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Cryogenic soil processes in a changing climate
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
2016 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Kryogena mark processer i ett föränderligt klimat (Swedish)
Abstract [en]

A considerable part of the global pool of terrestrial carbon is stored in high latitude soils. In these soils, repeated cycles of freezing and thawing creates soil motion (cryoturbation) that in combination with other cryogenic disturbance processes may play a profound role in controlling the carbon balance of the arctic soil. Conditions for cryogenic soil processes are predicted to dramatically change in response to the ongoing climate warming, but little is known how these changes may affect the ability of arctic soils to accumulate carbon. In this thesis, I utilize a patterned ground system, referred to as non-sorted circles, as experimental units and quantify how cryogenic soil processes affect plant communities and carbon fluxes in arctic soils. I show that the cryoturbation has been an important mechanism for transporting carbon downwards in the studied soil over the last millennia. Interestingly, burial of organic material by cryoturbation appears to have mainly occurred during bioclimatic events occurring around A.D. 900-1250 and A.D. 1650-1950 as indicated by inferred 14C ages. Using a novel photogrammetric approach, I estimate that about 0.2-0.8 % of the carbon pool is annually subjected to a net downward transport induced by the physical motion of soil. Even though this flux seems small, it suggests that cryoturbation is an important transporter of carbon over centennial and millennial timescales and contributes to translocate organic matter to deeper soil layers where respiration proceeds at slow rates. Cryogenic processes not only affect the trajectories of the soil carbon, but also generate plant community changes in both species composition and abundance, as indicated by a conducted plant survey on non-sorted circles subjected to variable differential frost heave during the winter. Here, disturbance-tolerant plant species, such as Carex capillaris and Tofieldia pusilla, seem to be favoured by disturbance generated by the differential heave. Comparison with findings from a previous plant survey on the site conducted in the 1980s suggest that the warmer temperatures during the last decades have resulted in decreased differential heave in the studied non-sorted circles. I argue that this change in cryogenic activity has increased abundance of plants present in the 1980s. The fact that the activity and function of the non-sorted circles in Abisko are undergoing changes is further supported by their contemporary carbon dioxide (CO2) fluxes. Here, my measurements of CO2 fluxes suggest that all studied non-sorted circles act as net CO2 sources and thus that the carbon balance of the soils are in a transition state. My results highlight the complex but important relationship between cryogenic soil processes and the carbon balance of arctic soils.

Place, publisher, year, edition, pages
Umeå: Umeå universitet , 2016. , 18 p.
Keyword [en]
Non-sorted circle, Carbon storage, Soil motion, Terrestrial photogrammetry, Cryoturbation, Carbon fluxes
National Category
Other Earth and Related Environmental Sciences
Research subject
Physical Geography
Identifiers
URN: urn:nbn:se:umu:diva-112509ISBN: 978-91-7601-361-8 (print)OAI: oai:DiVA.org:umu-112509DiVA: diva2:882724
Public defence
2016-01-22, Stora Hörsalen, KBC-huset, Umeå universitet, Umeå, 10:00 (English)
Opponent
Supervisors
Available from: 2015-12-18 Created: 2015-12-09 Last updated: 2015-12-17Bibliographically approved
List of papers
1. Buried soil organic inclusions in non-sorted circles fields in northern Sweden: Age and Paleoclimatic context
Open this publication in new window or tab >>Buried soil organic inclusions in non-sorted circles fields in northern Sweden: Age and Paleoclimatic context
2013 (English)In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 118, no 1, 104-111 p.Article in journal (Refereed) Published
Abstract [en]

Although burial of surface organic soil horizons into deeper mineral soil layers helps drive the long-term buildup of carbon in arctic soils, when and why buried horizons formed as result of cryoturbation in northern Sweden remain unclear. In this study, we used C-14 and Pb-210 dating to assess when organic matter was buried within non-sorted circles fields near Abisko in northern Sweden. In addition, we used aerial photos from 1959 and 2008 to detect eventual trends in cryogenic activities during this period. We found that organic matter from former organic horizons (stratigraphically intact or partly fragmented) corresponds to three major periods: 0-100 A. D., 900-1250 A. D., and 1650-1950 A. D. The latter two periods were indicated by several dated samples, while the extent of the oldest period is more uncertainty (indicated by only one sample). The aerial photos suggest a net overgrowth by shrub vegetation of previously exposed mineral soil surfaces since 1959. This overgrowth trend was seen in most of the studied fields (92 out of 137 analyzed fields), indicating that the cryogenic activity has mainly decreased in studied non-sorted circles fields since the 1950s. This latter interpretation is also supported by the absence of buried organic layers formed during the last decades. We suggest that the organic matter was buried during the transition from longer cold periods to warmer conditions. We believe these climatic shifts could have triggered regional scale burial of soil organic matter and thus affected how these soils sequestered carbon.

