute an important control on nitrogen (N) turnover and retention in arctic ecosystems where N availability is the main constraint on primary production. Ectomycorrhizal (ECM) symbioses may facilitate plant competition for the specific N pools available in various arctic ecosystems. We report here our study on the N uptake patterns of coexisting plants and microbes at two tundra sites with contrasting dominance of the circumpolar ECM shrub Betula nana. We added equimolar mixtures of glycine-N, NH4+ -N and NO3(-) -N, with one N form labelled with 15N at a time, and in the case of glycine, also labelled with 13C, either directly to the soil or to ECM fungal ingrowth bags. After 2 days, the vegetation contained 5.6, 7.7 and 9.1% (heath tundra) and 7.1, 14.3 and 12.5% (shrub tundra) of the glycine-, NH4+ - and NO3 (-) -(15)N, respectively, recovered in the plant-soil system, and the major part of 15N in the soil was immobilized by microbes (chloroform fumigation-extraction). In the subsequent 24 days, microbial N turnover transferred about half of the immobilized 15N to the non-extractable soil organic N pool, demonstrating that soil microbes played a major role in N turnover and retention in both tundra types. The ECM mycelial communities at the two tundras differed in N-form preferences, with a higher contribution of glycine to total N uptake at the heath tundra; however, the ECM mycelial communities at both sites strongly discriminated against NO3 (-) . Betula nana did not directly reflect ECM mycelial N uptake, and we conclude that N uptake by ECM plants is modulated by the N uptake patterns of both fungal and plant components of the symbiosis and by competitive interactions in the soil. Our field study furthermore showed that intact free amino acids are potentially important N sources for arctic ECM fungi and plants as well as for soil microorganisms.

Increased ectomycorrhizal fungal abundance after long-term fertilization and warming of two arctic tundra ecosystems.

Shrub abundance is expected to increase with enhanced temperature and nutrient availability in the Arctic, and associated changes in abundance of ectomycorrhizal (EM) fungi could be a key link between plant responses and longer-term changes in soil organic matter storage. This study quantifies the response in EM fungal abundance to long-term warming and fertilization in two arctic ecosystems with contrasting responses of the EM shrub Betula nana. Ergosterol was used as a biomarker for living fungal biomass in roots and organic soil and ingrowth bags were used to estimate EM mycelial production. We measured 15N and 13C natural abundance to identify the EM-saprotrophic divide in fungal sporocarps and to validate the EM origin of mycelia in the ingrowth bags. Fungal biomass in soil and EM mycelial production increased with fertilization at both tundra sites, and with warming at one site. This was caused partly by increased dominance of EM plants and partly by stimulation of EM mycelial growth. We conclude that cycling of carbon and nitrogen through EM fungi will increase when strongly nutrient-limited arctic ecosystems are exposed to a warmer and more nutrient-rich environment. This has potential consequences for below-ground litter quality and quantity, and for accumulation of organic matter in arctic soils.

Long-term experimental warming, shading and nutrient addition affect the concentration of phenolic compounds in arctic-alpine deciduous and evergreen dwarf shrubs.

Environmental changes are likely to alter the chemical composition of plant tissues, including content and concentrations of secondary compounds, and thereby affect the food sources of herbivores. After 10 years of experimental increase of temperature, nutrient levels and light attenuation in a sub-arctic, alpine ecosystem, we investigated the effects on carbon based secondary compounds (CBSC) and nitrogen in one dominant deciduous dwarf shrub, Salix herbacea x polaris and two dominant evergreen dwarf shrubs, Cassiope tetragona and Vaccinium vitis-idaea throughout one growing season. The main aims were to compare the seasonal course and treatment effects on CBSC among the species, life forms and leaf cohorts and to examine whether the responses in different CBSC were consistent across compounds. The changes in leaf chemistry both during the season and in response to the treatments were higher in S. herbacea x polaris than in the corresponding current year's leaf cohort of the evergreen C. tetragona. The changes were also much higher than in the 1-year-old leaves of the two evergreens probably due to differences in dilution and turnover of CBSC in growing and mature leaves paired with different rates of allocation. Most low molecular weight phenolics in the current year's leaves decreased in all treatments. Condensed tannins and the tannin-to-N ratio, however, either increased or decreased, and the strength and even direction of the responses varied among the species and leaf cohorts, supporting views of influential factors additional to resource-based or developmental controls, as e.g. species specific or genetic controls of CBSC. The results indicate that there is no common response to environmental changes across species and substances. However, the pronounced treatment responses imply that the quality of the herbivore forage is likely to be strongly affected in a changing arctic environment, although both the direction and strength of the responses will be different among plant species, tissue types and substances.

