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Importance of winter climate and soil frost for dissolved organic carbon (DOC) in boreal forest soils and streams: - implications for a changing climate
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Dissolved organic carbon (DOC) is a defining feature of surface waters in a large part of the world and it influences a variety of physical, chemical and biological processes in aquatic ecosystems. Riparian soils exert a major control on stream water chemistry in many northern latitude regions and provide a local source of DOC to adjacent streams. As the winter climate in northern regions is predicted to be particularly affected by climate change, it is important to investigate the sensitivity of DOC in riparian soils and the potential implications for adjacent streams’ chemistry in a changing winter climate. The effects of a changing winter climate on riparian soil DOC production and export to streams has received little attention to date, and this is the focus of my thesis.

In this thesis I first evaluate the hydro-climatic drivers of inter-annual variation in spring snowmelt DOC concentrations in two boreal forest streams of northern Sweden. By accounting for the effects of seasonal hydrology, I was able to statistically evaluate the importance of antecedent winter climatic conditions to inter-annual variation in stream DOC concentrations during the spring snow-melt periods. This descriptive work was complemented by a long-term field experiment where snow packs were manipulated to investigate the impacts of soil freezing on the concentration and quality of DOC in soil water. The effect of soil freezing on DOC was further addressed in a multi-factor laboratory experiment on soil samples taken from the riparian zone. The laboratory experiment was designed based on a central composite face-centered (CCF) model which applied three levels of four freezing-related factors: temperature, water content, duration and frequency of freeze-thaw cycles. The responses of soil microbial- activity and composition to the same experimental factors as well as their potential link to frost induced changes in DOC were also tested.

Large inter-annual variations were observed in spring snow-melt DOC concentrations in streams. Lower export of DOC during the preceding seasons and longer, and colder, winters resulted in higher spring snow-melt DOC concentrations. Soil water DOC concentrations and lability were significantly enhanced in the upper soil horizons which experienced extensive soil frost and longer frost duration. In the laboratory experiment, similar responses of soil water DOC were observed in that the higher concentrations and greater lability were found in samples incubated at the lowest temperatures (-12°C). The fungal to bacterial growth ratio also increased in the lower temperature treatments. In addition, fungal growth rate and soil basal respiration responded positively to frost-induced increases in DOC concentration. The frequency of freeze-thaw cycle did not appear to be an influential factor in the laboratory experiment. Several significant interactions of the factors were also detected.

By conducting and integrating field and laboratory experiments I highlight the importance of soil frost regime and winter climatic conditions for regulating DOC in riparian soils and their adjacent streams in areas with seasonally frozen soils. However, in a changing future winter climate, alterations in soil frost should be assessed as the result of changes in air temperature and snow-pack formation and extent, and implications for streams should be investigated with regard to changes in hydrology and export processes in soil-water interface.  

Place, publisher, year, edition, pages
meå: Department of Ecology and Environmental Sciences, Umeå University , 2011. , 22 p.
Identifiers
URN: urn:nbn:se:umu:diva-43171ISBN: 978-91-7459-219-1OAI: oai:DiVA.org:umu-43171DiVA: diva2:412279
Public defence
2011-05-20, KBC huset, KB3A9 (lilla hörsalen), Umeå universitet, Umeå, 10:00 (English)
Opponent
Supervisors
Available from: 2011-04-29 Created: 2011-04-21 Last updated: 2011-04-28Bibliographically approved
List of papers
1. Cold winter soils enhance dissolved organic carbon concentrations in soil and stream water
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2010 (English)In: Geophysical Research Letters, ISSN 0094-8276, Vol. 37, L08501- p.Article in journal (Refereed) Published
Abstract [en]

Concentrations of dissolved organic carbon ([DOC]) have increased in lakes, streams and rivers across a large part of the northern hemisphere and raised an animated scientific debate about the underlying mechanisms. The lack of consensus about the role of climate in controlling the DOC trends highlights the need for understanding the regulation of surface water DOC. We found that longer and colder winters result in higher [DOC] in a boreal headwater stream during the subsequent snowmelt. In addition, prolonged soil frost increases the spring and summer [DOC] in the riparian soil water, which is a major contributor of stream water DOC in the studied area. We conclude that winter climatic conditions can play a substantial role in controlling stream [DOC] in ways not previously understood. These findings are especially important for northern latitude regions expected to be most affected by climate change.

Place, publisher, year, edition, pages
American Geophysical Union, 2010
Identifiers
urn:nbn:se:umu:diva-33610 (URN)10.1029/2010GL042821 (DOI)000277033200002 ()
Available from: 2010-04-29 Created: 2010-04-29 Last updated: 2011-04-28Bibliographically approved
2. Regulation of stream water dissolved organic carbon (DOC) concentrations during snowmelt: the role of discharge, winter climate and memory effects
Open this publication in new window or tab >>Regulation of stream water dissolved organic carbon (DOC) concentrations during snowmelt: the role of discharge, winter climate and memory effects
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2010 (English)In: Biogeosciences, ISSN 1726-4170, Vol. 7, no 9, 2901-2913 p.Article in journal (Refereed) Published
Abstract [en]

Using a 15 year stream record from a northern boreal catchment, we demonstrate that the inter-annual variation in dissolved organic carbon (DOC) concentrations during snowmelt was related to discharge, winter climate and previous DOC export. A short and intense snowmelt gave higher stream water DOC concentrations, as did long winters, while a high previous DOC export during the antecedent summer and autumn resulted in lower concentrations during the following spring. By removing the effect of discharge we could detect that the length of winter affected the modeled soil water DOC concentrations during the following snowmelt period, which in turn affected the concentrations in the stream. Winter climate explained more of the stream water DOC variations than previous DOC export during the antecedent summer and autumn.

