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

Soper, Fiona M., Richards, Anna E., Siddique, Ilyas, Aidar, Marcos P. M., Cook, Garry D., Hutley, Lindsay B., Robinson, Nicole and Schmidt, Susanne (2014) Natural abundance (δ15N) indicates shifts in nitrogen relations of woody taxa along a savanna–woodland continental rainfall gradient. Oecologia, 178 1: 297-308. doi:10.1007/s00442-014-3176-3


Author Soper, Fiona M.
Richards, Anna E.
Siddique, Ilyas
Aidar, Marcos P. M.
Cook, Garry D.
Hutley, Lindsay B.
Robinson, Nicole
Schmidt, Susanne
Title Natural abundance (δ15N) indicates shifts in nitrogen relations of woody taxa along a savanna–woodland continental rainfall gradient
Formatted title
Natural abundance (δ15N) indicates shifts in nitrogen relations of woody taxa along a savanna–woodland continental rainfall gradient
Journal name Oecologia   Check publisher's open access policy
ISSN 0029-8549
1432-1939
Publication date 2014-12-13
Year available 2014
Sub-type Article (original research)
DOI 10.1007/s00442-014-3176-3
Open Access Status Not yet assessed
Volume 178
Issue 1
Start page 297
End page 308
Total pages 12
Place of publication Heidelberg Germany
Publisher Springer
Language eng
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 delta N-15(soil) and nitrate: ammonium ratios, as plant communities transitioned from N to water limitation. In this context, we tested the hypothesis that delta N-15(root xylem sap) provides a more direct measure of plant N acquisition than delta N-15(foliage). We found highly variable offsets between delta N-15(foliage) and delta N-15(root 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, delta N-15(foliage) overlapped between N-fixing Acacia and non-fixing Eucalyptus/Corymbia and could not be used to reliably identify biological N fixation (BNF). However, Acacia delta N-15(root 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. delta N-15(root 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 delta N-15(soil) and/or decreasing mycorrhizal colonisation with aridity. We propose that delta N-15(root xylem sap) is a more direct indicator of N source than delta N-15(foliage), with calibration required before it could be applied to quantify BNF.
Formatted 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.
Keyword Acacia
BNF
Eucalyptus
Proteaceae
Xylem sap
Q-Index Code C1
Q-Index Status Confirmed Code
Institutional Status UQ
Additional Notes Published online ahead of print 13 Dec 2014

Document type: Journal Article
Sub-type: Article (original research)
Collections: School of Agriculture and Food Sciences
Official 2015 Collection
 
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