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Mitchell, Edward

Nom
Mitchell, Edward
Affiliation principale
Site web
Fonction
Professeur ordinaire
Email
edward.mitchell@unine.ch
Identifiants
https://libra.unine.ch/handle/123456789/357
_
0000-0003-0358-506X

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  • Publication
    Accès libre
    Assessing the responses of Sphagnum micro-eukaryotes to climate changes using high throughput sequencing
    (2020-9-18)
    Reczuga, Monika 
    ;
    Seppey, Christophe Victor William
    ;
    Mulot, Matthieu 
    ;
    Jassey, Vincent E.J.
    ;
    Buttler, Alexandre
    ;
    Slowinska, Sandra
    ;
    Slowinski, Michal
    ;
    Lara, Enrique 
    ;
    Lamentowicz, Mariusz
    ;
    Mitchell, Edward 
    Current projections suggest that climate warming will be accompanied by more frequent and severe drought events. Peatlands store ca. one third of the world’s soil organic carbon. Warming and drought may cause peatlands to become carbon sources through stimulation of microbial activity increasing ecosystem respiration, with positive feedback effect on global warming. Micro-eukaryotes play a key role in the carbon cycle through food web interactions and therefore, alterations in their community structure and diversity may affect ecosystem functioning and could reflect these changes. We assessed the diversity and community composition of Sphagnum-associated eukaryotic microorganisms inhabiting peatlands and their response to experimental drought and warming using high throughput sequencing of environmental DNA. Under drier conditions, micro-eukaryotic diversity decreased, the relative abundance of autotrophs increased and that of osmotrophs (including Fungi and Peronosporomycetes) decreased. Furthermore, we identified climate change indicators that could be used as early indicators of change in peatland microbial communities and ecosystem functioning. The changes we observed indicate a shift towards a more “terrestrial” community in response to drought, in line with observed changes in the functioning of the ecosystem.
  • Publication
    Métadonnées seulement
    Factors modulating cottongrass seedling growth stimulation to enhanced nitrogen and carbon dioxide: compensatory tradeoffs in leaf dynamics and allocation to meet potassium-limited growth
    (2013)
    Siegenthaler, Andy
    ;
    Buttler, Alexandre
    ;
    Grosvernier, Philippe
    ;
    Gobat, Jean-Michel 
    ;
    Nilsson, Mats B.
    ;
    Mitchell, Edward 
    Eriophorum vaginatum is a characteristic species of northern peatlands and a keystone plant for cutover bog restoration. Understanding the factors affecting E. vaginatum seedling establishment (i.e. growth dynamics and allocation) under global change has practical implications for the management of abandoned mined bogs and restoration of their C-sequestration function. We studied the responses of leaf dynamics, above- and belowground biomass production of establishing seedlings to elevated CO2 and N. We hypothesised that nutrient factors such as limitation shifts or dilutions would modulate growth stimulation. Elevated CO2 did not affect biomass, but increased the number of young leaves in spring (+400 %), and the plant vitality (i.e. number of green leaves/total number of leaves) (+3 %), both of which were negatively correlated to [K+] in surface porewater, suggesting a K-limited production of young leaves. Nutrient ratios in green leaves indicated either N and K co-limitation or K limitation. N addition enhanced the number of tillers (+38 %), green leaves (+18 %), aboveground and belowground biomass (+99, +61 %), leaf mass-to-length ratio (+28 %), and reduced the leaf turnover (-32 %). N addition enhanced N availability and decreased [K+] in spring surface porewater. Increased tiller and leaf production in July were associated with a doubling in [K+] in surface porewater suggesting that under enhanced N production is K driven. Both experiments illustrate the importance of tradeoffs in E. vaginatum growth between: (1) producing tillers and generating new leaves, (2) maintaining adult leaves and initiating new ones, and (3) investing in basal parts (corms) for storage or in root growth for greater K uptake. The K concentration in surface porewater is thus the single most important factor controlling the growth of E. vaginatum seedlings in the regeneration of selected cutover bogs.
  • Publication
    Métadonnées seulement
    Seasonal patterns of testate amoeba diversity, community structure and species-environment relationships in four Sphagnum-dominated peatlands along a 1300 m altitudinal gradient in Switzerland
    (2013)
    Lamentowicz, Mariusz
    ;
    Bragazza, Luca
    ;
    Buttler, Alexandre
    ;
    Jassey, V. E. J.
