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Storage, landscape partitioning and lability of soil organic matter in permafrost terrain
Stockholm University, Faculty of Science, Department of Physical Geography.
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Recent estimates indicate that soils in the northern circumpolar permafrost region store substantial amounts of soil organic carbon (SOC). This reservoir has accumulated over 10-100.000 years and is often preserved in a relatively undecomposed state because frozen and often water-logged conditions prevented microbial degradation. Under a projected future climate change caused by rising greenhouse gases, permafrost thaw and rapid decomposition of vulnerable soil organic matter (SOM) could provide a positive feedback on global warming by releasing large amounts of carbon dioxide and/or methane into the atmosphere.

SOC pools have large regional and landscape-level variability depending on topographic, ecoclimatic and edaphic factors. As a consequence, large scale maps and even regional data sets describing SOC storage should be taken with caution since they are highly simplified. The purpose of this thesis is to improve our knowledge on quantity and quality of SOM in different areas of continuous permafrost and provide regional high quality data from hitherto under-sampled regions for future assessment of the potential remobilization of SOC under global warming. A special focus is put on SOC partitioning within the landscape and soil horizon levels as well as on soil forming processes under periglacial conditions. Throughout the five different study areas presented in this thesis the landscape mean SOC storage ranges between 8 and 30 kg C m-2, while site differences are in the order of 0 to 80 kg C m-2. Paper I presents new SOC data from contrasting areas in continuous permafrost: a mountainous High Arctic site in Zackenberg (NE Greenland) and lowland sites in Shalaurovo and Cherskiy lower Kolyma (NE Siberia). The main difference is that about 60% of the Zackenberg area is higher elevation terrain with mostly barren ground and very low SOC content, resulting in a much lower landscape-level mean SOC storage compared to the Siberian sites. In addition, Paper II shows that even when comparing two lowland sites located only 150 km apart in Taymyr Peninsula (N Siberia) the mean SOC storage differs with 40% between the areas. This emphasizes that even in lowlands on a regional scale not only different landforms and land cover but also microrelief, soil moisture and especially parent material play a very important role for obtaining more accurate SOC storage estimates.

Throughout this thesis a special emphasis is put on understanding the role of cryoturbation for SOC storage. Signs of cryoturbation were observed at all sites and 14C dates show that this process is occurring since at least the early Holocene. On average, 30% of all SOC in the top meter of soil is located in buried C-enriched pockets. The only exception is Zackenberg, with only 12%, where slope processes were the dominant mechanism for burying C-enriched material into deeper layers.

We use the weight ratio of Carbon/Nitrogen (C/N) to gain information about SOM decomposability. Generally, all sites show the same trend that the C/N ratio decreases with soil depth. Top organic soil and peat samples have always the highest C/N ratios, suggesting little decomposed SOM. Except for the Zackenberg site, the buried C-enriched pockets have significantly higher C/N ratios than the adjacent mineral subsoil samples. We assume that this C-enriched material was exposed over longer time periods to aerobic decomposition and was therefore relatively well decomposed before it was buried by reactivated slope processes.

Place, publisher, year, edition, pages
Stockholm: Stockholm University, 2015.
National Category
Earth and Related Environmental Sciences
Research subject
Physical Geography
Identifiers
URN: urn:nbn:se:su:diva-115707OAI: oai:DiVA.org:su-115707DiVA: diva2:799658
Presentation
2015-04-24, Ahlmannsalen, 13:00 (English)
Opponent
Supervisors
Available from: 2015-05-27 Created: 2015-03-27 Last updated: 2016-12-22Bibliographically approved
List of papers
1. Storage, Landscape Distribution, and Burial History of Soil Organic Matter in Contrasting Areas of Continuous Permafrost
Open this publication in new window or tab >>Storage, Landscape Distribution, and Burial History of Soil Organic Matter in Contrasting Areas of Continuous Permafrost
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2015 (English)In: Arctic, Antarctic and Alpine research, ISSN 1523-0430, E-ISSN 1938-4246, Vol. 47, no 1, 71-88 p.Article in journal (Refereed) Published
Abstract [en]

