Permafrost thaw causes large carbon loss in boreal peatlands while changes to peat quality are limited

Rapid, ongoing permafrost thaw of peatlands in the discontinuous
permafrost zone is exposing a globally significant store of soil carbon
(C) to microbial processes. Mineralisation and release of this peat C to
the atmosphere as greenhouse gases is a potentially important feedback to
climate change. Here we investigated the effects of permafrost thaw on
peat C at a peatland complex in western Canada. We collected 15 complete
peat cores (between 2.7 abd 4.5 m deep) along four chronosequences, from
elevated permafrost plateaus to saturated thermokarst bogs that thawed up
to 600 years ago. The peat cores were analysed for peat C storage and peat
quality, as indicated by decomposition proxies (FTIR and C/N ratios) and
potential decomposability using a 200-day aerobic incubation. Our results
suggest net C loss following thaw, with average total peat C stocks
decreasing by ~19.3 +/- 7.2 kg C m-2 over <600 years (~13% loss).
Average post-thaw accumulation of new peat at the surface over the same
period was ~13.1 +/- 2.5 kg C m-2. We estimate ~19% (+/- 5.8%) of deep
peat (>40 cm below surface) C is lost following thaw (average 26
+/- 7.9 kg C m-2 over <600 years). Our FTIR analysis shows peat
below the thaw transition in thermokarst bogs is slightly more decomposed
than peat of a similar type and age in permafrost plateaus, but we found
no significant changes to the quality or lability of deeper peat across
the chronosequences. Our incubation results also showed no increase in C
mineralisation of deep peat across the chronosequences. While these
limited changes in peat quality in deeper peat following permafrost thaw
highlight uncertainty in the exact mechanisms and processes for C loss,
our analysis of peat C stocks shows large C losses following permafrost
thaw in peatlands in western Canada.