    ;
    Mitchell, Edward 
    Altitudinal gradients are useful to study the potential effects of climate change on ecosystems. Historically, studies on elevation gradients have primarily focused on macro-organisms and ecosystem processes, while microorganisms have been mostly ignored despite their ubiquity and functional importance. We studied the temporal (about every two months from June 2008 until May 2009) variation of testate amoeba communities in four Sphagnum-dominated peatlands along a 1300 to elevation gradient in the Swiss Mountains (580-1880 m) in relation to water table depth and hydrochemistry with special focus on dissolved organic carbon (DOC), a useful proxy for changes in C-cycling in peatlands. The lowest site had significantly (P < 0.01) lowest testate amoeba density, species richness. The highest site had highest testate amoeba density (38 ind mg(-1) dry mass of Sphagnum). Seasonal fluctuations in testate amoeba species richness and diversity were not consistent among sites but density tended to peak in spring at all sites, autumn in the three highest sites and mid-winter in the upper two sites. In a redundancy analysis (RDA) community structure was more strongly correlated to altitude (33.8% of variance explained in living community) than to soil hydrological and hydro-chemical variables (together explaining 16.2% of variance). In a partial RDA with altitude used as covariable, the four sites were separated by DOP, DOC, DON, pH and average depth to water table. The abundance of high trophic level testate amoeba species (shell-aperture over their body size >0.20; i.e. primarily predators of protists and micro-metazoa) as well as the community size structure increased from lowest to highest elevation (respectively by 3.7x and 6x) and followed the seasonal patterns of total density, while DOC, DON, and DOC/DON decreased with elevation. These results agree with previously reported alteration of peatland microbial food chains in response to experimental warming, suggesting that climate-induced changes in microbial community structure (here a shortening of microbial food chains) represent a mechanism controlling the carbon balance of peatlands. (C) 2013 Elsevier Ltd. All rights reserved.
  • Publication
    Métadonnées seulement
    Above- and belowground linkages in Sphagnum peatland: climate warming affects plant-microbial interactions
    (2013)
    Jassey, Vincent E. J.
    ;
    Chiapusio, Genevieve
    ;
    Binet, Philippe
    ;
    Buttler, Alexandre
    ;
    Laggoun-Defarge, Fatima
    ;
    Delarue, Frederic
    ;
    Bernard, Nadine
    ;
    Mitchell, Edward 
    ;
    Toussaint, Marie-Laure
    ;
    Francez, Andre-Jean
    ;
    Gilbert, Daniel
    Peatlands contain approximately one third of all soil organic carbon (SOC). Warming can alter above- and belowground linkages that regulate soil organic carbon dynamics and C-balance in peatlands. Here we examine the multiyear impact of in situ experimental warming on the microbial food web, vegetation, and their feedbacks with soil chemistry. We provide evidence of both positive and negative impacts of warming on specific microbial functional groups, leading to destabilization of the microbial food web. We observed a strong reduction (70%) in the biomass of top-predators (testate amoebae) in warmed plots. Such a loss caused a shortening of microbial food chains, which in turn stimulated microbial activity, leading to slight increases in levels of nutrients and labile C in water. We further show that warming altered the regulatory role of Sphagnum-polyphenols on microbial community structure with a potential inhibition of top predators. In addition, warming caused a decrease in Sphagnum cover and an increase in vascular plant cover. Using structural equation modelling, we show that changes in the microbial food web affected the relationships between plants, soil water chemistry, and microbial communities. These results suggest that warming will destabilize C and nutrient recycling of peatlands via changes in above- and belowground linkages, and therefore, the microbial food web associated with mosses will feedback positively to global warming by destabilizing the carbon cycle. This study confirms that microbial food webs thus constitute a key element in the functioning of peatland ecosystems. Their study can help understand how mosses, as ecosystem engineers, tightly regulate biogeochemical cycling and climate feedback in peatlands
  • Publication
    Métadonnées seulement
    Climatic modifiers of the response to nitrogen deposition in peat-forming Sphagnum mosses: a meta-analysis
    (2011)
    Limpens, J.
