POTENTIAL PRECISION OF THE DELTA-N-15 NATURAL-ABUNDANCE METHOD IN-FIELD ESTIMATES OF NITROGEN-FIXATION BY CROP AND PASTURE LEGUMES IN SOUTH-WEST AUSTRALIA

Precision of estimation of the proportion of legume N derived from Nz fixation (%Ndfa) was assessed in relation to subterranean clover (Trifolium subterraneum L.) pastures and crops of pea (Pisum sativum L.) and lupin (Lupinus angustifolius L.) under south-west Australian conditions. By using a standardized 10-point sampling procedure of paired sampling of legume and reference plant and reference plant N-15 natural abundance (delta(15)N) values in the range from +2.9 to +4.0%, degrees Ndfa of sample crops of lupin and field pea and a clover pasture were assessed with respective precisions of 93+/-0.6 degrees, 76+/-2.4% and 91+/-1.3% (+/-s.e, n=10). Effects on delta 15N due to isotope discrimination during fixation and subsequent distribution of N by the three study legumes were studied using sand-cultured, fully symbiotic plant material. The resulting delta(15)N 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 degrees Ndfa estimates due to incorrect choice of B value was indicated. By using data for reference plant delta(15)N values from field surveys and previously assessed error factors in mass spectrometric measurement of delta 15N, precision of estimation of %Ndfa by using bulked material from the 10-point field sampling procedure was predicted for situations ranging from where a legume was obtaining only minimal amounts (10%) through to the bulk (90%) of its N by atmospheric fixation.