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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
    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
    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
    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
    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
    Accès libre
    Plant Litter Decomposition and Nutrient Release in Peatlands
    (2008)
    Bragazza, Luca
    ;
    Buttler, Alexandre
    ;
    Siegenthaler, Andy
    ;
    Mitchell, Edward 
    Decomposition of plant litter is a crucial process in controlling the carbon balance of peatlands. Indeed, as long as the rate of litter decomposition remains lower than the rate of above- and belowground litter production, a net accumulation of peat and, thus, carbon will take place. In addition, decomposition controls the release of important nutrients such as nitrogen, phosphorus, and potassium, the availability of which affects the structure and the functioning of plant communities. This chapter describes the role of the main drivers in affecting mass loss and nutrient release from recently deposited plant litter. In particular, the rate of mass loss of Sphagnum litter and vascular plant litter is reviewed in relation to regional climatic conditions, aerobic/anaerobic conditions, and litter chemistry. The rate of nutrient release is discussed in relation to the rate of mass loss and associated litter chemistry by means of a specific case study.
  • 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
    Accès libre
    Contrasted effects of increased N and CO2 supply on two keystone species in peatland restoration and implications for global change
    (2002)
    Mitchell, Edward 
    ;
    Buttler, Alexandre
    ;
    Grosvernier, Philippe
    ;
    Rydin, HÃ…kan
    ;
    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
    Accès libre
    Effects of elevated atmospheric CO2 and mineral nitrogen deposition on litter quality, bioleaching and decomposition in a sphagnum peat bog
    (: Kluwer Academic Publishers, 2001)
    Siegenthaler, Andy
    ;
    Mitchell, Edward 
    ;
    Van der Heijden, Edvin
    ;
    Buttler, Alexandre
    ;
    Grosvernier, Philippe
    ;
    Gobat, Jean-Michel 
  • Publication
    Métadonnées seulement
    Effects of elevated atmospheric CO2 and mineral nitrogen deposition on litter quality, bioleaching and decomposition in a sphagnum peat bog
    (: Springer, 1999-5-2)
    Siegenthaler, Andy
    ;
    Mitchell, Edward 
    ;
    Van der Heijden, Edvin
    ;
    Buttler, Alexandre
    ;
    Grosvernier, Philippe
    ;
    Gobat, Jean-Michel 
    ;
    Visconti, Guido
    ;
    Beniston, Martin
    ;
    Iannorelli, Emilio D
    ;
    Barba, Diego
    A brief overview of an attempt to link the effect of elevated CO2 and nitrogen deposition on litter quality and decomposition in a Sphagnum peat bog is given. Litter of three common species (Eriophorum vaginatum, Polytrichum strictum and Sphagnum fallax) was collected from field plots after two years of pre-treatment in two parallel experiments: a) Elevated atmospheric CO2 experiment, b) mineral nitrogen fertilisation experiment. The litters were put into litterbags, leached and inserted into field plots for 3 months, where they decomposed under specific treatment. Distinction between effects of initial litter quality and decomposition on mass loss in the bioleaching and/or in field decomposition process could be tested using a particular set-up in which cross-effects of pre-treatment and treatment were considered.