    ;
    Granath, G.
    ;
    Gunnarsson, U.
    ;
    Aerts, R.
    ;
    Bayley, S.
    ;
    Bragazza, Luca
    ;
    Bubier, J.
    ;
    Buttler, Alexandre
    ;
    van den Berg, L. J. L.
    ;
    Francez, Andre-Jean
    ;
    Gerdol, R.
    ;
    Grosvernier, Philippe
    ;
    Heijmans, M. M. P. D.
    ;
    Hoosbeek, M. R.
    ;
    Hotes, S.
    ;
    Ilomets, M.
    ;
    Leith, I.
    ;
    Mitchell, Edward 
    ;
    Moore, T.
    ;
    Nilsson, Mats B.
    ;
    Nordbakken, J. F.
    ;
    Rochefort, L.
    ;
    Rydin, H.
    ;
    Sheppard, L. J.
    ;
    Thormann, M.
    ;
    Wiedermann, M. M.
    ;
    Williams, B. L.
    ;
    Xu, B.
    Peatlands in the northern hemisphere have accumulated more atmospheric carbon (C) during the Holocene than any other terrestrial ecosystem, making peatlands long-term C sinks of global importance. Projected increases in nitrogen (N) deposition and temperature make future accumulation rates uncertain. Here, we assessed the impact of N deposition on peatland C sequestration potential by investigating the effects of experimental N addition on Sphagnum moss. We employed meta-regressions to the results of 107 field experiments, accounting for sampling dependence in the data. We found that high N loading (comprising N application rate, experiment duration, background N deposition) depressed Sphagnum production relative to untreated controls. The interactive effects of presence of competitive vascular plants and high tissue N concentrations indicated intensified biotic interactions and altered nutrient stochiometry as mechanisms underlying the detrimental N effects. Importantly, a higher summer temperature (mean for July) and increased annual precipitation intensified the negative effects of N. The temperature effect was comparable to an experimental application of almost 4 g N m(-2) yr(-1) for each 1 degrees C increase. Our results indicate that current rates of N deposition in a warmer environment will strongly inhibit C sequestration by Sphagnum-dominated vegetation.
  • Publication
    Accès libre
    Seasonal Net Ecosystem Carbon Exchange of a Regenerating Cutaway Bog: How Long Does it Take to Restore the C-Sequestration Function?
    (2011)
    Samaritani, Emanuela 
    ;
    Siegenthaler, Andy
    ;
    Yli-Petays, Mika
    ;
    Buttler, Alexandre
    ;
    Christin, Pascal-Antoine
    ;
    Mitchell, Edward 
    We measured the net ecosystem exchange (NEE) and respiration rates and modeled the photosynthesis and respiration dynamics in a cutover bog in the Swiss Jura Mountains during one growing season at three stages of regeneration (29, 42, and 51 years after peat cutting; coded sites A, B, and C) to determine if reestablishment of Sphagnum suffices to restore the C-sequestration function. From the younger to the older stage Sphagnum cover increased, while net primary Sphagnum production over the growing season decreased (139, 82, and, 67 g m−2 y−1 for A, B, and C respectively), and fen plant species were replaced by bog species. According to our NEE estimations, over the vegetation period site A was a net CO2-C source emitting 40 g CO2-C/m2 while sites B and C were accumulating CO2-C, on average 222 and 209 g CO2-C/m2, respectively. These differences are due to the higher respiration in site A during the summer, suggesting that early regeneration stages may be more sensitive to a warmer climate. Methane fluxes increased from site A to C in parallel with Eriophorum vaginatum cover and vascular plant leaf area. Our results show that reestablishing a Sphagnum cover is not sufficient to restore a CO2-sequestrating function but that after circa 50 years the ecosystem may naturally regain this function over the growing season.