Past changes in Arctic terrestrial ecosystems, climate and UV radiation.

At the last glacial maximum, vast ice sheets covered many continental areas. The beds of some shallow seas were exposed thereby connecting previously separated landmasses. Although some areas were ice-free and supported a flora and fauna, mean annual temperatures were 10-13 degrees C colder than during the Holocene. Within a few millennia of the glacial maximum, deglaciation started, characterized by a series of climatic fluctuations between about 18,000 and 11,400 years ago. Following the general thermal maximum in the Holocene, there has been a modest overall cooling trend, superimposed upon which have been a series of millennial and centennial fluctuations in climate such as the "Little Ice Age spanning approximately the late 13th to early 19th centuries. Throughout the climatic fluctuations of the last 150,000 years, Arctic ecosystems and biota have been close to their minimum extent within the most recent 10,000 years. They suffered loss of diversity as a result of extinctions during the most recent large-magnitude rapid global warming at the end of the last glacial stage. Consequently, Arctic ecosystems and biota such as large vertebrates are already under pressure and are particularly vulnerable to current and projected future global warming. Evidence from the past indicates that the treeline will very probably advance, perhaps rapidly, into tundra areas, as it did during the early Holocene, reducing the extent of tundra and increasing the risk of species extinction. Species will very probably extend their ranges northwards, displacing Arctic species as in the past. However, unlike the early Holocene, when lower relative sea level allowed a belt of tundra to persist around at least some parts of the Arctic basin when treelines advanced to the present coast, sea level is very likely to rise in future, further restricting the area of tundra and other treeless Arctic ecosystems. The negative response of current Arctic ecosystems to global climatic conditions that are apparently without precedent during the Pleistocene is likely to be considerable, particularly as their exposure to co-occurring environmental changes (such as enhanced levels of UV-B, deposition of nitrogen compounds from the atmosphere, heavy metal and acidic pollution, radioactive contamination, increased habitat fragmentation) is also without precedent.

Biodiversity, distributions and adaptations of Arctic species in the context of environmental change.

The individual of a species is the basic unit which responds to climate and UV-B changes, and it responds over a wide range of time scales. The diversity of animal, plant and microbial species appears to be low in the Arctic, and decreases from the boreal forests to the polar deserts of the extreme North but primitive species are particularly abundant. This latitudinal decline is associated with an increase in super-dominant species that occupy a wide range of habitats. Climate warming is expected to reduce the abundance and restrict the ranges of such species and to affect species at their northern range boundaries more than in the South: some Arctic animal and plant specialists could face extinction. Species most likely to expand into tundra are boreal species that currently exist as outlier populations in the Arctic. Many plant species have characteristics that allow them to survive short snow-free growing seasons, low solar angles, permafrost and low soil temperatures, low nutrient availability and physical disturbance. Many of these characteristics are likely to limit species' responses to climate warming, but mainly because of poor competitive ability compared with potential immigrant species. Terrestrial Arctic animals possess many adaptations that enable them to persist under a wide range of temperatures in the Arctic. Many escape unfavorable weather and resource shortage by winter dormancy or by migration. The biotic environment of Arctic animal species is relatively simple with few enemies, competitors, diseases, parasites and available food resources. Terrestrial Arctic animals are likely to be most vulnerable to warmer and drier summers, climatic changes that interfere with migration routes and staging areas, altered snow conditions and freeze-thaw cycles in winter, climate-induced disruption of the seasonal timing of reproduction and development, and influx of new competitors, predators, parasites and diseases. Arctic microorganisms are also well adapted to the Arctic's climate: some can metabolize at temperatures down to -39 degrees C. Cyanobacteria and algae have a wide range of adaptive strategies that allow them to avoid, or at least minimize UV injury. Microorganisms can tolerate most environmental conditions and they have short generation times which can facilitate rapid adaptation to new environments. In contrast, Arctic plant and animal species are very likely to change their distributions rather than evolve significantly in response to warming.

Responses to projected changes in climate and UV-B at the species level.

