|Published in||Plant Ecology & Diversity, v. 7(1-2)|
Nardoto, G.B., Quesada, C.A., Pati no, S., Saiz, G., Baker, T.R., Schwarz, M., Schrodt, F., Feldpausch, T.R., Domingues, T.F., Marimon, B.S., Marimon Junior, B.H., Vieira, I.C.G., Silveira, M., Bird, M.I., Phillips, O.L., Lloyd, J. and Martinelli, L.A.
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 &delta15N (&delta15NF) for Amazon forest trees, and to evaluate the extent to which putatively N2-fixing Fabaceae acquire nitrogen from the atmosphere.
Methods: Upper-canopy &delta15NF values were determined for 1255 trees sampled across 65 Amazon forest plots. Along with plot inventory data, differences in &delta15NF between nodule-forming Fabaceae and other trees were used to estimate the extent of N2 fixation.
Results: &delta15NF ranged from &minus12.1&permil to +9.3&permil. 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&minus1 year&minus1.
Conclusion: Plant &delta15N 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.