  • Publication
    Accès libre
    Litter- and ecosystem-driven decomposition under elevated CO2 and enhanced N deposition in a Sphagnum peatland
    (2010)
    Siegenthaler, Andy
    ;
    Buttler, Alexandre
    ;
    Bragazza, Luca
    ;
    van der Heijden, Edwin
    ;
    Grosvernier, Philippe
    ;
    Gobat, Jean-Michel 
    ;
    Mitchell, Edward 
    Peatlands represent massive global C pools and sinks. Carbon accumulation depends on the ratio between net primary production and decomposition, both of which can change under projected increases of atmospheric CO2 and N deposition. The decomposition of litter is influenced by 1) the quality of the litter, and 2) the microenvironmental conditions in which the litter decomposes. This study aims at experimentally testing the effects of these two drivers in the context of global change. We studied the in situ litter decomposition from three common peatland species (Eriophorum vaginatum, Polytrichum strictum and Sphagnum fallax) collected after one year of litter production under pre-treatment conditions (elevated CO2: 560 ppm or enhanced N: 3 g m−2 y−1 NH4NO3) and decomposed the following year under treatment conditions (same as pre-treatment). By considering the cross-effects between pre-treatments and treatments, we distinguished between the effects on mass loss of 1) the pre-treatment-induced litter quality and 2) the treatment conditions under which the litters were decomposing. The combination between CO2 pre-treatment and CO2 treatment reduced Polytrichum decomposition by −24% and this can be explained by litter quality-driven decomposition changes brought by the pre-treatment. CO2 pre-treatment reduced Eriophorum litter quality, although this was not sufficient to predict decomposition. The N addition pre-treatment reduced the decomposition of Eriophorum, due to enhanced lignin and soluble phenols concentrations in the initial litter, and reduced litter-driven losses of starch and enhanced litter-driven losses of soluble phenols. While decomposition indices based on initial litter quality provide a broad explanation of quantitative and qualitative decomposition, they can only be taken as first approximations. Indeed, the microbial ATP activity, the litter N loss and resulting litter quality, were strongly altered irrespective of the compounds' initial concentration and by means of processes that occurred independently of the initial litter-qualitative changes. The experimental design was valuable to assess litter- and ecosystem-driven decomposition pathways simultaneously or independently. The ability to separate these two drivers makes it possible to attest the presence of litter-qualitative changes even without any litter biochemical determinations, and shows the screening potential of this approach for future experiments dealing with multiple plant species.
  • Publication
    Accès libre
    Species-specific effects of polyploidisation and plant traits of Centaurea maculosa and Senecio inaequidens on rhizosphere microorganisms
    (2010)
    Thébault, Aurélie
    ;
    Frey, Beat
    ;
    Mitchell, Edward 
    ;
    Buttler, Alexandre
    Invasive plant species represent a threat to terrestrial ecosystems, but their effects on the soil biota and the mechanisms involved are not yet well understood. Many invasive species have undergone polyploidisation, leading to the coexistence of various cytotypes in the native range, whereas, in most cases, only one cytotype is present in the introduced range. Since genetic variation within a species can modify soil rhizosphere communities, we studied the effects of different cytotypes and ranges (native diploid, native tetraploid and introduced tetraploid) of Centaurea maculosa and Senecio inaequidens on microbial biomass carbon, rhizosphere total DNA content and bacterial communities of a standard soil in relation to plant functional traits. There was no overall significant difference in microbial biomass between cytotypes. The variation of rhizosphere total DNA content and bacterial community structure according to cytotype was species specific. The rhizosphere DNA content of S. inaequidens decreased with polyploidisation in the native range but did not vary for C. maculosa. In contrast, the bacterial community structure of C. maculosa was affected by polyploidisation and its diversity increased, whereas there was no significant change for S. inaequidens. Traits of S. inaequidens were correlated to the rhizosphere biota. Bacterial diversity and total DNA content were positively correlated with resource allocation to belowground growth and late flowering, whereas microbial biomass carbon was negatively correlated to investment in reproduction. There were no correlations between traits of the cytotypes of C. maculosa and corresponding rhizosphere soil biota. This study shows that polyploidisation may affect rhizosphere bacterial community composition, but that effects vary among plant species. Such changes may contribute to the success of invasive polyploid genotypes in the introduced range.
  • Publication
    Accès libre
    Functional microbial diversity in regenerating cutover peatlands responds to vegetation succession
    (2008)
    Artz, Rebekka R. E.
    ;
    Chapman, Stephen J.