Environmental manipulation experiments showed that species respond individualistically to each environmental-change variable. The greatest responses of plants were generally to nutrient, particularly nitrogen, addition. Summer warming experiments showed that woody plant responses were dominant and that mosses and lichens became less abundant. Responses to warming were controlled by moisture availability and snow cover. Many invertebrates increased population growth in response to summer warming, as long as desiccation was not induced. CO2 and UV-B enrichment experiments showed that plant and animal responses were small. However, some microorganisms and species of fungi were sensitive to increased UV-B and some intensive mutagenic actions could, perhaps, lead to unexpected epidemic outbreaks. Tundra soil heating, CO2 enrichment and amendment with mineral nutrients generally accelerated microbial activity. Algae are likely to dominate cyanobacteria in milder climates. Expected increases in winter freeze-thaw cycles leading to ice-crust formation are likely to severely reduce winter survival rate and disrupt the population dynamics of many terrestrial animals. A deeper snow cover is likely to restrict access to winter pastures by reindeer/caribou and their ability to flee from predators while any earlier onset of the snow-free period is likely to stimulate increased plant growth. Initial species responses to climate change might occur at the sub-species level: an Arctic plant or animal species with high genetic/racial diversity has proved an ability to adapt to different environmental conditions in the past and is likely to do so also in the future. Indigenous knowledge, air photographs, satellite images and monitoring show that changes in the distributions of some species are already occurring: Arctic vegetation is becoming more shrubby and more productive, there have been recent changes in the ranges of caribou, and "new" species of insects and birds previously associated with areas south of the treeline have been recorded. In contrast, almost all Arctic breeding bird species are declining and models predict further quite dramatic reductions of the populations of tundra birds due to warming. Species-climate response surface models predict potential future ranges of current Arctic species that are often markedly reduced and displaced northwards in response to warming. In contrast, invertebrates and microorganisms are very likely to quickly expand their ranges northwards into the Arctic.

Effects on the structure of Arctic ecosystems in the short- and long-term perspectives.

Species individualistic responses to warming and increased UV-B radiation are moderated by the responses of neighbors within communities, and trophic interactions within ecosystems. All of these responses lead to changes in ecosystem structure. Experimental manipulation of environmental factors expected to change at high latitudes showed that summer warming of tundra vegetation has generally led to smaller changes than fertilizer addition. Some of the factors manipulated have strong effects on the structure of Arctic ecosystems but the effects vary regionally, with the greatest response of plant and invertebrate communities being observed at the coldest locations. Arctic invertebrate communities are very likely to respond rapidly to warming whereas microbial biomass and nutrient stocks are more stable. Experimentally enhanced UV-B radiation altered the community composition of gram-negative bacteria and fungi, but not that of plants. Increased plant productivity due to warmer summers may dominate food-web dynamics. Trophic interactions of tundra and sub-Arctic forest plant-based food webs are centered on a few dominant animal species which often have cyclic population fluctuations that lead to extremely high peak abundances in some years. Population cycles of small rodents and insect defoliators such as the autumn moth affect the structure and diversity of tundra and forest-tundra vegetation and the viability of a number of specialist predators and parasites. Ice crusting in warmer winters is likely to reduce the accessibility of plant food to lemmings, while deep snow may protect them from snow-surface predators. In Fennoscandia, there is evidence already for a pronounced shift in small rodent community structure and dynamics that have resulted in a decline of predators that specialize in feeding on small rodents. Climate is also likely to alter the role of insect pests in the birch forest system: warmer winters may increase survival of eggs and expand the range of the insects. Insects that harass reindeer in the summer are also likely to become more widespread, abundant and active during warmer summers while refuges for reindeer/caribou on glaciers and late snow patches will probably disappear.

Effects on the function of Arctic ecosystems in the short- and long-term perspectives.

