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

Abstract. Climate change is predicted to put most of the permafrost habitats in the discontinuous zone, at risk of disappearing within the next few decades. On a decadal scale, abrupt permafrost thaw may result in larger C losses than gradual permafrost thaw, but drivers of C emissions are poorly understood. To investigate this, we measured C emissions from a palsa under simulated abrupt and gradual thaw scenarios. We continuously measured CO₂ and CH₄ emissions while deepening the permafrost table under flooded (abrupt) and non-flooded (gradual) conditions. Higher soil-moisture during permafrost thaw is commonly associated with decreasing CO₂ and increasing CH₄ emissions. Interestingly, our results showed consistent CH₄ uptake across all the cores from the palsa and a twofold increase in CO₂ emissions under abrupt thaw (flooded conditions). Peat quality analysis (FTIR) showed a higher degradation of C compounds at the permafrost table, likely due to the physical disruption of soil organic matter and the redox changes in the active layer caused by flooding. Averaged CO₂ emissions were significantly higher under abrupt thaw (150 mg-CO₂. m^-2.h^-1) compared to gradual thaw (70 mg-CO₂. m^-2.h^-1), with limited permafrost peat contribution. Conversely, permafrost thaw under gradual thaw contributed to a twofold increase in CO₂ emissions (57 to 98 mg-CO₂. m^-3.h^-1). Finally, CO₂ emissions increased with depth in saturated fens, suggesting that deep-rooted vegetation could be a transport pathway for CO₂ outside the growing season. Our findings underline the potential for increased CO₂ emissions during the transition to fen conditions under abrupt thaw scenarios and therefore the need for in-situ measurements.