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  • 1.
    Boström, Björn
    et al.
    Örebro University, Department of Natural Sciences.
    Comstedt, Daniel
    Örebro University, Department of Natural Sciences.
    Ekblad, Alf
    Örebro University, Department of Natural Sciences.
    Can isotopic fractionation during respiration explain the 13C-enriched sporocarps of ectomycorrhizal and saprotrophic fungi?2008In: New Phytologist, ISSN 0028-646X, E-ISSN 1469-8137, Vol. 177, no 4, p. 1012-1019Article in journal (Refereed)
    Abstract [en]

    • The mechanism behind the 13C enrichment of fungi relative to plant materials is unclear and constrains the use of stable isotopes in studies of the carbon cycle in soils.

    • Here, we examined whether isotopic fractionation during respiration contributes to this pattern by comparing δ13C signatures of respired CO2, sporocarps and their associated plant materials, from 16 species of ectomycorrhizal or saprotrophic fungi collected in a Norway spruce forest.

    • The isotopic composition of respired CO2 and sporocarps was positively correlated. The differences in δ13C between CO2 and sporocarps were generally small, < ±1‰ in nine out of 16 species, and the average shift for all investigated species was 0.04‰. However, when fungal groups were analysed separately, three out of six species of ectomycorrhizal basidiomycetes respired 13C-enriched CO2 (up to 1.6‰), whereas three out of five species of polypores respired 13C-depleted CO2 (up to 1.7‰; P < 0.05). The CO2 and sporocarps were always 13C-enriched compared with wood, litter or roots.

    • Loss of 13C-depleted CO2 may have enriched some species in 13C. However, that the CO2 was consistently 13C-enriched compared with plant materials implies that other processes must be found to explain the consistent 13C-enrichment of fungal biomass compared with plant materials.

  • 2.
    Boström, Björn
    et al.
    Örebro University, Department of Natural Sciences.
    Comstedt, Daniel
    Örebro University, Department of Natural Sciences.
    Ekblad, Alf
    Örebro University, Department of Natural Sciences.
    Isotope fractionation and 13C enrichment in soil profiles during the decomposition of soil organic matter2007In: Oecologia, ISSN 0029-8549, E-ISSN 1432-1939, Vol. 153, no 1, p. 89-98Article in journal (Refereed)
    Abstract [en]

    The mechanisms behind the 13C enrichment of organic matter with increasing soil depth in forests are unclear. To determine if 13C discrimination during respiration could contribute to this pattern, we compared d13C signatures of respired CO2 from sieved mineral soil, litter layer and litterfall with measurements of d13C and d15N of mineral soil, litter layer, litterfall, roots and fungal mycelia sampled from a 68-year-old Norway spruce forest stand planted on previously cultivated land. Because the land was subjected to ploughing before establishment of the forest stand, shifts in d13C in the top 20 cm reflect processes that have been active since the beginning of the reforestation process. As 13C-depleted organic matter accumulated in the upper soil, a 1.0 o/oo d13C gradient from –28.5 o/oo in the litter layer to –27.6 o/oo at a depth of 2–6 cm was formed. This can be explained by the 1 o/oo drop in d13C of atmospheric CO2 since the beginning of reforestation together with the mixing of new C (forest) and old C (farmland). However, the isotopic change of the atmospheric CO2 explains only a portion of the additional 1.0& increase in d13C below a depth of 20 cm. The d13C of the respired CO2 was similar to that of the organic matter in the upper soil layers but became increasingly 13C enriched with depth, up to 2.5 o/oo relative to the organic matter. We hypothesise that this 13C enrichment of the CO2 as well as the residual increase in d13C of the organic matter below a soil depth of 20 cm results from the increased contribution of 13C-enriched microbially derived C with depth. Our results suggest that 13C discrimination during microbial respiration does not contribute to the 13C enrichment of organic matter in soils. We therefore recommend that these results should be taken into consideration when natural variations in d13C of respired CO2 are used to separate different components of soil respiration or ecosystem respiration.

