The short-term belowground transfer of nitrogen from nitrogen-fixing trees to companion trees has never been studied in the field. A 15 N pulse-labeling study was conducted in a mixed plantation of Acacia mangium and Eucalyptus grandis at the peak of leaf area, 26 months after planting. 15 NeNO 3 À was injected into the stem of one big Acacia tree in three plots. 15 N was traced over 2 months in the labeled Acacia tree as well as in neighboring Eucalyptus trees. For both species, young leaves were sampled, as well as fine roots and the rhizosphere at a distance of 0.75 m and 2.25 m from the labeled tree. The 15 N atom% was also determined in the wood, bark, branches and total foliage of the 3 labeled Acacia trees and 9 Eucalyptus trees, 60 days after labeling. Most of the leaves, fine roots and rhizosphere samples of both species were 15 N enriched from 5 days after labeling. The d 15 N values were higher at a distance of 0.75 m than at 2.25 m in Acacia roots, but were similar at both distances in Eucalyptus roots and the rhizospheres. The wood and bark of Eucalyptus trees sampled at a distance of 6.2 m from the labeled Acacia trees were 15 N enriched. This shows belowground N transfer from Acacia to Eucalyptus trees in the field during the first few days after labeling. This facilitation process may provide a significant amount of the nitrogen requirements of trees close to N-fixing trees in mixed forests.

RESUMOEste estudo objetivou avaliar a deposição e decomposição da serapilheira e a produção de biomassa radicular de três fragmentos florestais em diferentes estágios sucessionais (avançado, médio e inicial) localizados no município de Pinheiral, RJ. Para interceptação da serapilheira instalaram-se em cada fragmento, dez coletores cônicos. Avaliou-se a biomassa radicular em dois períodos (chuvoso e seco), amostrando-se pelo método do monólito, em 0-10, 10-20 e 20-30 cm e dez repetições por profundidade. Não ocorreram diferenças na deposição de serapilheira entre as áreas, mas apenas uma tendência de aumento com a evolução sucessional. Não foram verificadas variações temporais de serapilheira entre as áreas mais jovens, inicial e médio no decorrer do ano, com tendência de aumento nos meses de julho e agosto. Em estágio avançado, o aporte da serapilheira pôde ser agrupado em dois períodos: "fevereiro a julho", menores valores e "agosto a janeiro", maiores valores. A deposição das frações folhas e material reprodutivo diminuíram, enquanto ramos e outros aumentaram com o avançar sucessional. A decomposição da serapilheira mostrou em estágio sucessional médio a menor constante k (0,0038 g.g -1 .dia) e maior tempo de meia vida (182 dias), seguida por inicial (0,044 g.g -1 .dia -1 e 154 dias) e avançado (0,0064 g.g -1 .dia -1 e 108 dias). A biomassa radicular nas diferentes áreas e profundidades, no período seco e chuvoso, apresentou maiores valores em estágio avançado, intermediários em médio e menores em inicial. As diferenças sazonais da biomassa radicular ocorreram sobretudo em estágio médio, com maiores valores na coleta de junho. Palavras-chave: serapilheira; ciclagem de nutrientes; raízes. ABSTRACTThe aim of this study was to evaluate litter input and decomposition and root biomass of three forest fragments at different successional stages (advanced, medium and initial) in Pinheiral Municipality, in the state of RJ, Brazil. To evaluate litter input, 10 conic collectors were installed in each fragment. The root biomass was quantified by monolith method in two periods (wet and dry) at depths of 0-10, 10-20 and 20-30 cm, and 10 samples were collected in each area. To evaluate the decomposition rate in each area, 12

RESUMO O presente estudo foi realizado em três formações florestais de restinga que se encontram periodicamente inundadas, na Restinga da Marambaia, RJ. Para a quantificação da produção de serapilheira foram utilizados dez coletores cônicos por formação, durante outubro de 2005 a setembro de 2006. A avaliação da decomposição se deu por meio de sacolas de decomposição (litter bags), sendo distribuídas 15 sacolas por formação. A produção anual de serapilheira estimada para cada uma das formações florestais, variou de 11,3, 10,8 e 11,1 Mg ha-1 ano-1 , para F1, F2 e F3 respectivamente, com maiores valores de deposição na estação chuvosa. A fração folhas foi a mais representativa em todas as formações. O nitrogênio foi o nutriente devolvido em maior quantidade a esses solos, 130,8 kg ha-1 ano-1 em F3. A decomposição se apresentou como uma curva do tipo exponencial, sendo a constante de decomposição (k) e o tempos de meia vida foi de 0,0032 g g-1 e 217 dias em F2, 0,0031 g g-1 e 227 dias em F3 e 0,0028 g g-1 ; 247 dias em F1 Palavras-chave: folhedo; deposição; restinga; ciclagem de nutrientes.

