No abstract

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.

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.

& 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.

No abstract

Forest residues (i.e. harvest residues and litter) are important nutrient sources for trees because soils of poor fertility are used for eucalypt plantations in Brazil. Understanding the dynamics of decomposition for these residues and their effects on soil fertility are relevant for the management of forest plantations. The objectives with this study were to assess the effects of forest residue management and fertilizer, both applied in the establishment of a Eucalyptus grandis Hill ex Maiden stand, on the decomposition rate (k), nutrient release and soil fertility after harvest of this stand. The treatments were applied to a plantation (R1). After 8 yr, R1 was clear‐cut and all of the treatments were reapplied to a new planting (R2). At the end of R1 (age of 8 yr), there was a reduction of 10% in the wood volume in the treatment with the forest residues removed and 36% reduction with no fertilizer application. At the end of 1 yr for plantation R2, these reductions were 30 and 57%, respectively. Residue decomposition under R2 was assessed by a new approach that involved collecting samples directly on the site without using litter bags. The k of forest residues was 3.6 for leaves, 1.2 for bark and 0.8 for branches. The application of small rates of N and P fertilizer did not influence the k of forest residues. In the first 300 days after clear‐cutting, approximately 50% of N, P, Ca, Mg and S and 80% of K in the forest residues were released. Even so, this did not result in significant changes in soil fertility levels. Only small reductions in soil N and P contents over time and changes in topsoil pH resulted from forest residue removal.

No abstract

The goals of this study were: (i) to compare sampling of fine roots with 4 and 8 cm of diameter augers (Area 1); (ii) to compare sampling of fine with auger and monolith method (Area 2); (iii) characterization of the fine root distribution in the soil profile, over summer and winter, of genotypes (Euca/yptus grandis Coft's Harbour and the hybrid E. grandis vs. E. urophy/la) propagated by seed and rooted cutting (Area 2).