ROLES OF ROOT SYMBIOSES IN AFRICAN WOODLAND AND FOREST - EVIDENCE FROM N-15 ABUNDANCE AND FOLIAR ANALYSIS

1 To gain insight into the nutrition of trees with ectomycorrhizas (ECM), VA-mycorrhizas (VAM) or VAM plus N,-fixing legume root nodule symbioses (NOD) in tropical woodland and forest ecosystems, we measured foliar nutrient concentrations and N-15 abundances in miombo woodland in Zambia (22 spp.) and in lowland rain forest in Cameroon (20 spp.). 2 In miombo woodland, confirmed NOD species had low N-15 abundances (mean N-15 = 0.2 parts per thousand), high % N and high N:P ratios. Baphia bequaertii, a species thought likely to be NOD, shared these characteristics, but so did Cassia abbreviata which is less likely to be NOD. 3 Among VAM and ECM species in miombo there were, in general, positive correlations between N-15 abundance and % N, and between % N and % P. Dominant ECM species (Caes alpinioideae-Dip tero carpaceae) had slightly higher % N, but not higher N-15 abundance than VAM species. The N-15 data do not agree with data on ECM and VAM species previously obtained from Tanzanian miombo. 4 In the rain forest, there were no large differences between the three symbiotic groups. For NOD species delta(15)N was almost 4 parts per thousand above that of atmospheric N-2 and only slightly lower than that of non-NOD species. NOD species also had relatively high foliar % N. 5 Data on species composition, foliar N-15 abundance and N:P ratios support the idea that N-2-fixation carried out by N-2-fixing trees is more important in the woodland than in the rain forest.