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

Published source details Kemmers R.H, van Delft S.P.J. & Jansen P.C. (2003) Iron and sulphate as possible key factors in the restoration ecology of rich fens in discharge areas. Wetlands Ecology and Management, 11, 367-381.
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Summary

In the Netherlands, brook valleys and carr peat areas are of particular interest in terms of conservation because of their potentially high biodiversity. It has been shown that a major factor controlling the vegetation development of these systems is their hydrology. The biodiversity of these areas has declined dramatically since people began drastically altering the hydrological cycle in the 1950s through drainage and water extraction.

Fens in undisturbed landscapes are supplied with calcium and iron by groundwater which gives soils a high base status. In the Netherlands, discharge areas have been drained for agricultural purposes, and in many recharge areas pumping stations have been installed to supply public drinking water. Both activities have resulted in less discharge to brook valley systems and a fall in the regional drainage base. Early measures to counter these problems in brook valley systems sought to keep fens wet by the retention of rainwater. This management practice resulted in the base-rich groundwater being replaced by base-poor rain water. It also brought about a reversed groundwater flow in these fens, turning them from discharge areas into recharge areas, with associated cation leaching and soil acidification. Many present-day nature restoration projects in the Netherlands therefore aim to restore the original high base status of these soils by hydrological modification and/or by the removal of acidified sods.

Study sites: Field research was carried out at sites in seven reference areas where restoration measures (e.g. altering hydrology and removing acidified sod) have been successful and at sites where they have failed. The study sites predominantly had mineral soil horizons (terrestrial) or organic horizons (semi-terrestrial). In each area several sites with different degrees of acidification were selected. The composition of the vegetation had been described and was used as a criterion for success of restoration measures.

Methods: Soil and interstitial water was sampled from distinct soil horizons and analysed for variables involved in geohydrochemical processes. Data was collected on the chemical composition of pore water, exchangeable cations, exchange characteristics, soil acidity, redox potentials and iron oxides. Prior to data collection, the humus form profile was described, distinguishing horizons consisting of predominantly labile, refractory or intermediate humus. The data were used to parameterise a chemical speciation model in order to calculate equilibrium concentrations involved in redox, dissolution, precipitation and exchange reactions of chemical components to identify key factors and processes involved in acid–base regulation. Several variables were monitored for one year and used to calibrate model outputs.

Key processes: It appeared that soil pH, Ca²+ saturation and iron contents were significantly lower at sites where restoration efforts had failed. At the same time, the humus profiles 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. 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.

Hypothesis: From our results we hypothesise that the soil’s cationic exchange complex 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.

Conclusions: We conclude that proper understanding of the pedological and geohydrochemical processes that control the base status of soils is a prerequisite for successful 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.

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