  • 3.
    Comstedt, Daniel
    Örebro University, Department of Natural Sciences.
    Explaining temporal variations in soil respiration rates and delta13C in coniferous forest ecosystems2008Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Soils of Northern Hemisphere forests contain a large part of the global terrestrial carbon (C) pool. Even small changes in this pool can have large impact on atmospheric [CO2] and the global climate. Soil respiration is the largest terrestrial C flux to the atmosphere and can be divided into autotrophic (from roots, mycorrhizal hyphae and associated microbes) and heterotrophic (from decomposers of organic material) respiration. It is therefore crucial to establish how the two components will respond to changing environmental factors. In this thesis I studied the effect of elevated atmospheric [CO2] (+340 ppm, 13C-depleted) and elevated air temperature (2.8-3.5 oC) on soil respiration in a whole-tree chamber (WTC) experiment conducted in a boreal Norway spruce forest. In another spruce forest I used multivariate modelling to establish the link between day-to-day variations in soil respiration rates and its δ13C, and above and below ground abiotic conditions. In both forests, variation in δ13C was used as a marker for autotrophic respiration. A trenching experiment was conducted in the latter forest in order to separate the two components of soil respiration. The potential problems associated with the trenching, increased root decomposition and changed soil moisture conditions were handled by empirical modelling. The WTC experiment showed that elevated [CO2] but not temperature resulted in 48 to 62% increased soil respiration rates. The CO2-induced increase was in absolute numbers relatively insensitive to seasonal changes in soil temperature and data on δ13C suggest it mostly resulted from increased autotrophic respiration. From the multivariate modelling we observed a strong link between weather (air temperature and vapour pressure deficit) and the day-to-day variation of soil respiration rate and its δ13C. However, the tightness of the link was dependent on good weather for up to a week before the respiration sampling. Changes in soil respiration rates showed a lag to weather conditions of 2-4 days, which was 1-3 days shorter than for the δ13C signal. We hypothesised to be due to pressure concentration waves moving in the phloem at higher rates than the solute itself (i.e., the δ13C–label). Results from the empirical modelling in the trenching experiment show that autotrophic respiration contributed to about 50% of total soil respiration, had a great day-to-day variation and was correlated to total soil respiration while not to soil temperature or soil moisture. Over the first five months after the trenching, an estimated 45% of respiration from the trenched plots was an artefact of the treatment. Of this, 29% was a water difference effect and 16% resulted from root decomposition. In conclusion, elevated [CO2] caused an increased C flux to the roots but this C was rapidly respired and has probably not caused changes in the C stored in root biomass or in soil organic matter in this N-limited forest. Autotrophic respiration seems to be strongly influenced by the availability of newly produced substrates and rather insensitive to changes in soil temperature. Root trenching artefacts can be compensated for by empirical modelling, an alternative to the sequential root harvesting technique.

    List of papers
    1. Effects of elevated atmospheric carbon dioxide and temperature on soil respiration in a boreal forest using δ13C as a labeling tool
    Open this publication in new window or tab >>Effects of elevated atmospheric carbon dioxide and temperature on soil respiration in a boreal forest using δ13C as a labeling tool
    Show others...
    2006 (English)In: Ecosystems (New York. Print), ISSN 1432-9840, E-ISSN 1435-0629, Vol. 9, no 8, p. 1266-1277Article in journal (Refereed) Published
    National Category
    Environmental Sciences Natural Sciences Chemical Sciences
    Research subject
    Environmental Chemistry
    Identifiers
    urn:nbn:se:oru:diva-2965 (URN)10.1007/s10021-006-0110-5 (DOI)
    Note

    Part of thesis: http://urn.kb.se/resolve?urn=urn:nbn:se:oru:diva-2055

    Available from: 2008-04-14 Created: 2008-04-14 Last updated: 2017-12-14Bibliographically approved
    2. Forest soil respiration rate and d13C is regulated by recent above ground weather conditions
    Open this publication in new window or tab >>Forest soil respiration rate and d13C is regulated by recent above ground weather conditions
    2005 (English)In: Oecologia, ISSN 0029-8549, E-ISSN 1432-1939, Vol. 143, no 1, p. 136-142Article in journal (Refereed) Published
    Abstract [en]

