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Achieving carbon isotope mass balance in northern forest soils, soil respiration and fungi
Örebro University, Department of Natural Sciences.
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Northern forests contain a large part of the global terrestrial carbon pool and it is unclear whether they will be sinks or sources for atmospheric carbon if the climate warms as predicted. Stable isotope techniques provide unique tools to study the carbon cycle at different scales but the interpretation of the isotope data is impaired by our inability to close the carbon isotope mass balance of ecosystems. This involves the paradox that the soil organic matter becomes increasingly 13C-enriched with increasing soil depth relative to the carbon input, plant litter, at the same time as soil respiration, the major carbon outflow from the soil, and fungi, organisms dependent on plant derived carbon, both are relatively 13C-enriched. I have determined the δ13C of the respired CO2 and the organic matter from different ecosystem components in a Norway spruce forest aiming at finding an explanation to the observed carbon isotope pattern.

In the first study the soil surface respiration rate and isotopic composition was found to be governed by aboveground weather conditions the preceding 1-6 days. This suggests there is a fast flux of recent photosynthates to root respiration. In the second study I compared the respired CO2 from decomposition with the δ13C of the root free soil organic matter sampled from the litter layer down to 50 cm depth. Discrimination against 13C during respiration could not explain the 13C enrichment of soil organic matter with depth because the δ13C of the respired CO2 became increasingly 13C-enriched relative to the organic matter with soil depth. However, ~1.5‰ of the 2‰ 13C-gradient could be explained by the 13C depletion of atmospheric CO2 that has proceeded since the beginning of the 18th century due to the burning of fossil fuels and deforestation. The remaining shift was hypothesized to be due to a belowground contribution of 13C-enriched ectomycorrhizal derived carbon. In the third study I compared the δ13C of respired CO2 and sporocarps of ectomycorrhizal and saprotrophic fungi sampled in the spruce forest. The δ13C of respired CO2 and sporocarps were positively correlated and the differences in δ13C between CO2 and sporocarps were small, <±1‰ in nine out of 16 species, although 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). Loss of 13C-depleted CO2 may have enriched the biomass of some fungal species in 13C. However, the consistent 13C enrichment of fungal sporocarps and respired CO2 relative to the plant materials implies that other processes must be found to explain the consistent 13C-enrichment of fungal biomass compared to plant materials. In the final study, compound specific stable isotope analyses provided further evidence for the hypothesis that the biomass of ectomycorrhizal fungi are 13C-enriched relative to host biomass because the carbon provided by the host is 13C-enriched Furthermore, ectomycorrhizal fungi showed lower average δ13C values of metabolites than saprotrophs which gives further support for the so-called saprotrophic-mycorrhizal divide. I conclude that a belowground allocation of 13C-enriched carbon to ectomycorrhizal fungi closes the carbon isotope mass balance in boreal and temperate forest soils and explains the 13C-enriched soil respiration.

Place, publisher, year, edition, pages
Örebro: Örebro universitet , 2008. , p. 60
Series
Örebro Studies in Biology, ISSN 1650-8793 ; 5
Keywords [en]
13C, Carbon cycle, Ectomycorrhizal fungi, Forest soil, Microbial respiration, Soil respiration, Stable isotopes
National Category
Biological Sciences
Research subject
Biology
Identifiers
URN: urn:nbn:se:oru:diva-2101ISBN: 978-91-7668-594-5 (print)OAI: oai:DiVA.org:oru-2101DiVA, id: diva2:135886
Public defence
2008-05-23, Hörsal T, Teknik, Örebro universitet, Örebro, 10:00
Opponent
Supervisors
Available from: 2008-04-24 Created: 2008-04-24 Last updated: 2017-10-18Bibliographically approved
List of papers
1. 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
2. Isotope fractionation and 13C enrichment in soil profiles during the decomposition of soil organic matter
Open this publication in new window or tab >>Isotope fractionation and 13C enrichment in soil profiles during the decomposition of soil organic matter
2007 (English)In: Oecologia, ISSN 0029-8549, E-ISSN 1432-1939, Vol. 153, no 1, p. 89-98Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Berlin: Springer, 2007
Keywords
C:N ratio, d13C, Forest soil organic matter, Isotopic discrimination, Microbial respiration
National Category
Natural Sciences Chemical Sciences Environmental Sciences
Research subject
Environmental Chemistry
Identifiers
urn:nbn:se:oru:diva-4241 (URN)10.1007/s00442-007-0700-8 (DOI)17401582 (PubMedID)
Available from: 2007-12-07 Created: 2007-12-07 Last updated: 2017-12-14Bibliographically approved
3. Can isotopic fractionation during respiration explain the 13C-enriched sporocarps of ectomycorrhizal and saprotrophic fungi?
Open this publication in new window or tab >>Can isotopic fractionation during respiration explain the 13C-enriched sporocarps of ectomycorrhizal and saprotrophic fungi?
2008 (English)In: New Phytologist, ISSN 0028-646X, E-ISSN 1469-8137, Vol. 177, no 4, p. 1012-1019Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Cambridge: Cambridge University Press, 2008
Keywords
Carbon/*metabolism, Carbon Dioxide/metabolism, Carbon Isotopes, Fungi/*metabolism, Nitrogen/metabolism, Nitrogen Isotopes, Oxygen Consumption/*physiology, Picea/microbiology, Time Factors, Trees/microbiology
National Category
Ecology Soil Science Natural Sciences Agricultural Sciences
Research subject
biologi
Identifiers
urn:nbn:se:oru:diva-4647 (URN)10.1111/j.1469-8137.2007.02332.x (DOI)18086229 (PubMedID)
Available from: 2008-10-20 Created: 2008-10-20 Last updated: 2017-12-14Bibliographically approved
4. Carbon isotope ratios in ectomycorrhizal and saprotrophic metabolites in relation to the δ13C of substrate, sporocarps and respired CO2
Open this publication in new window or tab >>Carbon isotope ratios in ectomycorrhizal and saprotrophic metabolites in relation to the δ13C of substrate, sporocarps and respired CO2
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Biological Sciences
Research subject
Biology
Identifiers
urn:nbn:se:oru:diva-15539 (URN)
Available from: 2011-05-11 Created: 2011-05-11 Last updated: 2017-10-17Bibliographically approved

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