This study describes and compares soil organic matter (SOM) quantity and characteristics in two areas of continuous permafrost, a mountainous region in NE Greenland (Zackenberg study site) and a lowland region in NE Siberia (Cherskiy and Shalaurovo study sites). Our assessments are based on stratified-random landscape-level inventories of soil profiles down to 1 m depth, with physico-chemical, elemental, and radiocarbon-dating analyses. The estimated mean soil organic carbon (SOC) storage in the upper meter of soils in the NE Greenland site is 8.3 ± 1.8 kg C m-2 compared to 20.3 ± 2.2 kg C m-2 and 30.0 ± 2.0 kg C m-2 in the NE Siberian sites (95% confidence intervals). The lower SOC storage in the High Arctic site in NE Greenland can be largely explained by the fact that 59% of the study area is located at higher elevation with mostly barren ground and thus very low SOC contents. In addition, SOC-rich fens and bogs occupy a much smaller proportion of the landscape in NE Greenland (∼3%) than in NE Siberia (∼20%). The contribution of deeper buried C-enriched material in the mineral soil horizons to the total SOC storage is lower in the NE Greenland site (∼13%) compared to the NE Siberian sites (∼24%–30%). Buried SOM seems generally more decomposed in NE Greenland than in NE Siberia, which we relate to different burial mechanisms prevailing in these regions.

Keyword
soil organic carbon, degree of decomposition in soil organic matter, radiocarbon dating, C/N ratios, continuous permafrost, slope processes, cryoturbation
National Category
Physical Geography
Research subject
Physical Geography
Identifiers
urn:nbn:se:su:diva-115639 (URN)10.1657/AAAR0014-027 (DOI)000350219000007 ()
Available from: 2015-03-27 Created: 2015-03-27 Last updated: 2017-12-04Bibliographically approved
2. Controls on the storage of organic carbon in permafrost soil in northern Siberia
Open this publication in new window or tab >>Controls on the storage of organic carbon in permafrost soil in northern Siberia
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2016 (English)In: European Journal of Soil Science, ISSN 1351-0754, E-ISSN 1365-2389, Vol. 67, no 4, 478-491 p.Article in journal (Refereed) Published
Abstract [en]

This research examined soil organic carbon (SOC), total nitrogen (TN) and aboveground phytomass carbon(PhC) stocks in two areas of the Taymyr Peninsula, northern Siberia.We combined field sampling, chemical and14C radiocarbon dating analyses with land cover classifications for landscape-level assessments. The estimatedmean for the 0–100-cm depth SOC stocks was 14.8 and 20.8 kgCm−2 in Ary-Mas and Logata, respectively. Thecorresponding values for TN were 1.0 and 1.3 kgNm−2. On average, about 2% only (range 0–12%) of the totalecosystem C is stored in PhC. In both study areas about 34% of the SOC at 0–100 cm is stored in cryoturbatedpockets, which have formed since at least the early Holocene. The larger carbon/nitrogen (C/N) ratio of thiscryoturbated material indicates that it consists of relatively undecomposed soil organic matter (SOM). Thereare substantial differences in SOC stocks and SOM properties within and between the two study areas, whichemphasizes the need to consider both geomorphology and soil texture in the assessment of landscape-level andregional SOC stocks.

Highlights

• This research addresses landscape-scale and regional variation in SOC stocks.

• Landform and soil texture are taken into account in the analysis.

• The contribution of phytomass to total ecosystem C stored is limited.

• Large SOC stocks are susceptible to decomposition following permafrost thaw.

Keyword
soil organic carbon, total nitrogen, phytomass carbon, permafrost, cryoturbation, C/N ratios, radiocarbon dating, landscape inventories
National Category
Earth and Related Environmental Sciences
Research subject
Physical Geography
Identifiers
urn:nbn:se:su:diva-115705 (URN)10.1111/ejss.12357 (DOI)000384745900012 ()
Projects
The ESF CryoCARB projectthe Nordforsk NCoE DEFROST projectthe EU FP7 PAGE21 project
Funder
European Science Foundation (ESF)EU, FP7, Seventh Framework Programme
Available from: 2015-03-27 Created: 2015-03-27 Last updated: 2017-12-04Bibliographically approved

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