SOIL ORGANIC-MATTER ACCUMULATION AND ITS IMPLICATIONS FOR NITROGEN MINERALIZATION AND PLANT-SPECIES COMPOSITION DURING SUCCESSION IN COASTAL DUNE SLACKS

Vegetation and soil development during succession in coastal dune slac ks on Terschelling island, the Netherlands, was investigated, by compa ring neighbouring ecosystems on similar substrates that had been devel oping for 1, 5, 35 and 76 years since the vegetation and organic soil layer had been removed. In this successional sequence, soil organic ma tter accumulated rapidly due to the production of litter and dead root s. N mineralization was extremely low, increasing from 0.2 g m(-2) yr( -1) after 5 years to 0.8 g m(-2) yr(-1) after 76 years. It was accompa nied by a decline in the pH (KCl) in the upper 10 cm of the soil from 6.8 to 4.4. Most of the above-ground biomass accumulated in the shrub species Oxycoccus macrocarpos and Salix repens. The 5- year-old plots harboured many plant species (18 species per 0.25 m(2)), but plant spe cies diversity was much lower in the older plots. It is concluded that most changes in species composition and the decline in diversity occu rred because early successional plant species were gradually outshaded by the thick litter layer and the accumulated shrub biomass.