The Impact of Permafrost Thaw on Soil Carbon Cycling in Boreal Peatlands

• Author / Creator

  Heffernan, William

• Permafrost peatlands in northern regions store a significant proportion of global soil carbon and have historically played an important role in global carbon cycling. Recent warming is accelerating permafrost thaw and causing thermokarst formation. Thermokarst formation in peatlands leads to a drastic increase in wetness and the colonization of Sphagnum mosses which may increase the capacity of peatlands to sequester carbon dioxide, as organic matter in newly formed peat. Thermokarst formation also exposes vast stores of previously frozen soil organic carbon to microbial decomposition, and potential mineralization and release as greenhouse gases to the atmosphere. The net effect of thermokarst formation on peatland carbon storage, greenhouse gas exchange, and the controls on decomposition in peatlands have been poorly understood, at both the short- and long-term perspective.
  In this thesis I aimed to use several complimentary approaches to assess the net effect of thermokarst formation on carbon storage, greenhouse gas exchange, and the controls on decomposition at a peatland site in the sporadic-discontinuous permafrost zone of boreal western Canada (59.5°N, 117.2°W). I used a space-for-time chronosequence approach along two thaw transects to determine the impacts of thawing on carbon storage and the microbial constraints on peat decomposition. I used carbon stocks and annual carbon balances calculated from carbon dioxide (CO2) and methane (CH4) surface fluxes to measure the former, and soil enzyme activity to measure the latter. All three approaches yielded consistent findings, suggesting that mineralization of recently thawed peat was slow and largely balanced by rapid peat accumulation at the surface.
  Peat core analysis showed that surficial peat accumulation offset losses of previously frozen carbon. Peat humification indices did not indicate any significant increase in decomposition of previously frozen peat upon thawing. Over a three-year measurement period, the functional response and controls of greenhouse gas fluxes differed between the recently thawed and mature bog stages. This resulted in asynchronous annual C fluxes. Due to the interannual variability of environmental drivers, this approach was unable to determine whether the long-term net carbon balance of recently thawed and more mature thermokarst bogs differed. However, it did demonstrate that the recently thawed thermokarst bogs in the study region are unlikely to experience rapid net C losses, although they do represent an area of high CH4 emissions which may have significant implications for radiative forcing.
  Soil enzyme activity, peat humification indices, and C stores showed that previously frozen peat was largely unaffected at this site by permafrost thaw. Annual carbon balances and microbial decomposition were governed by biotic and abiotic conditions at the surface. These findings demonstrate that 1) previously frozen carbon may not be vulnerable to rapid decomposition following permafrost thaw and 2) the large carbon stores found at this boreal peatland complex in western Canada, are not at risk to enhanced loss following thaw.
  Permafrost peatlands within the sporadic-discontinuous permafrost zones of western Canada are unlikely to experience rapid mineralization of previously frozen carbon following thaw. Recently thawed thermokarst bogs within this region are unlikely to represent a period of rapid net carbon loss on the landscape.

• Subjects / Keywords

  • Peatland
  • Carbon
  • Biogeochemistry
  • Permafrost
  • Boreal

• Graduation date

  Spring 2020

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-53x3-0b54

• License

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