Nodulated legume trees comprised 43% of the stand basal area in the low, most frequently flooded microsites, and 23% in higher, drier microsites in a tropical freshwater swamp forest in French Guiana. Dinitrogen fixation in Pterocarpus officinalis, Hydrochorea corymbosa and Inga pilosula was confirmed by acetylene reduction assay (ARA), presence of leghaemoglobin in nodules and the 15N natural abundance method. The results for Zygia cataractae were inconclusive but suggested N2 fixation in drier microsites. Nodulated Inga disticha had a 15N-to-14N ratio similar to non-N2-fixing trees, but ARA indicated nitrogenase activity and leghaemoglobin was present in nodules. All bacterial strains were identified as Bradyrhizobium spp. according to the partial 16S rDNA sequences, and they were infective in vitro in the model species Macroptilium atropurpureum. About 35-50% of N in the leaves of P. officinalis, H. corymbosa and I. pilosula was fixed from the atmosphere. Dinitrogen fixation was estimated to contribute at least 8-13% and 17-28% to whole-canopy N in high and low microsites, respectively. Symbiotic N2 fixation appears to provide both a competitive advantage to legume trees under N-limited, flooded conditions and an important N input to neotropical freshwater swamp forests.

Nodulation of Pterocarpus officinalis appears to be widely distributed in the swamp forest of Guadeloupe even under conditions of permanent waterlogging. The large majority of the nodules were found above the water table and as such were located on large aerial buttresses or on root hairs which develop mainly in emerged soil. Further, large trees modify their environment by accumulating litter between the buttresses to the point of ensuring sufficient topographic elevation to keep a volume of soil regularly emerged. Consequently the nodules were concentrated in a small circle (5 m in diameter) around the oldest trees. A few nodules (5%) survived under the water table, provided that healthy root hairs were present, and this kept the rate of nodulation constant over a vertical gradient (0.9 nodule g−1 root DW). The C2H4-evolution rate of P. officinalis, 2.6–5.9 μmol h−1 g−1, appeared to be quite low compared to that of the potted Gliricidia sepium cuttings (19.4 μmol h−1 g−1), but was considerably higher than that of the woody liana Dalbergia monetaria (1.0 μmol h−1 g−1) from the same swamp forest site as P. officinalis. It was concluded that P. officinalis is a N2-fixing tree species under the waterlogged conditions of tropical swamp forests, and was hypothesised that the symbiotic nitrogen fixation on P. officinalis buttresses and root hairs could be considered as an additional adaptation to flooded soil and a contribution to its success in the tropical swamp forests of the Caribbean region.