MASS-LOSS AND NITROGEN DYNAMICS IN DECOMPOSING ACID FOREST LITTER IN THE NETHERLANDS AT INCREASED NITROGEN DEPOSITION

Litterbag experiments were carried out in five forest ecosystems in th e Netherlands to study weight loss and nitrogen dynamics during the fi rst two years of decomposition of leaf and needle litter. All forests were characterized by a relatively high atmospheric nitrogen input by throughfall, ranging from 22-55 kg N ha-1 yr-1. Correlation analysis o f all seven leaf and needle litters revealed no significant relation b etween the measured litter quality indices (nitrogen and lignin concen tration, lignin-to-nitrogen ratio) and the decomposition rate A signif icant linear relation was found between initial lignin-to-nitrogen rat io and critical nitrogen concentration, suggesting an effect of litter quality on nitrogen dynamics. Comparison of the decomposition of oak leaves in a nitrogen-limited and a nitrogen-saturated forest suggested an increased nitrogen availability. The differences in capacities to retain atmospheric nitrogen inputs between these two sites could be ex plained by differences in net nitrogen immobilization in first year de composing oak leaves: in the nitrogen-limited oak forest a major part (55%) of the nitrogen input by throughfall was immobilized in the firs t year oak leaf litter. The three coniferous forests consisted of two monocultures of Douglas fir and a mixed stand of Douglas fir and Scots pine. Despite comparable litter quality in the Douglas fir needles in all sites, completely different nitrogen dynamics were found.