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Abstract:
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 δ15N (δ15NF) for Amazon forest trees, and to evaluate
the extent to which putatively N2-fixing Fabaceae acquire nitrogen from the atmosphere.
Methods: Upper-canopy δ15NF values were determined for 1255 trees sampled across 65 Amazon forest plots. Along with
plot inventory data, differences in δ15NF between nodule-forming Fabaceae and other trees were used to estimate the extent
of N2 fixation.
Results: δ15NF 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 δ15N 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.
