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

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 δ¹⁵N (δ¹⁵N_F) for Amazon forest trees, and to evaluate the extent to which putatively N₂-fixing Fabaceae acquire nitrogen from the atmosphere.

Methods: Upper-canopy δ¹⁵N_F values were determined for 1255 trees sampled across 65 Amazon forest plots. Along with plot inventory data, differences in δ¹⁵N_F between nodule-forming Fabaceae and other trees were used to estimate the extent of N₂ fixation.

Results: δ¹⁵N_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 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.

Keywords:

• ¹⁵N natural abundance
• Amazon
• Fabaceae
• nitrogen
• nitrogen fixation
• phosphorus
• tropical forest

Acknowledgements

We thank our many South American collaborators, also involved in the work described in Fyllas et al. (2009) and Patiño et al. (2009), for help with logistics and practical help in the field. Much of the data collection phase of this work was supported through the EU FP5 “LBACarbonsink” project and a UK Natural Environment Research Council (NERC) TROBIT Consortium grant. Support for Oliver Phillips, Carlos Quesada, and Ted Feldpausch came from the Gordon and Betty Moore Foundation with additional support for the RAINFOR project from the NERC-AMAZONICA Consortium. Shiela Lloyd provided invaluable help with the nodulation database and manuscript preparation.

Notes

^†Deceased 09 August 2011