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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
    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
    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
    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
    Métadonnées seulement
    Cut-over peatland regeneration assessment using organic matter and microbial indicators (bacteria and testate amoebae)
    (2008)
    Laggoun-Défarge, Fatima
    ;
    Mitchell, Edward 
    ;
    Gilbert, Daniel
    ;
    Disnar, Jean-Robert
    ;
    Comont, Laure
    ;
    Warner, Barry
    ;
    Buttler, Alexandre
    1. Cut-over peatlands cover large surfaces of high potential value for enhancing biodiversity and carbon sequestration if successfully restored. Unfortunately, evaluation of restoration success is not straightforward. We assessed the bioindicator value of organic matter (OM), testate amoebae (protozoa) and bacteria in peat from two regeneration stages and a reference site of a cut-over bog. 2. Contrasting biochemical signatures of peat OM were observed along the regenerating profiles, allowing clear differentiation between the newly regenerated peat and the old peat. Where peat macrofossils were absent sugar biomarkers were used to infer peat botanical origin and OM alteration. 3. Over the succession, the OM composition of the new peat differed. Peat from the more recent stage was dominated by Sphagnum-derived tissues and characterized by lower carbohydrate preservation and higher bacterial biomass than the advanced regeneration stage. 4. Surface testate amoeba communities also changed from the recent to the advanced stages of regeneration, indicating a shift from wet and moderately acidic conditions to drier and more acidic conditions. Over this regeneration sequence (i) the biomass and average size of species declined but were higher at the unexploited site and (ii) species richness and diversity increased but density declined. 5. Synthesis and applications. Although secondary succession in the cut-over bog led to an ecosystem similar to that of the reference site in terms of surface vegetation, OM and testate amoebae continued to reflect disturbances associated with peat harvesting. Nevertheless, the described dynamics of both microbial and biochemical variables over the succession showed similarities between the advanced stage and the reference site: a higher testate amoeba diversity was associated with better carbohydrate preservation and a more heterogeneous botanical composition of the peat. The inferred water table depth and pH based on testate amoebae indicators proved to be an alternative approach for assessing restoration processes, in contrast to labour-intensive repeated measurements in the field. The botanical and biochemical composition of peat OM provided additional information on past anthropogenic perturbations of the bog and could be used for restoration monitoring. The combination of several indicators therefore provides a more complete assessment of ecological conditions that could be valuable for the management of cut-over peatlands.
  • 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.
  • Publication
    Métadonnées seulement
    Reconciling commercial exploitation of peat with biodiversity in peatland ecosystems (EU Project RECIPE)
    (2004)
    Mitchell, Edward 
    ;
    Chapman, Steve
    ;
    Buttler, Alexandre
    ;
    Combe, Jean
    ;
    Francez, André-Jean
    ;
    Gilbert, Daniel
    ;
    Harms, Hauke
    ;
    Laggoun-Défarge, Fatima
    ;
    Schloter, Michael
    ;
    Vasander, Harri
  • Publication
    Métadonnées seulement
    Biochemical characteristics of peat organic matter and distribution of testate amoebae in two naturally regenerating cutover Sphagnum peatlands of the Jura Mountains
    (2004)
    Laggoun-Défarge, Fatima
    ;
    Mitchell, Edward 
    ;
    Gilbert, Daniel
    ;
    Warner, Barry
    ;
    Comont, Laure
    ;
    Disnar, Jean-Robert
    ;
    Buttler, Alexandre
  • Publication
    Métadonnées seulement
    Contrasted effects of increased N and CO2 supply on two keystone species in peatland restoration and implications for global change
    (2002-5-2)
    Mitchell, Edward 
    ;
    Buttler, Alexandre
    ;
    Grosvernier, Philippe
    ;
    Rydin, Hakan
    ;
    Siegenthaler, Andy
    ;
    Gobat, Jean-Michel 
    1 Significant areas of temperate bogs have been damaged by peat harvesting but may regenerate. These secondary mires, if well managed, may act as strong C sinks, regulate hydrology and buffer regional climate. 2 The potential effects of bog regeneration will, however, depend on the successful establishment of the principal peat formers - Sphagnum mosses. The influence of hydrology and microclimate on Sphagnum re-growth is well studied but effects of elevated CO2 and N deposition are not known. 3 We carried out two in-situ experiments in a cutover bog during three growing seasons in which we raised either CO2 (to 560 p.p.m.) or N (by adding NH4NO3, 3 g m(-2) year(-1)). The two treatments had contrasting effects on competition between the initial coloniser Polytrichum strictum (favoured by high N) and the later coloniser Sphagnum fallax (favoured by high CO2). 4 Such changes may have important consequences for bog regeneration and hence for carbon sequestration in cutover bogs, with potential feedback on regional hydrological and climatic processes.
  • Publication
    Métadonnées seulement
    Ruptures multiples dans les tourbières du Jura: changements climatiques et hydrologiques, successions végétales et impacts humains
    (: DIALOGUES D'HISTOIRE ANCIENNE, 2002)
    Buttler, Alexandre
    ;
    Mitchell, Edward 
    ;
    Freléchoux, François
    ;
    Van der Knaap, Willem
    ;
    Van Leeuwen, Jacqueline
    ;
    Warner, Barry
    ;
    Gobat, Jean-Michel 
    ;
    Schweingruber, Fritz
    ;
    Amman, Brigitte
  • Publication
    Métadonnées seulement
    Can testate amoebae (Protozoa) and other micro-organisms help to overcome biogeographic bias in large scale global change research?
    (: Kluwer Academic Publishers, 2001)
    Mitchell, Edward 
    ;
    Gilbert, Daniel
    ;
    Butler, Alexandre
    ;
    Grosvernier, Philippe
    ;
    Albinsson, Christer
    ;
    Rydin, Hakan
    ;
    Buttler, Alexandre
    ;
    Heijmans, Monique
    ;
    Hoosbeek, Marcel
    ;
    Greenup, Alisson
    ;
    Foot, Jonathan
    ;
    Saarinen, Timo
    ;
    Vasander, Harri
    ;
    Gobat, Jean-Michel