ATMOSPHERIC DEPOSITION OF ACIDIFYING AND EUTROPHICATING SUBSTANCES IN DUNE SLACKS

摘要

The atmospheric input of nitrogen and potantial acid, and the composition of the leachate from the topsoil layer are quantified for eight dune slack sites differing in plant composition, on the Frisian islands of Terschelling and Schiermonnikoog. Basiphilous mesotrophic communities with many rare plant species grow in one site on Terschelling and two sites on Schiermonnikoog. Samples were taken in the period mid May l992 -- mid May l993. Bulk atmospheric nitrogen deposition mea- sured in an open collector was between 15--20 kg ha~- l yr~- l for the eight sites. On Terschelling, the deposition was slightly higher than on Schiermonnikoog. Ammonium was a third of the total amount of nitrogen deposited on Terschelling and half the amount of nitrogen deposited on Schiermonnikoog. The deposition of potential acid varied from 490 to 8l0 mo1 ha~- 1 yr~- l. On Terschelling, the amount of potential acid was highest and around 800 mol ha~-l yr~-l for both sites. On Schiermonnikoog, the amount of potential acid was lowest for the dune slacks in the north--western part of the island and it showed an increase for slacks towards the eastern part of the island. Calcium is the main cation leached from the topsoil followed by sodium, magnesium and potassium. Nitrogen or ammonium hardly leached from the topsoil layer. The shift from basiphilous pioneer communities toward older and more acid stages of dune slack succession appears to be caused primarily by leaching of base cations rather than eutrophication.