Historically, the function of Arctic ecosystems in terms of cycles of nutrients and carbon has led to low levels of primary production and exchanges of energy, water and greenhouse gases have led to low local and regional cooling. Sequestration of carbon from atmospheric CO2, in extensive, cold organic soils and the high albedo from low, snow-covered vegetation have had impacts on regional climate. However, many aspects of the functioning of Arctic ecosystems are sensitive to changes in climate and its impacts on biodiversity. The current Arctic climate results in slow rates of organic matter decomposition. Arctic ecosystems therefore tend to accumulate organic matter and elements despite low inputs. As a result, soil-available elements like nitrogen and phosphorus are key limitations to increases in carbon fixation and further biomass and organic matter accumulation. Climate warming is expected to increase carbon and element turnover, particularly in soils, which may lead to initial losses of elements but eventual, slow recovery. Individual species and species diversity have clear impacts on element inputs and retention in Arctic ecosystems. Effects of increased CO2 and UV-B on whole ecosystems, on the other hand, are likely to be small although effects on plant tissue chemisty, decomposition and nitrogen fixation may become important in the long-term. Cycling of carbon in trace gas form is mainly as CO2 and CH4. Most carbon loss is in the form of CO2, produced by both plants and soil biota. Carbon emissions as methane from wet and moist tundra ecosystems are about 5% of emissions as CO2 and are responsive to warming in the absence of any other changes. Winter processes and vegetation type also affect CH4 emissions as well as exchanges of energy between biosphere and atmosphere. Arctic ecosystems exhibit the largest seasonal changes in energy exchange of any terrestrial ecosystem because of the large changes in albedo from late winter, when snow reflects most incoming radiation, to summer when the ecosystem absorbs most incoming radiation. Vegetation profoundly influences the water and energy exchange of Arctic ecosystems. Albedo during the period of snow cover declines from tundra to forest tundra to deciduous forest to evergreen forest. Shrubs and trees increase snow depth which in turn increases winter soil temperatures. Future changes in vegetation driven by climate change are therefore, very likely to profoundly alter regional climate.

Synthesis of effects in four Arctic subregions.

An assessment of impacts on Arctic terrestrial ecosystems has emphasized geographical variability in responses of species and ecosystems to environmental change. This variability is usually associated with north-south gradients in climate, biodiversity, vegetation zones, and ecosystem structure and function. It is clear, however, that significant east-west variability in environment, ecosystem structure and function, environmental history, and recent climate variability is also important. Some areas have cooled while others have become warmer. Also, east-west differences between geographical barriers of oceans, archipelagos and mountains have contributed significantly in the past to the ability of species and vegetation zones to relocate in response to climate changes, and they have created the isolation necessary for genetic differentiation of populations and biodiversity hot-spots to occur. These barriers will also affect the ability of species to relocate during projected future warming. To include this east-west variability and also to strike a balance between overgeneralization and overspecialization, the ACIA identified four major sub regions based on large-scale differences in weather and climate-shaping factors. Drawing on information, mostly model output that can be related to the four ACIA subregions, it is evident that geographical barriers to species re-location, particularly the distribution of landmasses and separation by seas, will affect the northwards shift in vegetation zones. The geographical constraints--or facilitation--of northward movement of vegetation zones will affect the future storage and release of carbon, and the exchange of energy and water between biosphere and atmosphere. In addition, differences in the ability of vegetation zones to re-locate will affect the biodiversity associated with each zone while the number of species threatened by climate change varies greatly between subregions with a significant hot-spot in Beringia. Overall, the subregional synthesis demonstrates the difficulty of generalizing projections of responses of ecosystem structure and function, species loss, and biospheric feedbacks to the climate system for the whole Arctic region and implies a need for a far greater understanding of the spatial variability in the responses of terrestrial arctic ecosystems to climate change.

Effects of changes in climate on landscape and regional processes, and feedbacks to the climate system.

Biological and physical processes in the Arctic system operate at various temporal and spatial scales to impact large-scale feedbacks and interactions with the earth system. There are four main potential feedback mechanisms between the impacts of climate change on the Arctic and the global climate system: albedo, greenhouse gas emissions or uptake by ecosystems, greenhouse gas emissions from methane hydrates, and increased freshwater fluxes that could affect the thermohaline circulation. All these feedbacks are controlled to some extent by changes in ecosystem distribution and character and particularly by large-scale movement of vegetation zones. Indications from a few, full annual measurements of CO2 fluxes are that currently the source areas exceed sink areas in geographical distribution. The little available information on CH4 sources indicates that emissions at the landscape level are of great importance for the total greenhouse balance of the circumpolar North. Energy and water balances of Arctic landscapes are also important feedback mechanisms in a changing climate. Increasing density and spatial expansion of vegetation will cause a lowering of the albedo and more energy to be absorbed on the ground. This effect is likely to exceed the negative feedback of increased C sequestration in greater primary productivity resulting from the displacements of areas of polar desert by tundra, and areas of tundra by forest. The degradation of permafrost has complex consequences for trace gas dynamics. In areas of discontinuous permafrost, warming, will lead to a complete loss of the permafrost. Depending on local hydrological conditions this may in turn lead to a wetting or drying of the environment with subsequent implications for greenhouse gas fluxes. Overall, the complex interactions between processes contributing to feedbacks, variability over time and space in these processes, and insufficient data have generated considerable uncertainties in estimating the net effects of climate change on terrestrial feedbacks to the climate system. This uncertainty applies to magnitude, and even direction of some of the feedbacks.

