Iron and sulphate as possible key factors in the restoration ecology of rich fens in discharge areas

Seven reference areas in the Netherlands were selected to trial restoration measures in acidified rich fens in discharge areas. In about half of the projects the measures that aimed to restore the high base status of the topsoil failed. The aim of the present study was to identify the key factors and processes in base regulation of rich fen systems, in order to underpin future restoration. We sampled soil and interstitial water from distinct soil horizons and analysed it for variables involved in geohydrochemical processes. We monitored interstitial water chemistry and redox potentials to calibrate and validate a chemical speciation model, that we used for the interpretation of our observations. It appeared that soil pH, Ca2+ saturation and iron contents were significantly lower at sites where restoration efforts had failed. At the same time, soils of these sites were very stratified instead of homogenous. Only soils with high iron contents recovered a high Ca2+ saturation. All sites were characterised by considerable downward water fluxes through the soil. Chemical speciation modelling was a useful tool for the interpretation of processes underlying our observations and helped elucidate the factors and processes that control the recharge of the CEC by base cations. The modelling results suggested that the main process in proton neutralisation of successful sites is the production of internal alkalinity by reduction of iron oxides. Additional redox capacity can be supplied by the ample presence of sulphates. From our results we hypothesise that the CEC will only be recharged successfully with base cations in the presence of sufficient redox capacity of the soil. It seems that redox processes facilitate the ionic exchange of protons for Ca2+ ions. Sites where restoration efforts failed changed from discharge areas to recharge areas, which caused iron depletion by leaching. We conclude that proper understanding of the pedological and geohydrochemical processes that control the base status of soils is a prerequisite for successful nature restoration. The role of soil processes cannot be ignored as it seems that the production of internal alkalinity upon reduction exceeds the external supply of alkalinity by groundwater flow.