Abstract

Background: Patterns in tropical forest nitrogen cycling are poorly understood. In particular, the extent to which leguminous trees in these forests fix nitrogen is unclear. Aims: We aimed to determine factors that explain variation in foliar delta N-15 (delta N-15(F)) for Amazon forest trees, and to evaluate the extent to which putatively N-2-fixing Fabaceae acquire nitrogen from the atmosphere. Methods: Upper-canopy delta N-15(F) values were determined for 1255 trees sampled across 65 Amazon forest plots. Along with plot inventory data, differences in delta N-15(F) between nodule-forming Fabaceae and other trees were used to estimate the extent of N-2 fixation. Results: delta N-15(F) ranged from -12.1% to + 9.3%. Most of this variation was attributable to site-specific conditions, with extractable soil phosphorus and dry-season precipitation having strong influences, suggesting a restricted availability of nitrogen on both young and old soils and/or at low precipitation. Fabaceae constituted fewer than 10% of the sampled trees, and only 36% were expressed fixers. We estimated an average Amazon forest symbiotic fixation rate of 3 kg N ha(-1) year(-1). Conclusion: Plant delta N-15 indicate that low levels of nitrogen availability are only likely to influence Amazon forest function on immature or old weathered soils and/or where dry-season precipitation is low. Most Fabaceae species that are capable of nodulating do not fix nitrogen in Amazonia.