Introducing N-fixing species in the understorey of fast-growing plantations might be an attractive option to improve soil N status. Intensive fine root sampling was performed in a complete randomized block design to investigate the ability of Eucalyptus grandis and Acacia mangium seedlings in monospecific stands and mixed-species plantations to take up complementary resources from niche exploration of soil layers. The same soil layers were explored by the two species down to a depth of 2 m in monospecific stands. Whilst the development of E. grandis fine roots was not affected by A. mangium trees in mixed-species plantations, A. mangium fine roots were excluded from the upper soil layer from 18 months after planting onwards, despite the paramount importance of that horizon for tree nutrition in highly weathered soils, and were only found deeper and close to A. mangium trees 30 months after planting.