Potential precision of the delta 15N natural abundance method in field estimates of nitrogen fixation by crop and pasture legumes in south-west Australia [Trifolium subterraneum; Pisum sativum; Lupinus angustifolius; Western Australia]
1994
Unkovich, M.J. | Pate, J.S. (Western Australia Univ., Nedlands. Dept. of Botany and Co-operative Research Centre for Legumes in Mediterranean Agriculture) | Sanford, P. (Western Australian Dept. of Agriculture, Albany) | Armstrong, E.L. (New South Wales Agriculture, Wagga Wagga (Australia). Agricultural Research Inst.)
By using a standardized 10-point sampling procedure of paired sampling of legume and reference plant and reference plant 15N natural abundance (delta 15N) values in the range from 2.9 to 4.0 percent, the proportion of legume N derived from N fixation (LN) of sample crops of lupin and field pea and a clover pasture were assessed. Effects on delta 15N due to isotope discrimination during fixation and subsequent distribution of N by the 3 study legumes were studied using sand-cultured, fully symbiotic plant material. The resulting delta 15N data (B values) showed consistently more negative values for shoots than roots (all species), no significant effects of cultivar on B values (all species), a marked effect of rhizobial strain on B value (subclover) and a tendency for B values to fall with plant age (pea and lupin). The likely magnitude of errors in LN estimates due to incorrect choice of B value was indicated. By using data for reference plant delta 15N values from field surveys and previously assessed error factors in mass spectrometric measurement of delta 15N, precision of estimation of LN by using bulked material from the 10-point field sampling procedure was predicted for situations ranging from where a legume was obtained only minimal amounts (10 percent) through to the bulk (90 percent) of its N by atmospheric fixation.
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