Belowground biodiversity is formed by fungi, bacteria, archaea, animals and plants that altogether affect soil functioning, particularly by controlling rates of production and decomposition of organic matter. Peat soils, being the most concentrated stores of soil carbon, are formed by a long-term net exceedance of production over decomposition. The REPEAT project addresses the mechanisms contributing to peat formation in fen peatlands in order to improve restoration prospects of these threatened ecosystems that provide vital ecosystem services for mitigation of climate change, regional hydrology, nutrient retention and biodiversity conservation (Bonn et al. 2016) . In Europe most fen peatlands have been severely degraded by land use. Drainage has turned the peatlands from carbon sinks into significant sources of greenhouse gases (GHG) and has made Central Europe – after Indonesia - the second largest hot-spot of peat GHG emissions worldwide (Joosten 2009). Peat formation is a precondition to re-install the vital ecosystem services provided by the fen ecosystem. However, re-establishment of high groundwater tables alone is often not sufficient to restore peat formation (Grootjans et al. 2012). In spite of decades of trials, processes that control peat accumulation (including their rates, pathways and drivers) remain unknown. Previous research on peatland carbon cycling has focused almost exclusively on rainwater-fed bogs with upward growing peatmoss (Sphagnum) as the prevailing mode of peat formation. In contrast, in groundwater-fed fens roots and rhizomes of sedges and grasses grow into the older peat matrix to form 'displacement peat'. Therefore, peat formation models developed for bogs (Clymo et al. 1998, Frolking et al. 2001) do not fully apply to fens. REPEAT aims to clarify the mechanisms of peat formation in fens by linking biogeochemical processes to soil community structure and biodiversity, as well as to plant belowground litter quality, with special focus on the prospects of restoring peat formation mechanisms. Paludiculture (biomass harvest) will receive special attention because it has recently been recognized as an key management approach that allows sustainable use of wet and rewetted peatlands.