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Information on biological nitrogen fixation (BNF) in dry forests is scarce, in spite of the importance of the process to nitrogen cycling in these systems. it can be used to improve management and to select species for revegetation and agroforestry systems. The BNF of tree legumes was estimated in four areas of the Brazilian dry forest (caatinga), analyzing potentially fixing species (target species) and non fixing species (reference species), using the (15)N natural abundance methodology. The differences between the isotopic signals of target and reference species permit the safe identification of N(2)-fixing plants and reasonably precise calculations of proportions of N derived from the air (%Ndfa). Species with great N(2) fixation capacity were identified, notably Mimosa tenuiflora, Mimosa arenosa and Piptadenia stipulacea. The mean biological fixation contributions to the plants were high, varying from 27 to 68%. However, the estimated quantities of N added annually to the leaf biomass in the systems were low (2.5 and 11.2 kg ha(-1) year(-1)), due to low proportions of fixing plants in the vegetation communities. In situations of native vegetation regeneration, when the succession is dominated by fixing species, fixation could reach up to 130 kg ha(-1) year(-1). (C) 2009 Elsevier Ltd. All rights reserved.