& Key message Inter-specific interactions with eucalypts in mixed plantations increased N 2 fixation rate of acacia trees compared to monocultures. N 2 fixation was higher during the wet summer than during the dry winter both in acacia monocultures and in mixed plantations. & Context Introducing N-fixing trees in fast-growing tropical plantations may contribute to reducing the long-term requirements of N fertilizers. Management practices established in forest monocultures should be revisited in mixed-species plantations. & Aims This field experiment aimed to compare N 2 fixation rates of Acacia mangium Wild in monospecific stands and in mixedspecies stands with Eucalyptus grandis W. Hill ex Maiden. A secondary objective was to gain insight into the seasonal variations of N 2 fixation. & Methods 15 N was applied to acacia and eucalypt monocultures and mixed-species with a 1:1 ratio at mid rotation. Leaves were collected in autumn, winter, spring, and summer to determine the foliar N concentrations and 15 N atom fraction.

Functional specialization of fine roots was found for Eucalyptus grandis trees at harvesting age (6 years) on tropical soils. Aiming to elucidate whether functional specialization is a ubiquitous feature of eucalypts, we focused on its changes with ontogeny, tree nutrient status and soil depth. We studied the potential uptake of N, K and Ca by 2-year-old E. grandis trees, as a function of soil depth and NPK fertilization. We injected NO −3-15 N, Rb + (K + analogue) and Sr 2+ (Ca 2+ analogue) tracers simultaneously in a solution at depths of 10, 50, 150 and 300 cm in a sandy Ferralsol soil. A complete randomized block design was set up with three replicates of paired trees per injection depth, in fertilized and non-fertilized plots. Recently expanded leaves were sampled at 70 days after tracer injection. Determination of foliar Rb, Sr concentrations and x(15 N) allowed estimating the relative uptake potential (RUP) and the specific RUP (SRUP), defined as the ratio between RUP and fine root length density (RLD) in the corresponding soil layer. Various root traits were measured at each depth. Foliar N and K concentrations were higher in fertilized than in non-fertilized trees. The RUP of NO 3-15 N decreased sharply with soil depth and the highest values of the SRUP of NO 3-15 N were found at a depth of 50 cm. The RUP of Rb + and Sr 2+ did not change with soil depth, whilst the SRUP of Rb + and Sr 2+ were higher at the depth of 300 cm than in the topsoil, concomitant with an increase in root diameter and a decrease in root tissue density with depth. The SRUP of Rb + and Sr 2+ at a depth of 300 cm were on average 136 and 61% higher for fertilized trees than for non-fertilized trees, respectively. Fine roots of young E. grandis trees showed contrasting potential uptake rates with soil depth depending on the nutrient. Fertilization increased the uptake rate of Rb + and Sr 2+ by unit of root length in deep soil layers. Functional specialization of fine roots for cations of low mobility depending on depth previously shown at harvesting age also occurs in young E. grandis plantations and increases with fertilization application. This mechanism helps explaining very low amounts of cations lost by leaching in Eucalyptus plantations established in deep tropical soils, even in highly fertilized stands. High productivity of commercial eucalypt plantations in Brazil

Background and aims Fine-root functioning is a major driver of plant growth and strongly influences the global carbon cycle. While fine-root over-yielding has been shown in the upper soil layers of mixedspecies forests relative to monospecific stands, the consequences of tree diversity on fine-root growth in very deep soil layers is still unknown. Our study aimed to assess the consequences of mixing Acacia mangium and Eucalyptus grandis trees on soil exploration by roots down to the water table at 17 m depth in a tropical planted forest. Method Fine roots (diameter < 2 mm) were sampled in a randomized block design with three treatments: monospecific stands of Acacia mangium (100A), Eucalyptus grandis (100E), and mixed stands with 50% of each species (50A50E). Root ingrowth bags were installed at 4 depths (from 0.1 m to 6 m) in the three treatments within three different blocks, to study the fine-root production over 2 periods of 3 months. Results Down to 17 m depth, total fine-root biomass was 1127 g m −2 in 50A50E, 780 g m −2 in 100A and 714 g m −2 in 100E. Specific root length and specific root area were 110-150% higher in 50A50E than in 100A for Acacia mangium trees and 34% higher in 50A50E than in 100E for Eucalyptus grandis trees. Ingrowth bags showed that the capacity of fine roots to explore soil patches did not decrease down to a depth of 6 m for the two species. Conclusions Belowground interactions between Acacia mangium and Eucalyptus grandis trees greatly increased the exploration of very deep soil layers by fine roots, which is likely to enhance the uptake of soil resources. Mixing tree species might therefore increase the resilience of tropical planted forests through a better exploration of deep soils.

