Inger Kappel Schmidt

RESPONSES IN MICROBES AND PLANTS TO CHANGED TEMPERATURE, NUTRIENT, AND LIGHT REGIMES IN THE ARCTIC

Previous research has shown that experimental perturbations of arctic ecosystems simulating direct and indirect effects of predicted environmental changes have led to strong responses in the plant communities, mostly associated with increased plant nutrient availability. Similarly, changes in decomposition and nutrient mineralization are likely to occur if the soil warms and the soil moisture conditions are altered. Plant and microbial responses have usually been investigated separately, and few, if any, studies have addressed simultaneous responses to environmental changes in plants and soil microorganisms, except in models. We measured simultaneous responses in biomass, nitrogen (N), and phosphorus (P) incorporation in plants and microorganisms after five years of factorial fertilizer addition, air warming, and shading. We expected increased N and P uptake by microorganisms after fertilizer addition and also after warming, due to increases in mineralization rates in warmer soils. Plant productivity and N and P uptake were expected to increase after fertilizer addition but less after warming, because microbes were expected to absorb most of the extra released nutrients. Shading was expected to decrease plant production and also microbial biomass, due to the reduced production of labile carbon (C) in plant root exudates associated with reduced photosynthesis. We found that the plants responded strongly to fertilizer addition by increased biomass accumulation and N and P uptake. They responded less to warming, but more than expected, showing a decline in N and P concentrations in many cases. There were few significant responses to shading. The strongest response was found in combined fertilizer addition and warming treatments. All functional vascular plant groups responded similarly. However, mosses declined under those conditions when vascular plant growth was most pronounced. Contrary to our expectation, microbial C, N, and P did not increase after warming, but microbial N and P increased after shading. As expected, fertilizer addition led to increased microbial P content, whereas microbial N either increased or did not change. In general, microbial C did not change in any treatment. The microbes accumulated extra N and P only when soil inorganic N or P levels increased, suggesting that the soil microorganisms absorbed extra nutrients only in cases of declining N and P sink strength in plants.

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 NOinf3sup-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 NHinf4sup+-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: NHinf4sup+, NOinf3sup-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.

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.

The Response of Soil Processes to Climate Change: Results from Manipulation Studies of Shrublands Across an Environmental Gradient

Predicted changes in climate may affect key soil processes such as respiration and net nitrogen (N) mineralization and thus key ecosystem functions such as carbon (C) storage and nutrient availability. To identify the sensitivity of shrubland soils to predicted climate changes, we have carried out experimental manipulations involving ecosystem warming and prolonged summer drought in ericaceous shrublands across a European climate gradient. We used retractable covers to create artificial nighttime warming and prolonged summer drought to 20-m2 experimental plots. Combining the data from across the environmental gradient with the results from the manipulation experiments provides evidence for strong climate controls on soil respiration, net N mineralization and nitrification, and litter decomposition. Trends of 0%–19% increases of soil respiration in response to warming and decreases of 3%–29% in response to drought were observed. Across the environmental gradient and below soil temperatures of 20°C at a depth of 5–10 cm, a mean Q10 of 4.1 in respiration rates was observed although this varied from 2.4 to 7.0 between sites. Highest Q10 values were observed in Spain and the UK and were therefore not correlated with soil temperature. A trend of increased accumulated surface litter mass loss was observed with experimental warming (2%– 22%) but there was no consistent response to experimental drought. In contrast to soil respiration and decomposition, variability in net N mineralization was best explained by soil moisture rather than temperature. When water was neither limiting or in excess, a Q10 of 1.5 was observed for net N mineralization rates. These data suggest that key soil processes will be differentially affected by predicted changes in rainfall pattern and temperature and the net effect on ecosystem functioning will be difficult to predict without a greater understanding of the controls underlying the sensitivity of soils to climate variables.

