Mixed-species plantations of Acacia mangium and Eucalyptus grandis in Brazil -: 2:: Nitrogen accumulation in the stands and biological N2 fixation

The sustainability of plantation forests is closely dependent on soil nitrogen availability in short-rotation forests established on low-fertility soils. Planting an understorey of nitrogen-fixing trees might be an attractive option for maintaining the N fertility of soils. The development of mono-specific stands of Acacia mangium (100A:0E) and Eucalyptus grandis (0A:100E) was compared with mixed-species plantations, where A. mangium was planted in a mixture at a density of 50% of that of E. grandis (50A:100E). N(2) fixation by A. mangium was quantified in 100A:0E and 50A:100E at age 18 and 30 months by the (15)N natural abundance method and in 50A:100E at age 30 months by the (15)N dilution method. The consistency of results obtained by isotopic methods was checked against observations of nodulation, Specific Acetylene Reduction Activity (SARA), as well as the dynamics of N accumulation within both species. The different tree components (leaves, branches, stems, stumps, coarse roots, medium-sized roots and fine roots) were sampled on 5-10 trees per species for each age. Litter fall was assessed up to 30 months after planting and used to estimate fine root mortality. Higher N concentrations in A. mangium tree components than in E. grandis might be a result of N,, fixation. However, no evidence of N transfer from A. mangium to E. grandis was found. SARA values were not significantly different in 100A:0E and 50A: 100E but the biomass of nodules was 20-30 times higher in 100A:0E than in 50A: 100E. At age 18 months, higher delta(15)N values found in A. mangium tree components than in E. grandis components prevented reliable estimations of the percentage of N derived from atmospheric fixation (%Ndfa). At age 30 months, %Ndfa estimated by natural abundance and by 15N dilution amounted to 10-20 and 60%, respectively. The amount of N derived from N(2) fixation in the standing biomass was estimated at 62 kg N ha(-1) in 100A:0E and 3 kg N ha(-1) in 50A:100E by the (15)N natural abundance method, and 16 kg N ha(-1) in 50A:100E by the (15)N dilution method. The total amount of atmospheric N, fixed since planting (including fine root mortality and litter fall) was estimated at 66 kg N ha(-1) in 100A:0E and 7 kg N ha(-1) in 50A:100E by the (15)N natural abundance method, and 31 kg N ha(-1) in 50A:100E by the (15)N dilution method. The most reliable estimation of N(2) fixation was likely to be achieved using the (15)N dilution method and sampling the whole plant. (C) 2007 Elsevier B.V. All rights reserved.