In order to determine how dependent a woody legume can be upon potential N sources, the time course of N2fixation and N absorption was studied onGliricidia sepiumcv. ILG50,Leucaena leucocephalacv. K28, andAlbizia lebbeckin a screenhouse for 16 months usingSenna siameaas a reference tree. Dry matter and N yield in all four species increased steadily with time. The numbers and dry weight of nodules also increased with tree age. N-difference and15N dilution methods correlated well (r = 0.87–0.93) in defining an increasing pattern of N2fixation with time in all species, with the highest increment rate between sowing and 8 months. The percent N2fixed ranged from 17.9 to 74%, 27.7 to 71.9% and 43.6 to 83.6%, equivalent to 191 to 3385 mg, 321 to 2863 mg and 533 to 6419 mg N inLeucaena, GliricidiaandAlbizia, respectively, for the period between 4 and 16 months. In contrast to others,G. sepiumhad its peak of amount of N2fixed at 12 months after planting. In both the proportion and amount of N2fixed over time,A. lebbeckwas superior toL. leucocephalaandG. sepiumwhich were similar. The proportion of plant N derived from absorption (soil+fertilizer) over time decreased inversely to the increase in symbiotic N2fixation, with soil being the predominant source. This study evidenced the benefit of relying on N2-fixing woody legumes as sustainable N-supplying sources to soil-crop systems.