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Dinâmica de crescimento e funcionamento nutricional das raízes finas de Eucalyptus em função da fertilização e da associação com espécie fixadora de nitrogênio O objetivo geral dessa tese foi compreender melhor o funcionamento das raízes finas (diâmetro <2 mm) de Eucalyptus. Mais especificamente, nosso trabalho teve como objetivo obter informações sobre as mudanças no padrão de absorção de nutriente pelas raízes finas em função da fertilização e profundidade do solo. Foi também, avaliar a dinâmica de crescimento das raízes finas em plantações mistas com uma espécie fixadora de nitrôgenio para testar a hípotese do gradiente de estresse. O capítulo 1 teve como objetivo estudar o efeito da fertilização mineral na especialização funcional das raízes finas de Eucalyptus grandis jovens em camadas profundas do solo (Itatinga-SP). Foram injetados macadores de NO 3-15 N, Rb + (K +) e Sr 2+ (Ca 2+) simultaneamente em uma solução a 10, 50, 150 e 300 cm de profundidade. A determinação das concentrações foliares de Rb + , Sr 2+ e a porcentagem de átomos de 15 N permitiu estimar o potencial de absorção relativo (PAR) e o PAR específico, definido como PARE, obtido pela razão entre o PAR e a unidade de densidade do comprimento de raízes finas por camada de solo correspondente. O PAR de NO 3-15 N diminuiu rapidamente com a profundidade e os valores mais altos do PARE de NO 3-15 N foram encontrados a uma profundidade de 50 cm. O PARE de Rb + e Sr 2+ foi maior a 300 cm de profundidade em relação à camada superficial do solo, com um aumento do diâmetro da raiz e uma diminuição da densidade do tecido radicular com a profundidade. O PARE de Rb + e Sr 2+ a 300 cm de profundidade foi, em média, 88% maior para árvores fertilizadas quando comparado com as árvores não fertilizadas. Os resultados sugerem que a especialização funcional das raízes finas para a absorção de nutrientes é uma característica estável do eucalipto e que pode ser reforçada pela aplicação de fertilizantes. O capítulo 2 focou nos processos ecológicos entre Acacia mangium e Eucalyptus em um gradiente de estresse nutricional. Raízes finas foram amostradas aos 16 e 34 meses após o plantio em blocos casualisados com dois tratamentos: uma mistura com 50% de cada espécie (50A:50E) com e sem fertilização. Durante este período, dois tubos de minirhizotron, perto de eucalipto e acacia, em cada tratamento e bloco, foram utilizados para monitorar o crescimento e o tempo de vida das raízes finas. Aos 16 e 34 meses após o plantio, a DRF de Eucalyptus foi maior em relação à Acacia e maior em F+ do que em F-na camada superior do solo. Este resultado mostrou que, provavelmente, há uma maior competição das raízes de eucalipto nas raízes de acacia em F+ do que em F-. Na camada superficial, a DRF de Eucalyptus em F-foi maior aos 34 meses e perto de árvores de Acacia comparado aos eucaliptos, o que seria consistente com uma maior facilitação de N da Acacia para os Eucalyptus em ambiente com maior deficiência de N (hipótese de gradiente de estresse). A mesma concentração de N nas folhas de...