Background and Aim: In Guadeloupe’s swamp forests, smallholder farmers traditionally cultivate flooded Colocasia esculenta (taro) monocultures under the canopy of Pterocarpus officinalis stands, without the use of pesticides and fertilizers. To better understand functioning of this traditional agroforestry system, we estimated the ability of the tree legume P. officinalis to fix atmospheric nitrogen (N) and to transfer fixed N to C. esculenta under natural and controlled conditions. Methods: We use the 15N natural abundance and the 15N isotope dilution methods to determine the ability of P. officinalis to fix atmospheric N and to transfer fixed N to C. esculenta under three natural sites and controlled conditions via arbuscular mycorrhizal (AM) inoculation, respectively. Results: The proportions of fixed N (%Ndfa) in P. officinalis in swamp forest varied from 72 to 95% in mature trees, from 52 to 81% in seedlings associated with C. esculenta, and from 78 to 84% in seedlings not associated with C. esculenta. The proportion of fixed N transferred from P. officinalis seedlings to C. esculenta neighboring plants was estimated to 27%. The AM inoculation had a significantly positive effect on N2 fixation in P. officinalis, leaf dry weight and leaf P concentration in both P. officinalis and C. esculenta plants in cropping systems. The proportion of fixed N transferred from P. officinalis to C. esculenta in the AM inoculated plants reached 18%. Conclusion: The development of AM mycorrhizal networks may have facilitated the transfer of fixed N from P. officinalis to C. esculenta in field and pot experiments.