Gradual drying of permafrost peat decreases carbon dioxide in drier peat plateaus but not in wetter fens and bogs

Abstract. Permafrost thawing of northern peatlands can cause local collapse of peat plateaus into much wetter thermokarst bogs and fens, dominated by Sphagnum mosses and graminoids, respectively. However, permafrost thaw can also improve landscape drainage and thus lead to regional drying of peatlands. How gradual drying of these thawing permafrost peatlands affects the subsequent microbial production of carbon dioxide (CO₂) and nitrous oxide (N₂O) is uncertain because of landscape heterogeneity in moisture, peat quality, and vegetation. Here, we collected near-surface peat samples (5–20 cm) from Alberta, Canada, across transects representing a thaw gradient from peat plateaus to a fen or bog. We incubated the samples for two weeks at either field moisture conditions or under gradual drying, which reduced moisture by ~80 %. Only the fen sites, which had high moisture and % total N, produced N₂O (0.06−6.7 μg N₂O-N g^-1 dry peat) but were unaffected by the drying treatments. Peat CO₂ production was greatest from the fen and the youngest stage of the thermokarst bog despite having the most water-saturated field conditions, likely reflecting their more labile plant inputs and, thus more decomposable peat. We found that CO₂ respiration was enhanced by drying in relatively wet sites like the fens and young bog but was suppressed by drying in relatively drier peat plateaus. Further, gradual drying increased ¹³C-CO₂ respiration, suggesting a possible shift to more decomposed, older C being lost with peat drying. Our study thus suggests that future peat CO₂ and N₂O production from peatlands will depend on whether peat plateaus thaw into fens or bogs and on their diverging responses of peat respiration to more moisture-limited conditions.