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Pseudosamanea guachapele (guachapele), a nitrogen fixing leguminous tree, is an alternative for mixed forest plantations in the tropics. As little information is available for guachapele (Mimosoideae) in mixed plantation with eucalyptus considering the Brazilian edaphoclimatic conditions, an experiment was carried out to evaluate the contribution of biological nitrogen fixation to guachapele and leaf litter decomposition rates and nutrient release of eucalyptus and guachapele residues (pure and mixed). The percentage of nitrogen derived from atmospheric N2 (% Ndfa) was estimated by comparing the natural 15N abundance (d15N,‰) in guachapele tissues with that of Eucalyptus grandis, a non-nitrogen fixing species, both with seven years after planting. Decomposition constants (k) and litter half-lives (t1/2) were estimated by fitting a single exponential model to litter bag data. The estimation of %Ndfa for guachapele in pure stand fell within a narrower range (17-36 %) in relation to mixed conditions (35-60 %). Nitrogen concentration in leaf litter was positively related to the decomposition rate, decreasing from pure guachapele to pure eucalyptus. Half-lives (t1/2) were significantly different (p < 0.05) among residues with 148, 185 and 218 days, for guachapele leaves, mixture of both species and for pure eucalyptus, respectively. Nutrient release rates followed the same sequence of t1/2 due to the initial residues quality (mainly N). It was observed that a fast release of N, K and Mg occurred from the residues tested, mainly for guachapele and mixed stand. These results indicate that guachapele could benefit the mixed system from the N addition and a faster decomposition rate of a richer litter.