Sustainable production of eucalypt plantations forests is closely dependent on conservative management of soil organic matter stocks and N availability, mainly in short rotation stands set up on low fertility sites. The aims of this study conducted in Brazil and Congo were: i) to examine the effect of mixtures of Eucalyptus sp. and Acacia mangium on stand growth, biomass allocation, net primary productivity and biological N2 fixation, and ii) to assess the behaviour of the two species in contrasted pedo-climatic conditions.