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© 1987 Oxford University Press

RESEARCH-ARTICLE

Evaluation of the 15N Natural Abundance Method and Xylem Sap Analysis for Assessing N2 Fixation of Understorey Legumes in Jarrah (Eucalyptus marginata Donn ex Sm.) Forest in S.W. Australia

ALEXANDER P. HANSEN1 and JOHN S. PATE2

1Department of Botany, The Australian National University Canberra, A.C.T., 2601, Australia
2Department of Botany, University of Western Australia Nedlands, W. Australia 6009

Hansen, A. P. and Pate, J. S. 1987. Evaluation of the 15N natural abundance method and xylem sap analysis for assessing N2 fixation of understorey legumes in jarrah (Eucalyptus marginata Donn ex Sm.) forest in S.W. Australia.—J. exp. Bot 38: 1446–1458.

Nodulated seedlings of Acacia pulchella, A. alata and A. extensa were grown in glasshouse sand culture under a range of levels (0–16 mol m3) of nitrate, supplied as 15NO3, or as unenriched laboratory grade nitrate ({delta}15N value 5·5%o). Nitrate at 8·0 mol m 3 or above was highly inhibitory to growth of all species. Using 15N dilution analysis of the 15N enriched cultures to measure symbiotic dependency, it was shown that {delta}15N values of the parallel unenriched cultures increased in near linear fashion from close to zero in fully symbiotic plants to values close to that of the supplied NO3 in plants experiencing nitrate levels (4·0 mol m3 or above) inhibiting N2 fixation by over 90%. Xylem sap analyses (0·4 mol m3 NO3 treatments) showed asparagine as the major nitrogenous solute, relatively little spill-over of free nitrate, and no evidence of major shifts in balance of amino compounds with increasing dependence on nitrate. This essentially invalidated use of the technique as a field assay for N2 fixation by the species.

{delta}15N values for total N of soil sampled at 64 widely distributed sites in jarrah forest ranged from – 2·15 to +5·4 (mean +2·1). Comparable values for soil mineral N (NH+4 and NO3) were +0·3 to + 14·2 (mean +5·1). {delta}15N values of the total plant N of the legumes and of non-N2-fixing reference species were also highly variable between sites, with little evidence of reference plant N accurately reflecting the 15N abundance of soil nitrogen, or of visibly well nodulated legume components showing consistently lower {delta}15N values than their companion reference plants. At one site it was possible to compare {delta}15N values of first season seedling legumes with previously published estimates of their progressive N2 fixation using C2H2 reduction assays.

It was concluded that heterogeneity in 15N discrimination of soil within the ecosystem precluded effective use of the 15N natural abundance technique for assessing legume N2 fixation.

Key words: Acacia spp., 15N natural abundance,, xylem sap analysis,, nitrogen fixation.


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