Mineralization and microbial immobilization of N and P in arctic soils in relation to season, temperature and nutrient amendment

Abstract In situ summer, winter and annual net N and P mineralization and microbial immobilization of mineralized nutrients were measured in a low-altitude subarctic/alpine dwarf shrub heath and in a high-altitude fellfield. Net mineralization was determined by using the buried-bag technique, which was combined with fumigation–extraction to recover microbial nutrients and estimate microbial nutrient immobilization. The measurements were carried out in unperturbed plots and in plots, which had been subjected to elevated temperature and fertilizer addition for 5 years before the experiment started. During the growing season, the microbes in the unperturbed plots immobilized the major part of the mineralized nutrients, which resulted in low net mineralization. This occurred also during winter, when we assumed that nutrients should be released during microbial die-back. However, we found no evidence for a large microbial winter die-back and the surviving microbial biomass even immobilized extra nutrients. Soil temperature enhancement by ca. 2°C in general increased, or tended to increase, net mineralization. However, there was not necessarily any strong correlation between net mineralization and temperature because temperature-induced increase in gross mineralized nutrients could either lead to nutrient immobilization in the microbes or the nutrients could be released to the soil inorganic pool. Fertilizer additions had no major effect on net nutrient mineralization or nutrient immobilization. However, the ligno-cellulose index, which has been used as a predictor of substrate quality and usually correlates negatively with decomposition rate, also appeared to be a good predictor of gross mineralization, but a poor predictor of net mineralization.

NITROGEN UPTAKE BY ARCTIC SOIL MICROBES AND PLANTS IN RELATION TO SOIL NITROGEN SUPPLY

In Alaska, evergreen and deciduous shrubs dominate the vegetation of moist acidic arctic tundra (soil pH 5.5). In this study we compare soil concentrations and microbial and plant uptake of amino acids, ammonium (NH4+), and nitrate (NO3−) in acidic and nonacidic tundra. The objective was to determine any differences between the tundra sites that may relate to the differences in vegetation. We sampled the water-extractable soil N pool over one growing season and found that it at all times was higher at the nonacidic than at the acidic site, while at both sites it was dominated by NH4+ followed in order by amino acid N and NO3−. In addition, we designed an experiment in which a mixture of aspartic acid, glycine, NH4+, and NO3− were injected into the soil in the middle of the growth period. In the mixture, one N form at a time was labeled with 15N and in the case of amino acids also with 13C. Soi...

Coupling of nutrient cycling and carbon dynamics in the Arctic, integration of soil microbial and plant processes

Abstract Most studies of nutrient cycling in arctic ecosystems have either addressed questions of plant nutrient acquisition or of decomposition and mineralization processes, while few studies have integrated processes in both the soil and plant compartments. Here, we synthesize information on nutrient cycling within, and between, the soil/microbial and the plant compartments of the ecosystems and integrate the cycling of nutrients with the turnover of organic matter and the carbon balance in tundra ecosystems. Based on this compilation and integration, we discuss implications for ecosystem function in response to predicted climatic changes. Many arctic ecosystems have high amounts of nutrients in the microbial biomass compared to the pools in the plant biomass both due to large nutrient-containing organic deposits in the soil and low plant biomass. The microbial pools of N and P, which are the most commonly limiting nutrients for plant production, may approach (N) or even exceed (P) the plant pools. Net nutrient mineralization is low, the residence time of nutrients in the soil is long and the nutrients are strongly immobilized in the soil microorganisms. This contributes to pronounced nutrient limitation for plant productivity, implies that the microbial sink strength for nutrients is strong and that the microbes may compete with plants for nutrients, but also that they are a potential source of plant nutrients during periods of declining microbial populations. The extent of this competition is poorly explored and it is uncertain whether plants mainly take up nutrients continuously during the summer when the microbial activity and, presumably, also the microbial sink strength is high, or whether the main nutrient uptake occurs during pulses of nutrient release when the microbial sink strength declines. Improved knowledge of mechanisms for plant-microbial interactions in these nutrient-limited systems is important, because it will form a basis also for our understanding of the C exchange between the ecosystems and the atmosphere under the predicted, future climatic change. High microbial nutrient immobilization, i.e. low release of plant-available nutrients, paired with high microbial decomposition of soil organic matter will lead to a loss of C from the soil to the atmosphere, which may not be compensated fully by increased plant C fixation. Hence, the system will be a net source of atmospheric C. Conversely, if plants are able to sequester extra nutrients efficiently, their productivity will increase and the systems may accumulate more C and turn into a C sink, particularly if nutrients are allocated to woody tissues of low nutrient concentrations.

