Resumo -O objetivo deste trabalho foi avaliar o crescimento inicial da parte aérea e do sistema radicular, a nutrição mineral e a fixação biológica de N 2 (FBN Growth, nutrition and biological fixation of nitrogen in mixed-species plantations of eucalypt with leguminous treesAbstract -The objective of this work was to assess interactions between species on the above and belowground growth, nitrogen uptake and biological nitrogen fixation (BNF) in mixed stands of Eucalyptus grandis and native leguminous N 2 -fixing trees. A complete randomized block design was installed with seven treatments and three blocks. Within the lines of the E. grandis seedlings, native leguminous N 2 -fixing trees -Peltophorum dubium, Inga sp., Mimosa scabrella, Acacia polyphylla, Mimosa caesalpiniaefolia -and one exotic leguminous plant, Acacia mangium, were intercropped. E. grandis was also solely planted. Mimosa scabrella and A. mangium were the legume trees that presented the highest growth. Although E. grandis showed a lower growth when combined with M. scabrella, this mixed-species stand exhibited the highest biomass accumulation. Eucalyptus grandis fine root densities (FRD) were 6-20 times higher than the FRD of the leguminous species in the upper soil layer (0-10 cm) 24 months after planting. The FRD of M. scabrella and M. caesalpiniaefolia in the 30-50 cm soil layer was higher than in the 10-30 cm layer. The δ 15N values of M. scabrella indicated that 90% of stocked N is derived from BNF.
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.
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