Journal of Tropical Ecology (2003) 19:655–666. Copyright  2003 Cambridge University Press DOI:10.1017/S0266467403006059 Printed in the United Kingdom Nodulation and dinitrogen fixation of legume trees in a tropical freshwater swamp forest in French Guiana Piia Koponen* 1 , Pekka Nygren* † , Anne Marie Domenach ‡2 , Christine Le Roux § , Etienne Saur ¶ and Jean Christophe Roggy # *Department of Forest Ecology, PO Box 57, 00014 University of Helsinki, Finland †Center for Agroforestry, 203 Natural Resources Bldg., University of Missouri, Columbia, MO 65211, USA ‡Laboratoire d’Ecologie Microbienne du Sol, UMR 5557, Universite ´ Lyon I, 43 Bd. du 11 novembre 1918, 69622 Villeurbanne, France §CIRAD-fore ˆt/LSTM, TA 10/J, Campus International de Baillarguet, 34398 Montpellier Ce ´dex 5, France ¶Ecole Nationale d’Inge ´nieurs des Travaux Agricoles de Bordeaux, BP 201, 33175 Gradignan, France #SILVOLAB Guyane, UMR ENGREF/INRA ‘fore ˆts tropı ˆcales de Guyane’, BP 709, 97387 Kourou, French Guiana (Accepted 19 November 2002) Abstract: 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 15 N natural abundance method. The results for Zygia cataractae were inconclusive but suggested N 2 fixation in drier microsites. Nodulated Inga disticha had a 15 N-to- 14 N ratio similar to non-N 2 -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 N 2 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. Key Words: acetylene reduction assay, Bradyrhizobium, flooding, microtopography, 15 N natural abundance method, 16S rDNA sequencing INTRODUCTION Large areas of neotropical freshwater floodplain and swamp forests are dominated by nodulated legume trees like Pentaclethra macroloba and Pterocarpus officinalis (Imbert et al. 2000, Norris 1969). In Amazonia, nodulated legume trees are more abundant in floodplains than in the unflooded lowland, or terra firme, forests (Moreira et al. 1992). Recent studies in Guadeloupe (Saur et al. 2000) and the Brazilian Pantanal (James et al. 2001) suggest that flood-tolerant legume symbioses may be rather common. Although N 2 fixation may provide advantage to legumes in the flooded ecosystems characterized by N losses in leaching and denitrification, it has seldom been considered as an adaptation to waterlogged soil (Barrios & Herrera 1994, Moreira & Franco 1994, Saur et al. 1998). 1 Corresponding author. Current address: Department of Ecology and Systematics, PO Box 65, 00014 University of Helsinki, Finland, Email: piia.koponen@helsinki.fi 2 Current address: UMR ECOFOG, Campus agronomique, BP 709, 97387 Kourou, French Guiana. Although the legume–rhizobia symbiosis is considered sensitive to excess of water, individual nodules are able to acclimate in a period of days to reduced oxygen. Accli- mation is both anatomical with enlarged infected cells, enhanced production of lenticels and aerenchyma, and physiological, involving changes in photosynthate alloca- tion (Arrese-Igor et al. 1993, Pankhurst & Sprent 1975, Pugh et al. 1995, Walsh 1995). Viable rhizobial cells are found in water, and they infect aquatic legumes such as Aeschynomene spp. and Sesbania spp. (Wang & Martı ´nez- Romero 2000). These species are often both stem and root nodulated under waterlogged conditions (Boivin et al. 1997). The three legume subfamilies are among the eight most species-rich families in French Guiana; nearly 15% of tree species in lowland rain forests are legumes (Roggy & Pre ´- vost 1999, Sabatier & Pre ´vost 1994). In a terra firme rain forest, 50, 71 and 77% of studied tree species in Caesalpi- nioideae, Mimosoideae and Papilionoideae, respectively, were nodulated (Roggy & Pre ´vost 1999). Roggy et al.