As the Arctic warms, the ~277 Pg of carbon stored in permafrost peatlands faces an uncertain fate. Arctic and Subarctic
peatlands are likely to release more methane (CH4) as permafrost thaw releases formerly-frozen carbon, thaw-induced land subsidence and
inundation lead to anaerobic conditions, and higher temperatures allow more rapid decomposition. In addition to these effects, CH4 and
carbon dioxide (CO2) emissions may also change due to shifts in plant inputs and consequent changes in organic matter quality, but the
exact relationships between organic matter and CH4 production are not well understood. In this study, we examined microbial CH4 and CO2
production and their relationship to organic matter chemistry in Stordalen Mire, a thawing Subarctic peatland in northern Sweden. We also
used stable carbon isotopes (δ13C) of CH4 and CO2, and their apparent fractionation factor (αC), to examine the effect of thaw on the
proportion of methanogenesis by hydrogenotrophic or acetoclastic pathways. At Stordalen, permafrost thaw causes dry, aerobic permafrost
plateaus (palsas) to collapse and become inundated. These wet depressions are then colonized first by Sphagnum mosses and then by sedges
as permafrost thaw and plant succession progress. In our study, we examined a chronosequence of sites with varying permafrost status and
plant community composition. These sites included dry, intact palsas; recently-thawed collapsed palsa sinkholes; partially-thawed
Sphagnum-dominated bogs; mostly-thawed poor fens with a combination of Sphagnum and tall sedges; and fully-thawed rich fens with mature
stands of tall sedges and no Sphagnum. The changes in potential CH4 and CO2 production along the thaw progression were examined with
anaerobic peat incubations, which were all performed with identical temperature and water saturation. These incubations showed increases
in potential decomposition rates and CH4/CO2 production ratios along the thaw progression. Methanogenesis pathways also shifted from
predominately hydrogenotrophic to acetoclastic, as revealed by lower αC in fens. These shifts are consistent with increasing organic
matter bioavailability along the thaw progression, which was confirmed by analyses of peat and dissolved organic matter (DOM) chemistry.
These analyses showed that compared to collapsed palsas and bogs, rich fens had lower peat C/N ratios, higher peat humification rates (as
determined by Fourier-transform infrared [FTIR] spectroscopy), and more labile DOM compounds (as determined by Fourier-transform ion
cyclotron resonance mass spectrometry [FT-ICR MS]). The validity of these incubations for revealing trends in in situ CH4 and CO2
production was determined by comparison with dissolved CH4 and CO2 in field-collected pore water. The incubation CH4/CO2 ratios were
compared to both the raw pore water CH4/CO2 concentration ratios, and to the pore water CH4/CO2 production ratios estimated with an
isotope mass balance model. In both cases, CH4/CO2 ratios were higher in the incubations than in the pore water; however, the same
increases in CH4/CO2 with thaw were observed in both cases. Incubation and field pore water αC were also compared. Incubation αC values
were slightly higher than field αC, but αC decreased with thaw in both the incubations and the field. We thus conclude that incubations
can reliably estimate relative CH4/CO2 ratios and αC between different sites, though their ability to estimate absolute CH4/CO2 and αC is
limited. The changes in DOM chemistry along the thaw progression were examined more closely with a combination of elemental composition
(via FT-ICR MS) and optical properties (via UV/Vis absorption and fluorescence spectroscopies). These techniques revealed that the
presence of dense Sphagnum moss, which is abundant in collapsed palsa, bog, and poor fen sites, is the main driver of DOM elemental
composition and optical properties at Stordalen. Compared to rich fens, DOM from sites with Sphagnum had greater aromaticity, higher
average molecular weights, and greater O/C ratios. These properties suggest a higher abundance of phenolic compounds, which are released
by Sphagnum and may inhibit decomposition at these sites. In contrast, rich fen DOM had greater saturation, lower O/C ratios, greater N/C
and S/C ratios, and optical properties suggesting a higher proportion of microbially-derived DOM. Overall, our results suggest that the
changes in plant community due to permafrost thaw at Stordalen lead to greater organic matter lability and higher CH4 production. In
inundated sites, these changes are primarily driven by the disappearance of Sphagnum as partially-thawed sites transition to fully-thawed
rich fens. Similar plant successions have been observed in other peatlands with thawing permafrost, highlighting the potential importance
of these shifts in driving future northern peatland greenhouse gas balances. Future models of climate feedbacks in permafrost peatlands
should thus take into account any changes in plant community composition, especially changes in Sphagnum cover, as permafrost
thaws. / A Dissertation submitted to the Department of Earth, Ocean, and Atmospheric Science in partial
fulfillment of the requirements for the degree of Doctor of Philosophy. / Spring Semester 2016. / February 29, 2016. / decomposition, dissolved organic matter, methane, peat, permafrost / Includes bibliographical references. / Jeffrey P. Chanton, Professor Directing Dissertation; Alan G. Marshall, University
Representative; William M. Landing, Committee Member; Yang Wang, Committee Member; William C. Burnett, Committee Member; Patrick M. Crill,
Committee Member.
| Identifer | oai:union.ndltd.org:fsu.edu/oai:fsu.digital.flvc.org:fsu_360372 |
| Contributors | Hodgkins, Suzanne Berenice (authoraut), Chanton, Jeffrey P. (professor directing dissertation), Marshall, Alan G. (Alan George) (university representative), Landing, William M. (committee member), Wang, Yang (committee member), Burnett, William C. (committee member), Crill, Patrick (committee member), Florida State University (degree granting institution), College of Arts and Sciences (degree granting college), Department of Earth, Ocean, and Atmospheric Science (degree granting department) |
| Publisher | Florida State University, Florida State University |
| Source Sets | Florida State University |
| Language | English, English |
| Detected Language | English |
| Type | Text, text |
| Format | 1 online resource (236 pages), computer, application/pdf |
| Rights | This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s). The copyright in theses and dissertations completed at Florida State University is held by the students who author them. |
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