Redox dynamics in fen peats during drying and flooding

Biogeosciences Discussions This discussion paper is/has been under review for the journal Biogeosciences (BG). Please refer to the corresponding final paper in BG if available. Abstract Climate change induced drying and flooding may alter the redox conditions of organic matter decomposition in peat soils. The seasonal and intermittent changes in pore water solutes (NO − 3 , Fe 2+ , SO 2− 4 , H 2 S, acetate) and dissolved soil gases (CO 2 , O 2 , CH 4 , H 2) under natural water table fluctuations were compared to the response under a rein-5 forced drying and flooding in fen peats. Oxygen penetration during dryings led to CO 2 and CH 4 degassing and to a regeneration of dissolved electron acceptors (NO − 3 , Fe 3+ and SO 2− 4). Drying intensity controlled the extent of the electron acceptor regeneration. Iron was rapidly reduced and sulfate pools ∼ 1 mmol L −1 depleted upon rewetting and CH 4 did not substantially accumulate until sulfate levels declined to ∼ 100 µmol l −1. The 10 post-rewetting recovery of soil methane concentrations to levels ∼ 80 µmol l −1 needed 40–50 days after natural drought. This recovery was prolonged after experimentally reinforced drought. A greater regeneration of electron acceptors during drying was not related to prolonged methanogenesis suppression after rewetting. Peat compaction, solid phase content of reactive iron and total reduced inorganic sulfur and organic mat-15 ter content controlled oxygen penetration, the regeneration of electron acceptors and the recovery of CH 4 production, respectively. Methane production was maintained despite moderate water table decline of 20 cm in denser peats. Flooding led to accumulation of acetate and H 2 , promoted CH 4 production and strengthened the co-occurrence of iron and sulfate reduction and methanogenesis. Mass balances during drying and 20 flooding indicated that an important fraction of the electron flow must have been used for the generation and consumption of electron acceptors in the solid phase or other mechanisms. In contrast to flooding, dry-wet cycles negatively affect methane production on a seasonal scale but this impact might strongly depend on drying intensity and on the peat matrix, whose structure and physical properties influence moisture con-25 tent.