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Abstract

Globally, large-scale land drainage has severely deteriorated the functioning and services of peatlands, making restoration plans of the utmost importance. Rewetting is essential for the restoration of drained peatlands, but the level of success including greenhouse gas (GHG) mitigation largely depends on the soil microbiome interactions under the prevailing biogeochemical conditions. Here, we investigated the effects of inundation of drained iron (Fe) -rich peat topsoils on nutrient release, surface water quality, GHG production and consumption, and on the composition and activity of the microbial community. The effect of the addition of different potential electron acceptors on methane (CH4) production and consumption were studied in incubation experiments. In response to inundation, porewater concentrations of Fe, total inorganic carbon, ammonium, and phosphorus increased. CH4 emissions increased in the control (i.e. without any additions) and Fe(III) oxide amended incubations upon inundation. This could be explained by the increase in the relative abundance of methanogens even though Fe(III) was previously hypothesized to lower methanogenic activity. In contrast, nitrite, nitrate, and sulfate-rich incubations inhibited methanogenesis. The prolonged exposure to nitrogen oxides stimulated denitrification with nitrous oxide (N2O) as the main gaseous product, together with an increase in the relative abundance of denitrifying microorganisms. Our results demonstrate that insights into the changes in microbial communities in relation to soil geochemistry explain differences in responses observed in different peat soils observed upon inundation. The increase in emissions of the potent GHGs CH4 and N2O from Fe-rich peat topsoils are a major adverse effect in the early stage of inundation.