MINERAL NUTRIENT STATUS OF COASTAL HILL DIPTEROCARP FOREST AND ADINANDRA-BELUKAR IN SINGAPORE - BIOASSAYS OF NUTRIENT LIMITATION

Bioassays of nutrient limitation were carried out for Melastoma malabathricum growing on soil from primary lowland dipterocarp rainforest in Singapore, and for Dillenia suffruticosa on soil from adinandra belukar, a nutrient-poor secondary forest type dominated by Adinandra dumosa. Three questions were addressed. 1. What is the nutrient most limiting to growth in primary forest? 2. What is the nature of nutrient limitation under conditions of adequate P supply? 3. Is there a qualitative difference in the nature of nutrient limitation under primary forest and adinandra belukar? Results showed that there was a strong limitation by P availability in both primary forest and adinandra belukar under the experimental conditions used. Once plants had an adequate P supply, all other nutrients became limiting to growth in primary forest soil. These findings are interpreted as support for the hypothesis that P availability would limit the productivity of moist tropical forests in general in the absence of mycorrhizas; tentative conclusions are drawn on the assumption that most woody tropical plants are mycorrhizal. It is argued that limitation by major cations may be common on old, highly leached tropical rainforest soils.