Peat capping: natural capping of wet landfills by peat formation

Given the bioremediation potential of peat, natural capping of landfills in wetlands with a “peat cap” could provide a sustainable addition to regular capping methods using basal liners with limited life-spans and sand. It is unknown, however, which initial conditions optimise growth of this “peat cap” on top of a sand layer. Here, we tested the combined effects of topsoil addition (clay or organic soil) and vegetation type (Typha latifolia, T. angustifolia, Stratiotes aloides and submerged spp.) on net ecosystem C exchange and water quality in 18 sandy basins situated in a constructed wetland on top of a landfill. Although the highest net C sequestration rates occurred in Typha stands on sand, due to lower decomposition-related C losses as compared to clay and organic topsoils, vegetation development was slow and its cover was very low (15%) compared to clay (40%) and organic topsoils (70%). As this strongly impeded the build-up of a uniform peat layer, we conclude that, within a restricted time frame, the application of nutrient-rich topsoils is still necessary for sufficient biomass production to accumulate organic material. By recycling local soils, the accompanying initial C loss becomes negligible on a landscape scale.