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.