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δ15N values of tropical savanna and monsoon forest species reflect root specialisations and soil nitrogen status

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Abstract

A large number of herbaceous and woody plants from tropical woodland, savanna, and monsoon forest were analysed to determine the impact of environmental factors (nutrient and water availability, fire) and biological factors (microbial associations, systematics) on plant δ15N values. Foliar δ15N values of herbaceous and woody species were not related to growth form or phenology, but a strong relationship existed between mycorrhizal status and plant δ15N. In woodland and savanna, woody species with ectomycorrhizal (ECM) associations and putative N2-fixing species with ECM/arbuscular (AM) associations had lowest foliar δ15N values (1.0–0.6‰), AM species had mostly intermediate δ15N values (average +0.6‰), while non-mycorrhizal Proteaceae had highest δ15N values (+2.9 to +4.1‰). Similar differences in foliar δ15N were observed between AM (average 0.1 and 0.2‰) and non-mycorrhizal (average +0.8 and +0.3‰) herbaceous species in woodland and savanna. Leguminous savanna species had significantly higher leaf N contents (1.8–2.5% N) than non-fixing species (0.9–1.2% N) indicating substantial N acquisition via N2 fixation. Monsoon forest species had similar leaf N contents (average 2.4% N) and positive δ15N values (+0.9 to +2.4‰). Soil nitrification and plant NO3 − use was substantially higher in monsoon forest than in woodland or savanna. In the studied communities, higher soil N content and nitrification rates were associated with more positive soil δ15N and plant δ15N. In support of this notion, Ficus, a high NO3 − using taxa associated with NO3 − rich sites in the savanna, had the highest δ15N values of all AM species in the savanna. δ15N of xylem sap was examined as a tool for studying plant δ15N relations. δ15N of xylem sap varied seasonally and between differently aged Acacia and other savanna species. Plants from annually burnt savanna had significantly higher δ15N values compared to plants from less frequently burnt savanna, suggesting that foliar 15N natural abundance could be used as marker for assessing historic fire regimes. Australian woodland and savanna species had low leaf δ15N and N content compared to species from equivalent African communities indicating that Australian biota are the more N depauperate. The largest differences in leaf δ15N occurred between the dominant ECM Australian and African savanna (miombo) species, which were depleted and enriched in 15N, respectively. While the depleted δ15N of Australian ECM species are similar to those of previous reports on ECM species in natural plant communities, the 15N-enriched δ15N of African ECM species represent an anomaly.

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Acknowledgements

We thank EPA of the Supervising Scientist, Jabiru, for logistic support and CSIRO Tropical Ecosystems Research Centre for access to the Kapalga fire trial. We are indebted to Drs. N. Ashwath, L. Hutley, M. Turnbull, P. Erskine, G. Woodall, I. Biggs and S. Richards for helping with field and laboratory work. G. Moss and J. Stewart provided excellent expert technical support with mass spectrometry. Many thanks to Prof. Peter Högberg for supplying additional data for the African communities. The Australian Research Council supported this study.

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Authors and Affiliations

  1. Department of Botany, University of Queensland, QLD 4072, St Lucia, Brisbane, Australia

    S. Schmidt

  2. Faculty of Life and Physical Sciences, The University of Western Australia, WA 6907, Nedlands, Australia

    G. R. Stewart

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  1. S. Schmidt
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  2. G. R. Stewart
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Correspondence to S. Schmidt.

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Schmidt, S., Stewart, G.R. δ15N values of tropical savanna and monsoon forest species reflect root specialisations and soil nitrogen status. Oecologia 134, 569–577 (2003). https://doi.org/10.1007/s00442-002-1150-y

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