    Soil respiration, a key component of the global carbon cycle, is a major source of uncertainty when estimating terrestrial carbon budgets at ecosystem and higher levels. Rates of soil and root respiration are assumed to be dependent on soil temperature and soil moisture yet these factors often barely explain half the seasonal variation in soil respiration. We here found that soil moisture (range 16.5-27.6% of dry weight) and soil temperature (range 8-17.5 degrees C) together explained 55% of the variance (cross-validated explained variance; Q2) in soil respiration rate (range 1.0-3.4 micromol C m(-2) s(-1)) in a Norway spruce (Picea abies) forest. We hypothesised that this was due to that the two components of soil respiration, root respiration and decomposition, are governed by different factors. We therefore applied PLS (partial least squares regression) multivariate modelling in which we, together with below ground temperature and soil moisture, used the recent above ground air temperature and air humidity (vapour pressure deficit, VPD) conditions as x-variables. We found that air temperature and VPD data collected 1-4 days before respiration measurements explained 86% of the seasonal variation in the rate of soil respiration. The addition of soil moisture and soil temperature to the PLS-models increased the Q2 to 93%. delta13C analysis of soil respiration supported the hypotheses that there was a fast flux of photosynthates to root respiration and a dependence on recent above ground weather conditions. Taken together, our results suggest that shoot activities the preceding 1-6 days influence, to a large degree, the rate of root and soil respiration. We propose this above ground influence on soil respiration to be proportionally largest in the middle of the growing season and in situations when there is large day-to-day shifts in the above ground weather conditions. During such conditions soil temperature may not exert the major control on root respiration.

    Keywords
    Air temperature, 13C, PLS time series analysis, Root respiration, Soil temperature
    National Category
    Biomaterials Science Chemical Sciences Environmental Sciences
    Research subject
    Environmental Chemistry
    Identifiers
    urn:nbn:se:oru:diva-2966 (URN)10.1007/s00442-004-1776-z (DOI)
    Available from: 2008-04-14 Created: 2008-04-14 Last updated: 2017-12-14Bibliographically approved
    3. A link between above ground weather conditions and the δ13C of forest soil respiration is not always observed
    Open this publication in new window or tab >>A link between above ground weather conditions and the δ13C of forest soil respiration is not always observed
    (English)Manuscript (Other academic)
    National Category
    Natural Sciences Biological Sciences
    Research subject
    Biology
    Identifiers
    urn:nbn:se:oru:diva-2967 (URN)
    Available from: 2008-04-14 Created: 2008-04-14 Last updated: 2017-10-18Bibliographically approved
    4. Autotrophic and heterotrophic soil respiration in a Norway spruce forest: estimating the root decomposition and soil moisture effects in a trenching experiment
    Open this publication in new window or tab >>Autotrophic and heterotrophic soil respiration in a Norway spruce forest: estimating the root decomposition and soil moisture effects in a trenching experiment
    (English)Manuscript (Other academic)
    National Category
    Natural Sciences Biological Sciences
    Research subject
    Biology
    Identifiers
    urn:nbn:se:oru:diva-2968 (URN)
    Available from: 2008-04-14 Created: 2008-04-14 Last updated: 2017-10-18Bibliographically approved
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  • 4.
    Comstedt, Daniel
    et al.
    Örebro University, Department of Natural Sciences.
    Boström, Björn
    Örebro University, Department of Natural Sciences.
    Ekblad, Alf
    Örebro University, Department of Natural Sciences.
    Autotrophic and heterotrophic soil respiration in a Norway spruce forest: estimating the root decomposition and soil moisture effects in a trenching experimentManuscript (Other academic)
  • 5.
    Comstedt, Daniel
    et al.
    Örebro University, School of Science and Technology.
    Boström, Björn
    Örebro University, School of Science and Technology.
    Ekblad, Alf
    Örebro University, School of Science and Technology.
    Autotrophic and heterotrophic soil respiration in a Norway spruce forest: estimating the root decomposition and soil moisture effects in a trenching experiment2011In: Biogeochemistry, ISSN 0168-2563, E-ISSN 1573-515X, Vol. 104, no 1-3, p. 121-132Article in journal (Refereed)
    Abstract [en]

