William (Liam) Heffernan | ALES Graduate Seminar

A graduate exam seminar is a presentation of the student’s final research project for their degree.
This is an ALES PhD Final Exam Seminar by William (Liam) Heffernan.

This seminar will not be open to the general public to attend.
Thesis Topic: The Impact of Permafrost Thaw on Soil Carbon Cycling in Boreal Peatlands

PhD with Dr. David Olefeldt

Seminar Abstract:

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. Soil enzyme activity, peat humification indices, and carbon 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.