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 (Trif olium subterraneum L.) pastures and crops of pea (Pisum sativum L.) an d lupin (Lupinus angustifolius L.) under south-west Australian conditi ons. By using a standardized 10-point sampling procedure of paired sam pling of legume and reference plant and reference plant N-15 natural a bundance (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 discrim ination 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 signific ant 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 i n degrees Ndfa estimates due to incorrect choice of B value was indica ted. By using data for reference plant delta(15)N values from field su rveys and previously assessed error factors in mass spectrometric meas urement 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 amo unts (10%) through to the bulk (90%) of its N by atmospheric fixation.