Uncertainties and recommendations.

An assessment of the impacts of changes in climate and UV-B radiation on Arctic terrestrial ecosystems, made within the Arctic Climate Impacts Assessment (ACIA), highlighted the profound implications of projected warming in particular for future ecosystem services, biodiversity and feedbacks to climate. However, although our current understanding of ecological processes and changes driven by climate and UV-B is strong in some geographical areas and in some disciplines, it is weak in others. Even though recently the strength of our predictions has increased dramatically with increased research effort in the Arctic and the introduction of new technologies, our current understanding is still constrained by various uncertainties. The assessment is based on a range of approaches that each have uncertainties, and on data sets that are often far from complete. Uncertainties arise from methodologies and conceptual frameworks, from unpredictable surprises, from lack of validation of models, and from the use of particular scenarios, rather than predictions, of future greenhouse gas emissions and climates. Recommendations to reduce the uncertainties are wide-ranging and relate to all disciplines within the assessment. However, a repeated theme is the critical importance of achieving an adequate spatial and long-term coverage of experiments, observations and monitoring of environmental changes and their impacts throughout the sparsely populated and remote region that is the Arctic.

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.

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.

Vascular plant 15N natural abundance in heath and forest tundra ecosystems is closely correlated with presence and type of mycorrhizal fungi in roots.

In this study we show that the natural abundance of the nitrogen isotope 15, δ15N, of plants in heath tundra and at the tundra-forest ecocline is closely correlated with the presence and type of mycorrhizal association in the plant roots. A total of 56 vascular plant species, 7 moss species, 2 lichens and 6 species of fungi from four heath and forest tundra sites in Greenland, Siberia and Sweden were analysed for δ15N and N concentration. Roots of vascular plants were examined for mycorrhizal colonization, and the soil organic matter was analysed for δ15N, N concentration and soil inorganic, dissolved organic and microbial N. No arbuscular mycorrhizal (AM) colonizations were found although potential host plants were present in all sites. The dominant species were either ectomycorrhizal (ECM) or ericoid mycorrhizal (ERI). The δ15N of ECM or ERI plants was 3.5-7.7‰ lower than that of non-mycorrhizal (NON) species in three of the four sites. This corresponds to the results in our earlier study of mycorrhiza and plant δ15N which was limited to one heath and one fellfield in N Sweden. Hence, our data suggest that the δ15N pattern: NON/AM plants > ECM plants ≥ ERI plants is a general phenomenon in ecosystems with nutrient-deficient organogenic soils. In the fourth site, a␣birch forest with a lush herb/shrub understorey, the differences between functional groups were considerably smaller, and only the ERI species differed (by 1.1‰) from the NON species. Plants of all functional groups from this site had nearly twice the leaf N concentration as that found in the same species at the other three sites. It is likely that low inorganic N availability is a prerequisite for strong δ15N separation among functional groups. Both ECM roots and fruitbodies were 15N enriched compared to leaves which suggests that the difference in δ15N between plants with different kinds of mycorrhiza could be due to isotopic fractionation at the␣fungal-plant interface. However, differences in δ15N between soil N forms absorbed by the plants could also contribute to the wide differences in plant δ15N found in most heath and forest tundra ecosystems. We hypothesize that during microbial immobilization of soil ammonium the microbial N pool could become 15N-depleted and the remaining, plant-available soil ammonium 15N-enriched. The latter could be a main source of N for NON/AM plants which usually have high δ15N. In contrast, amino acids and other soil organic N compounds presumably are 15N-depleted, similar to plant litter, and ECM and ERI plants with high uptake of these N forms hence have low leaf δ15N. Further indications come from the δ15N of mosses and lichens which was similar to that of ECM plants. Tundra cryptogams (and ECM and ERI plants) have previously been shown to have higher uptake of amino acid than ammonium N; their low δ15N might therefore reflect the δ15N of free amino acids in the soil. The concentration of dissolved organic N was 3-16 times higher than that of inorganic N in the sites. Organic nitrogen could be an important N source for ECM and, in particular, ERI plants in heath and forest tundra ecosystems with low release rate of inorganic N from the soil organic matter.

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.