    The two components of soil respiration, autotrophic respiration (from roots, mycorrhizal hyphae and associated microbes) and heterotrophic respiration (from decomposers), was separated in a root trenching experiment in a Norway spruce forest. In June 2003, cylinders (29.7 cm diameter) were inserted to 50 cm soil depth and respiration was measured both outside (control) and inside the trenched areas. The potential problems associated with the trenching treatment, increased decomposition of roots and ectomycorrhizal mycelia and changed soil moisture conditions, were handled by empirical modelling. The model was calibrated with respiration, moisture and temperature data of 2004 from the trenched plots as a training set. We estimate that over the first 5 months after the trenching, 45% of respiration from the trenched plots was an artefact of the treatment. Of this, 29% was a water difference effect and 16% resulted from root and mycelia decomposition. Autotrophic and heterotrophic respiration contributed to about 50% each of total soil respiration in the control plots averaged over the two growing seasons. We show that the potential problems with the trenching, decomposing roots and mycelia and soil moisture effects, can be handled by a modelling approach, which is an alternative to the sequential root harvesting technique.

  • 6.
    Comstedt, Daniel
    et al.
    Örebro University, Department of Natural Sciences.
    Boström, Björn
    Marshall, John
    Holm, Anders
    Slaney, Michelle
    Linder, Sune
    Ekblad, Alf
    Effects of elevated atmospheric carbon dioxide and temperature on soil respiration in a boreal forest using δ13C as a labeling tool2006In: Ecosystems (New York. Print), ISSN 1432-9840, E-ISSN 1435-0629, Vol. 9, no 8, p. 1266-1277Article in journal (Refereed)
  • 7.
    Comstedt, Daniel
    et al.
    Örebro University, Department of Natural Sciences.
    Boström, Björn
    Thompson, Matthew V.
    Ekblad, Alf
    A link between above ground weather conditions and the δ13C of forest soil respiration is not always observedManuscript (Other academic)
  • 8.
    Ekblad, Alf
    et al.
    Örebro University, Department of Natural Sciences.
    Boström, Björn
    Örebro University, Department of Natural Sciences.
    Holm, Anders
    Örebro University, Department of Natural Sciences.
    Comstedt, Daniel
    Örebro University, Department of Natural Sciences.
    Forest soil respiration rate and d13C is regulated by recent above ground weather conditions2005In: Oecologia, ISSN 0029-8549, E-ISSN 1432-1939, Vol. 143, no 1, p. 136-142Article in journal (Refereed)
    Abstract [en]

    Soil respiration, a key component of the global carbon cycle, is a major source of uncertainty when estimating terrestrial carbon budgets at ecosystem and higher levels. Rates of soil and root respiration are assumed to be dependent on soil temperature and soil moisture yet these factors often barely explain half the seasonal variation in soil respiration. We here found that soil moisture (range 16.5-27.6% of dry weight) and soil temperature (range 8-17.5 degrees C) together explained 55% of the variance (cross-validated explained variance; Q2) in soil respiration rate (range 1.0-3.4 micromol C m(-2) s(-1)) in a Norway spruce (Picea abies) forest. We hypothesised that this was due to that the two components of soil respiration, root respiration and decomposition, are governed by different factors. We therefore applied PLS (partial least squares regression) multivariate modelling in which we, together with below ground temperature and soil moisture, used the recent above ground air temperature and air humidity (vapour pressure deficit, VPD) conditions as x-variables. We found that air temperature and VPD data collected 1-4 days before respiration measurements explained 86% of the seasonal variation in the rate of soil respiration. The addition of soil moisture and soil temperature to the PLS-models increased the Q2 to 93%. delta13C analysis of soil respiration supported the hypotheses that there was a fast flux of photosynthates to root respiration and a dependence on recent above ground weather conditions. Taken together, our results suggest that shoot activities the preceding 1-6 days influence, to a large degree, the rate of root and soil respiration. We propose this above ground influence on soil respiration to be proportionally largest in the middle of the growing season and in situations when there is large day-to-day shifts in the above ground weather conditions. During such conditions soil temperature may not exert the major control on root respiration.

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