Mineralization and distribution of nutrients in plants and microbes in four arctic ecosystems: responses to warming

Mineralization and nutrient distribution in plants and microbes were studied in four arctic ecosystems at Abisko, Northern Sweden and Toolik Lake, Alaska, which have been subjected to long-term warming with plastic greenhouses. Net mineralization and microbial immobilization were studied by the buried bag method and ecosystem pool sizes of C, N and P were determined by harvest methods. The highest amounts of organic N and P were bound in the soil organic matter. Microbial N and P constituted the largest labile pools often equal to (N) or exceeding (P) the amounts stored in the vegetation. Despite large pools of N and P in the soil, net mineralization of N and P was generally low during the growing season, except in the wet sedge tundra, and in most cases lower than the plant uptake requirement. In contrast, the microorganisms immobilized high amounts of nutrients in the buried bags during incubation. The same high immobilization was not observed in the surrounding soil, where the microbial nutrient content in most cases remained constant or decreased over the growing season. This suggests that the low mineralization measured in many arctic ecosystems over the growing season is due to increased immobilization by soil microbes when competition from plant roots is prevented. Furthermore, it suggests that plants compete well with microbes for nutrients in these four ecosystems. Warming increased net mineralization in several cases, which led to increased assimilation of nutrients by plants but not by the microbes.

Quantifying global soil carbon losses in response to warming

The majority of the Earth’s terrestrial carbon is stored in the soil. If anthropogenic warming stimulates the loss of this carbon to the atmosphere, it could drive further planetary warming. Despite evidence that warming enhances carbon fluxes to and from the soil, the net global balance between these responses remains uncertain. Here we present a comprehensive analysis of warming-induced changes in soil carbon stocks by assembling data from 49 field experiments located across North America, Europe and Asia. We find that the effects of warming are contingent on the size of the initial soil carbon stock, with considerable losses occurring in high-latitude areas. By extrapolating this empirical relationship to the global scale, we provide estimates of soil carbon sensitivity to warming that may help to constrain Earth system model projections. Our empirical relationship suggests that global soil carbon stocks in the upper soil horizons will fall by 30 ± 30 petagrams of carbon to 203 ± 161 petagrams of carbon under one degree of warming, depending on the rate at which the effects of warming are realized. Under the conservative assumption that the response of soil carbon to warming occurs within a year, a business-as-usual climate scenario would drive the loss of 55 ± 50 petagrams of carbon from the upper soil horizons by 2050. This value is around 12–17 per cent of the expected anthropogenic emissions over this period. Despite the considerable uncertainty in our estimates, the direction of the global soil carbon response is consistent across all scenarios. This provides strong empirical support for the idea that rising temperatures will stimulate the net loss of soil carbon to the atmosphere, driving a positive land carbon–climate feedback that could accelerate climate change.

Simulated climate change affecting microorganisms, nematode density and biodiversity in subarctic soils

Arctic terrestrial ecosystems are strongly dominated by temperature, and global warming is expected to have a particularly strong impact in high latitudes. The Arctic will therefore be an important region for early detection of global change. In the present study the effects of environmental manipulations simulating climate change on soil microorganisms and nematode populations were investigated. Study sites were a dwarf shrub dominated tree-line heath (450 m a.s.l.) and a high altitude fellfield (1150 m a.s.l.) at Abisko, Swedish Lapland. Soil temperature was enhanced by using passive greenhouses and the impact on soil organisms with and without NPK fertilizer addition was assessed. The nematode community was strongly affected by warming and nutrient application. Population density was twice as high for all treatments at the fellfield as compared to controls. At the heath temperature enhancement with or without fertilizer application also led to a doubling of the population density, whereas fertilization alone caused an increase of about one third. The environmental manipulations resulted in a greater microbial biomass C and active fungal biomass in the heath soil. Increased density was also recorded for bacterial and fungal feeding nematodes at both sites. The results suggest that nematodes have an important impact on microbial biomass and turnover rates in the two subarctic systems. Elevated soil temperature apparently will lead to increased grazing on microorganisms, contributing to enhanced net N and P mineralization rates and plant nutrient availability. However, biodiversity was generally affected negatively by the environmental manipulations. The effects were more severe at the high altitude fellfield indicating that the influence of elevated temperature will be more pronounced in systems already stressed by extreme climatic conditions.