Abstract. Peatland restoration by inundation of drained areas can alter local greenhouse gas emissions as CO₂ and CH₄. Factors that can influence these emissions include the quality and amount of substrates available for anaerobic degradation processes and the sources and availability of electron acceptors. In order to learn about possible sources of high CO₂ and CH₄. emissions from a rewetted degraded fen grassland, we performed incubation experiments that tested the effects of fresh plant litter in the flooded peats on pore water chemistry and CO₂ and CH₄. production and emission.

The position in the soil profile of the pre-existing drained peat substrate affected initial rates of anaerobic CO₂ production subsequent to flooding, with the uppermost peat layer producing the greatest specific rates of CO₂ evolution. CH₄ production rates depended on the availability of electron acceptors and was significant only when sulfate concentrations were reduced in the pore waters. Very high specific rates of both CO₂ (maximum of 412 mg C d⁻¹ kg⁻¹ C) and CH₄ production (788 mg C d⁻¹ kg⁻¹ C) were observed in a new sediment layer that accumulated over the 2.5 years since the site was flooded. This new sediment layer was characterized by overall low C content, but represented a mixture of sand and relatively easily decomposable plant litter from reed canary grass killed by flooding. Samples that excluded this new sediment layer but included intact roots remaining from flooded grasses had specific rates of CO₂ (max. 28 mg C d⁻¹ kg⁻¹ C) and CH₄ (max. 34 mg C d⁻¹ kg⁻¹ C) production that were 10–20 times lower than for the new sediment layer and were comparable to those of a newly flooded upper peat layer. Lowest rates of anaerobic CO₂ and CH₄ production (range of 4–8 mg C d⁻¹ kg⁻¹ C and <1 mg C d⁻¹ kg⁻¹ C) were observed when all fresh organic matter sources (plant litter and roots) were excluded. In conclusion, the presence of fresh organic substrates such as plant and root litter originating from plants killed by inundation has a high potential for CH₄ production, whereas peat without any fresh plant-derived material is relatively inert. Significant anaerobic CO₂ and CH₄ production in peat only occurs when some labile organic matter is available, e.g. from remaining roots or root exudates.