Identifiers
urn:nbn:se:umu:diva-40597 (URN)10.5194/bg-7-2901-2010 (DOI)000282427800016 ()
Available from: 2011-03-01 Created: 2011-03-01 Last updated: 2011-04-28Bibliographically approved
3. The influence of soil frost on the quality of dissolved organic carbon in a boreal forest soil: combining field and laboratory experiments
Open this publication in new window or tab >>The influence of soil frost on the quality of dissolved organic carbon in a boreal forest soil: combining field and laboratory experiments
2012 (English)In: Biogeochemistry, ISSN 0168-2563, E-ISSN 1573-515X, Vol. 107, no 1-3, 95-106 p.Article in journal (Refereed) Published
Abstract [en]

Riparian soils exert a major control on stream water dissolved organic carbon (DOC) in northern latitudes. As the winter climate in northern regions is predicted to be particularly affected by climate change, we tested the sensitivity of DOC formation to winter conditions in riparian soils using an 8 year field-scale soil frost manipulation experiment in northern Sweden. In conjunction with the field experiment, we also carried out a laboratory experiment based on three levels of four winter climatic factors: frost intensity, soil water content, frost duration and frequency of freeze–thaw cycles. We evaluated changes in lability of DOC in soil solution from lysimeter samples taken at different depths (10–80 cm) as well as from DOC extracted from soils in the laboratory, using carbon-specific ultraviolet absorbance at 254 nm (sUVA254). In the field, significantly more labile DOC was observed during the spring and summer from upper horizons of frost-exposed soils, when compared to controls. In addition, the amount of labile DOC was positively correlated with frost duration at a soil depth of 10 cm. In the laboratory, frost intensity was the factor that had the greatest positive influence on DOC lability; it also reduced the C:N ratio which may indicate a microbial origin of the DOC. The laboratory experiment also demonstrated significant interactions between some of the applied climatic factors, such as frost intensity interacting with water content. In combination, field and laboratory experiments demonstrate that winter soil conditions have profound effects on DOC-concentration and quality during subsequent seasons.

Place, publisher, year, edition, pages
Springer, 2012
Keyword
Soil frost, sUVA254, Dissolved organic carbon, Water content, Riparian zone, CCF design
National Category
Biological Sciences
Identifiers
urn:nbn:se:umu:diva-43398 (URN)10.1007/s10533-010-9534-2 (DOI)000298226700007 ()
Note
Published online october 2010Available from: 2011-04-28 Created: 2011-04-28 Last updated: 2012-02-21Bibliographically approved
4. Effects of soil frost on growth, composition and respiration of the soil microbial decomposer community
Open this publication in new window or tab >>Effects of soil frost on growth, composition and respiration of the soil microbial decomposer community
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2011 (English)In: Soil Biology and Biochemistry, ISSN 0038-0717, E-ISSN 1879-3428, Vol. 43, no 10, 2069-2077 p.Article in journal (Refereed) Published
Abstract [en]

Most climate change scenarios predict that the variability of weather conditions will increase in coming decades. Hence, the frequency and intensity of freeze-thaw cycles in high-latitude regions are likely to increase, with concomitant effect on soil carbon biogeochemistry and associated microbial processes. To address this issue we sampled riparian soil from a Swedish boreal forest and applied treatments with variations in four factors related to soil freezing (temperature, treatment duration, soil water content and frequency of freeze-thaw cycles), at three levels in a laboratory experiment, using a Central Composite Face-centred (CCF) experimental design. We then measured bacterial (leucine incorporation) and fungal (acetate in ergosterol incorporation) growth, basal respiration, soil microbial phospholipid fatty acid (PLFA) composition, and concentration of dissolved organic carbon (DOC). Fungal growth was higher in soil exposed to freeze-thawing perturbations and freezing temperatures of −6 °C and −12 °C, than under more constant conditions (steady 0 °C). The opposite pattern was found for bacteria, resulting in an increasing fungal-to-bacterial growth ratio following more intensive winter conditions. Soil respiration increased with water content, decreased with treatment duration and appeared to mainly be driven by treatment-induced changes in the DOC concentration. There was a clear shift in the PLFA composition at 0 °C, compared with the two lower temperatures, with PLFA markers associated with fungi as well as a number of unsaturated PLFAs being relatively more common at 0 °C. Shifts in the PLFA pattern were consistent with those expected for phenotypic plasticity of the cell membrane to low temperatures. There were small declines in PLFA concentrations after freeze-thawing and with longer durations. However, the number of freeze-thaw events had no effect on the microbiological variables. The findings

National Category
Geology
Identifiers
urn:nbn:se:umu:diva-43405 (URN)10.1016/j.soilbio.2011.06.005 (DOI)
Available from: 2011-04-28 Created: 2011-04-28 Last updated: 2011-10-20Bibliographically approved

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