Basin-wide variations in Amazon forest nitrogen-cycling characteristics as inferred from plant and soil ¹⁵N:¹⁴N measurements

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 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.