NITROGEN-FIXATION IN CHICKPEA .2. COMPARISON OF N-15 ENRICHMENT AND N-15 NATURAL-ABUNDANCE METHODS FOR ESTIMATING NITROGEN-FIXATION

The N-15 enrichment and N-15 natural abundance methods for estimating N-2 fixation in chickpea were compared over a range of soil NO3-N levels at crop establishment varying from 10 to 326 kg N/ha (0-120 cm depth). Barley was used as a non-N-2 fixing control crop. Both methods estimated reduced N-2 fixation as soil NO3-N levels at crop establishment increased. Similar estimates of % N-2 fixation were obtained at high values, but at low values the N-15 enrichment method gave lower estimates, some of which were negative. The N-15 natural abundance method provided realistic estimates of % N-2 fixation across all soil NO3-N levels at crop establishment. An asymptotic curve described a close (R(2) = 0.95) relationship between these factors. Standard errors of estimates of means for the N-15 natural abundance method remained acceptable and relatively stable over the full range of measurements; however, with the N-15 enrichment method they became unacceptably large at low values of % N-2 fixation. These large errors may have been partly due to legume and control plants assimilating mineral N of differing N-15 enrichment. High mineral N levels associated with low values of % N-2 fixation were also shown to reduce reliability of N-2 fixation values estimated by the N-15 enrichment method. These errors caused potentially greater inaccuracy at low values of % N-2 fixation than at high values. To compare N-2 fixation means statistically, transformations were necessary to stabilize variance and to impart lower weightings to plots with low values of % N-2 fixation.