Dynamics of seasonal bicarbonate supply in a dune slack: effects on organic matter, nitrogen pool and vegetation succession

Abstract

The seasonal variation in groundwater composition was studied in an old dune slack complex on the Wadden Sea island of Schiermonnikoog that recently had lost practically all rare basiphilous plant species. In order to assess its restoration perspectives the groundwater acid neutralization capacity (ANC(aq)) of various sites was discussed in relation to the decalcification depth, amount of organic matter and amount of nitrogen in the topsoil layer. The decalcification depth reflected the (former) hydrological regime quite well; shallow in the exfiltration zone and deep in the infiltration part of the slack. The pattern of HCO3-, SO42- and pH reflected the functioning of a flow-through lake, even when the slack was not flooded. Relies of low productivity (mesotrophic) vegetation (Samolo-Littorelletum) with rare species were related to low amounts of organic matter and nitrogen in the topsoil layer. At this site the variation in HCO3- concentration was also relatively low during the year. It was hypothesized that a regular supply of acid buffering components from calcareous soil layers buffers the pH at a high level and slows down the accumulation of organic matter thus creating opportunities for the rare basiphilous plant species. A possible biological control mechanism for these processes was discussed. A reconstruction of the former, present and future eco-hydrological situation was made of the dune slack.