A case study on topsoil removal and rewetting for paludiculture: effect on biogeochemistry and greenhouse gas emissions from Typha latifolia, Typha angustifolia and Azolla filiculoides

Abstract. Rewetting drained peatlands for paludiculture purposes is a way to reduce peat oxidation (and thus CO2 emissions) while at the same time it could generate an income for landowners, who need to convert their traditional farming into wetland farming. The side effect of rewetting drained peatlands is that it potentially induces high methane (CH4) emission. Topsoil removal could reduce this emission due to the removal of easily degradable carbon and nutrients. Another way to limit CH4 emission is the choice in paludiculture species. In this study we conducted a field experiment in the coastal area of the Netherlands, in which a former non-intensively used drained peat grassland is rewetted to complete inundation (water table ~+18 cm) after a topsoil removal of ~20 cm. Two emergent macrophytes with a high potential of internal gas transport (Typha latifolia and Typha angustifolia), and a free floating macrophyte (Azolla filiculoides) were introduced and intensive measurement campaigns were conducted to capture CO2 and CH4 fluxes, soil and surface water chemistry. Greenhouse gas fluxes were compared to a high-productive peat meadow as reference site. Topsoil removal reduced the amount of phosphorus and iron in the soil to a large extent. The total amount of soil carbon per volume stayed more or less the same. The salinity of the soil was in general high defining the system as brackish. Despite the topsoil removal and salinity, we found very high CH4 emission for T. latifolia (84.7 g CH4 m-2 yr-1), compared to the much lower emissions from T. angustifolia (36.9 g CH4 m-2 yr-1) and Azolla (22.2 g CH4 m-2 yr-1). The high emission can be partly explained by the large input of dissolved organic carbon into the system, but it could also be caused by plant stress factors, like salinity level and herbivory. For the total CO2 flux (including C-export), the rewetting was effective, with a minor uptake of CO2 for Azolla (-0.13 kg CO2 m-2 yr-1) and a larger uptake for the Typa species (-1.14 and -1.26 kg CO2 m-2 yr-1 for T. angustifolia and T. latifolia, respectively) compared to the emission of 2.06 kg CO2 m-2 yr-1 for the reference site. Azolla and T. angustifolia seem to have the highest potential in reducing greenhouse gas emissions after complete rewetting of drained peatlands. When considering the total greenhouse gas balance, other factors like biomass use, and storage of topsoil after removal should be considered. Especially the latter could cause substantial carbon losses if not kept in anoxic conditions. For Azolla, a follow-up study without topsoil removal would be useful, to see if the biomass production would be high while keeping CH4 emissions low.