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Oecologia

, Volume 178, Issue 1, pp 297–308 | Cite as

Natural abundance (δ15N) indicates shifts in nitrogen relations of woody taxa along a savanna–woodland continental rainfall gradient

  • Fiona M. Soper
  • Anna E. Richards
  • Ilyas Siddique
  • Marcos P. M. Aidar
  • Garry D. Cook
  • Lindsay B. Hutley
  • Nicole Robinson
  • Susanne Schmidt
Ecosystem ecology - Original research

Abstract

Water and nitrogen (N) interact to influence soil N cycling and plant N acquisition. We studied indices of soil N availability and acquisition by woody plant taxa with distinct nutritional specialisations along a north Australian rainfall gradient from monsoonal savanna (1,600–1,300 mm annual rainfall) to semi-arid woodland (600–250 mm). Aridity resulted in increased ‘openness’ of N cycling, indicated by increasing δ15Nsoil and nitrate:ammonium ratios, as plant communities transitioned from N to water limitation. In this context, we tested the hypothesis that δ15Nroot xylem sap provides a more direct measure of plant N acquisition than δ15Nfoliage. We found highly variable offsets between δ15Nfoliage and δ15Nroot xylem sap, both between taxa at a single site (1.3–3.4 ‰) and within taxa across sites (0.8–3.4 ‰). As a result, δ15Nfoliage overlapped between N-fixing Acacia and non-fixing Eucalyptus/Corymbia and could not be used to reliably identify biological N fixation (BNF). However, Acacia δ15Nroot xylem sap indicated a decline in BNF with aridity corroborated by absence of root nodules and increasing xylem sap nitrate concentrations and consistent with shifting resource limitation. Acacia dominance at arid sites may be attributed to flexibility in N acquisition rather than BNF capacity. δ15Nroot xylem sap showed no evidence of shifting N acquisition in non-mycorrhizal Hakea/Grevillea and indicated only minor shifts in Eucalyptus/Corymbia consistent with enrichment of δ15Nsoil and/or decreasing mycorrhizal colonisation with aridity. We propose that δ15Nroot xylem sap is a more direct indicator of N source than δ15Nfoliage, with calibration required before it could be applied to quantify BNF.

Keywords

Acacia BNF Eucalyptus Proteaceae Xylem sap 

Notes

Acknowledgments

We thank Gordon Moss and Dr Andrew Fletcher for assistance with field sampling and sample analysis. This study was conducted with the support of an Australian Research Council Discovery grant to S.S. S.S. originally formulated the idea, S.S., N.R., G.D.C., L.H. and A.E.R. developed methodology, S.S., A.E.R., and M.P.M.A. conducted fieldwork, I.S. and F.M.S. performed statistical analyses, F.M.S. and S.S. interpreted data and wrote the manuscript, other authors provided editorial advice.

Supplementary material

442_2014_3176_MOESM1_ESM.pdf (231 kb)
Supplementary material 1 (PDF 230 kb)

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Copyright information

© Springer-Verlag Berlin Heidelberg 2014

Authors and Affiliations

  • Fiona M. Soper
    • 1
  • Anna E. Richards
    • 2
  • Ilyas Siddique
    • 3
  • Marcos P. M. Aidar
    • 4
  • Garry D. Cook
    • 2
  • Lindsay B. Hutley
    • 5
  • Nicole Robinson
    • 6
  • Susanne Schmidt
    • 6
  1. 1.Department of Ecology and Evolutionary BiologyCornell UniversityIthacaUSA
  2. 2.CSIRO Ecosystem SciencesWinnellieAustralia
  3. 3.Department of Crop ScienceUniversidade Federal de Santa CatarinaFlorianópolisBrazil
  4. 4.Institute of Botany, SMASão PauloBrazil
  5. 5.School of Environment, Research Institute for the Environment and LivelihoodsCharles Darwin UniversityDarwinAustralia
  6. 6.School of Agriculture and Food SciencesThe University of QueenslandBrisbaneAustralia

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