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Explaining temporal variations in soil respiration rates and delta13C in coniferous forest ecosystems
Örebro University, Department of Natural Sciences.
2008 (English)Doctoral 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.

Place, publisher, year, edition, pages
Örebro: Örebro universitet , 2008. , p. 49
Series
Örebro Studies in Biology, ISSN 1650-8793 ; 4
Keywords [en]
Autotrophic, Boreal forest, Elevated CO2, 13C, Natural abundance of stable isotopes, Picea abies, PLS, Root respiration, Soil respiration
National Category
Biological Sciences
Research subject
Biology
Identifiers
URN: urn:nbn:se:oru:diva-2055ISBN: 978-91-7668-591-4 (print)OAI: oai:DiVA.org:oru-2055DiVA, id: diva2:135827
Public defence
2008-05-09, Hörsal M, Musikhögskolan, Örebro Universitet, Örebro, 10:00
Opponent
Supervisors
Available from: 2008-04-14 Created: 2008-04-14 Last updated: 2017-10-18Bibliographically approved
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
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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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Comstedt, Daniel

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