Enhanced CO2 Emissions Driven by Flooding in a simulation of Palsa Degradation

With climate change, discontinuous permafrost is thawing rapidly and part of Permafrost-affected peatlands are at risk of disappearing within decades. Thawing in permafrost-affected peatlands can occur gradually (decades to centuries) or abruptly (weeks to years). While abrupt degradation is less frequent, it may contribute to higher carbon (C) loss.  However, predicting C loss with permafrost thaw from organic-rich soils, such as in Palsas, remains challenging. This highlights the need for laboratory studies focused on biogeochemical and hydrological changes, as well as C emissions during permafrost thaw. In this study, we simulated a gradual and an abrupt Palsa degradation under varying hydrological conditions to observe the impact of these different scenarios on the degradation rate of organic matter (OM). We used a meso-scale incubation setup to continuously measure CO₂ and CH₄ emissions, while deepening the permafrost table with three thaw stages over the 90 days duration of the incubation. This approach enabled the quantification of C contribution from deeper layers. Additionally, we assessed the OM degradation stage by using a FTIR approach. Our results showed a net CH₄ uptake for all the Palsa cores and a twofold increase in CO₂ emission rates following the thawing events for all the treatments simulating abrupt thaw (flooded conditions). We found that the physical disruptions of macro-agglomerates and redox changes due to the flooding enhanced OM lability in the active layer. In contrast, deepening the permafrost table increased emissions from Palsa cores under gradual thaw by a factor of two (dry conditions), while CO₂ emissions remained constant under the abrupt thaw simulation. This outcome supports higher C contribution from permafrost layers under dry conditions. Finally, the increase in CO₂ emissions with thaw from the saturated peatland highlight the potential role of deep-rooted vegetation as a transport pathway for CO₂ in water-saturated soils outside the growing season.