Brazil has large stretches of degraded land, mainly occupied by pasture. Recently, the Brazilian government created a program providing financial support to recover these lands throughout the country by establishing agroforestry systems, among others. To select the most appropriate initial spatial arrangements and tree planting density for such systems, the plant growth and leaf area index (LAI) of a Eucalyptus camaldulensis clone were evaluated in five spatial arrangements: 3.6 9 2.5 m, and 3.3 9 3.3 m (monoculture), and (2 9 2) ? 10 m, (3 9 3) ? 9 m, and 9 9 3 m (silvopastoral systems), with 1,111, 918, 833, 556 and 370 plants per hectare, respectively. The arrangement did not affect plant height growth (p [ 0.05) up to 50 months. The average diameter was affected by plant proximity in the planting line, being smaller (p \ 0.05) in the arrangements (2 9 2) ? 10 m, and 3.6 9 2.5 m, and by the number of lines in the rows, as the plants in the single line arrangement (9 9 3 m) attained the highest diameter. Individual volumes of high initial density stand were lower than in the silvopastoral systems. The arrangements (2 9 2) ? 10 m, 3.6 9 2.5 m, and 3.3 9 3.3 m did not differ (p [ 0.05) in volume per hectare, which was higher (p \ 0.05) than for the other arrangements studied. LAI decreased with age and it was higher (p \ 0.01) in the monoculture as compared to the silvopastoral systems. The silvopastoral system can be established in the (2 9 2) ? 10 m arrangement for high biomass production besides having the additional benefit of financial returns from the pasture, as compared with the monoculture. The 9 9 3 m arrangement is preferable for large diameter log production.

& 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.

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.

No abstract

RESUMOAs áreas de restingas são ambientes frágeis, que desde a colonização do país, vêm sendo submetidas a pressões antrópicas. Para que medidas mitigatórias possam ser tomadas, é necessário que sejam desenvolvidos estudos para uma melhor compreensão dos processos ecológicos nestes ambientes. Desta forma, este estudo teve como objetivo quantificar o aporte de serapilheira e nutrientes e sua decomposição numa floresta periodicamente inundada na restinga da Marambaia, RJ. Na área foram instalados dez coletores cônicos e 30 litter bags. O aporte anual de serapilheira foi de 7,64 Mg.ha -1 , sendo setembro o mês de maior aporte. O nitrogênio foi o elemento devolvido ao solo em maior quantidade (71,9 kg ha -1 ano -1 ), seguido pelo potássio (41,1 kg ha -1 ano -1 ). A velocidade de decomposição da serapilheira foi de 0,0015 g g -1 dia -1 e o tempo de meia vida de 462 dias. O K foi o elemento que apresentou as maiores perdas em comparação ao demais. A celulose apresentou-se como maior participante na estrutura da serapilheira foliar, seguida pela lignina, sendo essa última associada à textura coriácea das folhas nesta formação. Palavras-chave: matéria orgânica; decomposição; macronutrientes. ABSTRACTRestinga (sandbank) areas are fragile environments, which have been subjected to anthropogenic pressures since the country colonization. So that mitigate actions can be taken, it must be developed studies to better understand the ecological processes in these environments. Thus, this study aims to quantify litter and nutrients devolution and litter decomposition in a periodically flooded forest in 'Restinga da Marambaia', Rio de Janeiro. In the study area 10 conic collectors and 30 litter bags were installed. The annual litter devolution was 7.64 Mg.ha -1 , and September was the highest contribution month. Nitrogen was the element returned to the soil to a higher amount (71.9 kg ha -1 yr -1 ), followed by potassium (41.1 kg ha -1 yr -1 ). Litter decomposition rate 0.0015 g g -1 day -1 and the halflife were 462 days. Potassium was the element that showed the highest losses in comparison to the others.Cellulose appeared as a major participant in the structure of leaf litter, followed by lignin, the latter being associated with the leathery texture of the leaves in this formation.