Nardoto, Gabriela B., Quesada, Carlos A., Patiño, Sandra, Saiz, Gustavo, Baker, Tim R., Schwarz, Michael, Schrodt, Franziska, Feldpausch, Ted R., Domingues, Thomas F., Marimon, Beatriz S., Marimon, Ben-Hur, Vieira, Ima C.G., Silveira, Marcos, Bird, Michael I., Phillips, Oliver L., Lloyd, Jon, and Martinelli, Luiz A. (2014) Basin-wide variations in Amazon forest nitrogen-cycling characteristics as inferred from plant and soil 15N:14N measurements. Plant Ecology and Diversity, 7 (1-2). pp. 173-187.

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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 (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.

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