Enhanced CO2 emissions driven by flooding in a simulation of palsa degradation

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.