Keyword
non-sorted circles, frost creep, radiocarbon, Pb-210, arctic, soil
National Category
Geology
Identifiers
urn:nbn:se:umu:diva-71325 (URN)10.1002/jgrg.20016 (DOI)000317844700009 ()
Available from: 2013-05-28 Created: 2013-05-26 Last updated: 2017-12-06Bibliographically approved
2. The use of terrestrial photogrammetry to estimate soil motion rates in non-sorted circles
Open this publication in new window or tab >>The use of terrestrial photogrammetry to estimate soil motion rates in non-sorted circles
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Soil motion induced by cryogenic processes is known for creating soil surface structures (patterned ground) and redistributing carbon within Arctic soils. Lateral and vertical soil motion created by cryogenic processes proceeds over annual to millennial time-scales and is difficult to quantify without adopting disruptive soil sampling techniques. In this study, we evaluate the use of terrestrial close range photogrammetry to calculate soil motion rates within a patterned ground system (non-sorted circles). The measured rates of lateral and vertical motion were estimated and used to infer the importance of physical soil transport for the formation of non-sorted circles as well as the trajectories of soil carbon. Soil experiencing significant vertical displacement between years covered approximately 65% of the non-sorted circles and had surface levels fluctuating between 4 and -2.1 cm. Systematic lateral motion of surface stones allowed detection of lateral motion working outwards from the centre towards the sides, at rates ranging between 0.1 and 6.3 cm yr-1. We conclude that terrestrial close range photogrammetry can be used to identify the main soil movements within non-sorted circles and that this transport is an important factor controlling the trajectories of soil carbon over centennial to millennial timescales. 

Keyword
Soil motion, Terrestrial photogrammetry, Carbon storage, Non-sorted circles
National Category
Geology
Research subject
Physical Geography
Identifiers
urn:nbn:se:umu:diva-112508 (URN)
Available from: 2015-12-15 Created: 2015-12-09 Last updated: 2015-12-17Bibliographically approved
3. Decreased cryogenic disturbance: one of the potential mechanisms behind the shrubification of non-sorted circles in subarctic Sweden
Open this publication in new window or tab >>Decreased cryogenic disturbance: one of the potential mechanisms behind the shrubification of non-sorted circles in subarctic Sweden
(English)Manuscript (preprint) (Other academic)
Abstract [en]

During the last few decades, the arctic has experienced large-scale vegetation changes often evident as widespread greening (increased NDVI) in satellite images. Understanding the mechanisms behind this greening is crucial for our ability to predict future vegetation changes. The objective of this study was to test the hypothesis that decreased cryogenic disturbances have contributed to the greening of patterned ground fields (non-sorted circles) in the study site (Abisko, Northern Sweden) during the last few decades. The hypothesis was tested by surveying the composition of plant communities across a gradient in cryogenic disturbance and by reinvestigating plant communities previously surveyed in the 1980s to scrutinise how these communities changed in response to reduced cryogenic disturbance. Whereas the historical changes in species occurrence associated with decreased cryogenic disturbances were fairly consistent with the changes along the contemporary gradient of cryogenic disturbances, the species abundance revealed important transient changes highly dependent on the initial plant community composition. Our results imply that the decreased environmental stress from differential heave during winter in recent decades is one of the mechanisms that has contributed to the greening of the patterned ground in the study area, although more than a few decades are likely required before this decreased stress contributes in a substantial way to the on-going shrubification. 

Keyword
Patterned ground, Plant abundance, Non-sorted circles, Freeze/thaw-index, Cryogenic disturbance, Differential heave
National Category
Other Earth and Related Environmental Sciences
Research subject
Physical Geography
Identifiers
urn:nbn:se:umu:diva-112507 (URN)
Available from: 2015-12-15 Created: 2015-12-09 Last updated: 2015-12-17Bibliographically approved
4. Cryogenic disturbance and its impact on carbon fluxes in a subarctic heathland
Open this publication in new window or tab >>Cryogenic disturbance and its impact on carbon fluxes in a subarctic heathland
2015 (English)In: Environmental Research Letters, ISSN 1748-9326, E-ISSN 1748-9326, Vol. 10, no 11, 114006Article in journal (Refereed) Published
Abstract [en]

Differential frost heave, along with the associated cryogenic disturbance that accompanies it, is an almost universal feature of arctic landscapes that potentially influences the fate of the soil carbon (C) stored in arctic soils. In this study, we quantify how gross ecosystem photosynthesis (GEP), soil respiration (Re) and the resulting net ecosystem exchange (NEE) vary in a patterned ground system (non-sorted circles) at plot-scale and whole-patterned ground scales in response to cryogenic disturbances (differential heave and soil surface disruption). We found that: (i) all studied non-sorted circles (n=15) acted as net CO2 sources (positive NEE); (ii) GEP showed a weaker decrease than Re in response to increased cryogenic disturbance/decreased humus cover, indicating that undisturbed humus-covered sites are currently the main source of atmospheric CO2 in the studied system. Interestingly, Re fluxes normalized to C pools indicated that C is currently respired more rapidly at sites exposed to cryogenic disturbances; hence, higher NEE fluxes at less disturbed sites are likely an effect of a more slowly degrading but larger total pool that was built up in the past. Our results highlight the complex effects of cryogenic processes on the C cycle at various time scales. 

Place, publisher, year, edition, pages
IOP Publishing, 2015
Keyword
carbon fluxes, non-sorted circle, cryoturbation, NDVI
National Category
Earth and Related Environmental Sciences
Research subject
Physical Geography
Identifiers
urn:nbn:se:umu:diva-112505 (URN)10.1088/1748-9326/10/11/114006 (DOI)000367249900010 ()
Available from: 2015-12-15 Created: 2015-12-09 Last updated: 2017-12-01Bibliographically approved

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