    ;
    Siegenthaler, Andy
    ;
    Mitchell, Edward 
    ;
    Buttler, Alexandre
    ;
    Bortoluzzi, Estelle
    ;
    Gilbert, Daniel
    ;
    Yli-Petays, Mika
    ;
    Vasander, Harri
    ;
    Francez, André-Jean
    1. While establishment of vegetation is the most visual indicator of regeneration on cutover peatland, the reinstatement of belowground functions is less well understood. Vegetation succession results in differences in peat quality in terms of C availability. The respiratory response of the soil microbial community to ecologically relevant substrates (community-level physiological profile, CLPP) such as those found in rhizosphere exudates and litter hydrolysates, is thought to reflect the activity and functional diversity of the soil microbial community, especially those involved in turnover of soluble photosynthate-derived C.
    2. The relationship between CLPP and typical regeneration stages was investigated at five European peatlands, each with up to five sites representing a gradient of natural regeneration stages. We aimed to determine whether unaided revegetation consistently affected soil microbial CLPP, which environmental factors explained variation in CLPP on the scale of individual peatlands, and if these factors were consistent across different peatlands.
    3. Within each peatland, a decomposition index based on diagnostic bands in Fourier transform-infrared spectra indicated that regeneration had generally started from a common base and that the influence of vegetation on the decomposition index declined with depth. In parallel, differences in vegetation cover between regeneration stages resulted in significantly different CLPP, but this effect decreased rapidly with soil depth. The magnitudes of the effect of vegetation succession versus soil depth appeared to be linked with the age range of the regeneration gradients. Hence, the effect of vegetation on CLPP is effectively diluted due to the remaining organic matter. Specific plant species described significant proportions of CLPP variability but these species were not consistent across peatland types. The effects of soil depth appeared to be peatland-specific.
    4. Synthesis and applications. Together, the results indicate significant responses of the microbial community to vegetation succession, with the strength of the effect probably dependent on quantities of labile C allocation to the soil microbial community. Therefore, particularly in the early stages of regeneration of cutover peatlands, CLPP could provide vital information about the relative importance of different plant functional types on potential rates of labile C turnover.
  • Publication
    Métadonnées seulement
    Functional microbial diversity in regenerating cutover peatlands responds to vegetation succession
    (2008)
    Artz, Rebekka
    ;
    Chapman, Steve
    ;
    Siegenthaler, Andy
    ;
    Mitchell, Edward 
    ;
    Buttler, Alexandre
    ;
    Bortoluzzi, Estelle
    ;
    Gilbert, Daniel
    ;
    Yli-Petays, Mika
    ;
    Vasander, Harri
    ;
    Francez, André-Jean
    1. While establishment of vegetation is the most visual indicator of regeneration on cutover peatland, the reinstatement of belowground functions is less well understood. Vegetation succession results in differences in peat quality in terms of C availability. The respiratory response of the soil microbial community to ecologically relevant substrates (community-level physiological profile, CLPP) such as those found in rhizosphere exudates and litter hydrolysates, is thought to reflect the activity and functional diversity of the soil microbial community, especially those involved in turnover of soluble photosynthate-derived C. 2. The relationship between CLPP and typical regeneration stages was investigated at five European peatlands, each with up to five sites representing a gradient of natural regeneration stages. We aimed to determine whether unaided revegetation consistently affected soil microbial CLPP, which environmental factors explained variation in CLPP on the scale of individual peatlands, and if these factors were consistent across different peatlands. 3. Within each peatland, a decomposition index based on diagnostic bands in Fourier transform-infrared spectra indicated that regeneration had generally started from a common base and that the influence of vegetation on the decomposition index declined with depth. In parallel, differences in vegetation cover between regeneration stages resulted in significantly different CLPP, but this effect decreased rapidly with soil depth. The magnitudes of the effect of vegetation succession versus soil depth appeared to be linked with the age range of the regeneration gradients. Hence, the effect of vegetation on CLPP is effectively diluted due to the remaining organic matter. Specific plant species described significant proportions of CLPP variability but these species were not consistent across peatland types. The effects of soil depth appeared to be peatland-specific. 4. Synthesis and applications. Together, the results indicate significant responses of the microbial community to vegetation succession, with the strength of the effect probably dependent on quantities of labile C allocation to the soil microbial community. Therefore, particularly in the early stages of regeneration of cutover peatlands, CLPP could provide vital information about the relative importance of different plant functional types on potential rates of labile C turnover.