Peatlands cover 3-5% of the world's land area but store ∼30% of terrestrial soil carbon. Peat accumulates where plant net primary production (NPP) exceeds carbon losses mainly through microbial decay. Permafrost is discontinuous across the boreal forest, but near its southern limit is limited almost exclusively to peatlands. Southern permafrost in peatlands has been melting in response to climatic warming over the past ∼150 years following the Little Ice Age. Given that peatlands (1) currently serve as a major reservoir of terrestrial carbon and (2) are located primarily at northern latitudes expected to receive dramatic climate warming, my objectives are to determine the influence of permafrost melt on peatland carbon storage.

210 Pb-dated chronologies from peatlands across Alberta, Saskatchewan, and Manitoba show that rates of peat accumulation were 40% higher in internal lawns (areas of permafrost thaw) than in bogs (no permafrost) and frost mounds (intact permafrost). Permafrost melt stimulates carbon accumulation as peat, although regional variation in the nature of this carbon response is likely. Laboratory incubations and field-based carbon fluxes suggest that heterotrophic CO 2 respiration increases following thaw. Methane emissions across the peatland surface to the atmosphere also were elevated in internal lawns compared to bogs and frost mounds, but overall fluxes were low compared to previous studies. Increased peat accumulation and heterotrophic respiration in internal lawns points to high rates of NPP post-thaw. Mass losses of Sphagnum riparium , a common bryophyte in internal lawns, were greater than other moss species during a 1-year, field incubation. Concentrations of water-soluble carbohydrates were a strong predictor of bryophyte decay, and were higher in S. riparium tissue than in other mosses. Sphagnum riparium is allocating more of its carbon resources to the production of metabolic carbohydrates than other mosses, most likely to sustain high NPP.

Across Alberta, Saskatchewan, and Manitoba, I estimate that permafrost melt in peatlands creates an enhanced regional net sink of 160 ± 50 Gg C yr-1 . Future permafrost thaw may serve as a small buffer against carbon losses in peatlands due to increased fire severity and land-use (peat extraction, oil sand mining, hydroelectric reservoir creation).