A STATISTICAL COMPARISON OF THE 2-SOURCE DELTA-N-15 AND N-15 ISOTOPE-DILUTION METHODS FOR ESTIMATING PLANT N-2-FIXATION USING TRIFOLIUM-PRATENSE AND LOLIUM-PERENNE

In field studies, N-15-enriched and N-15-natural abundance methods may yield similar mean estimates for N-2-fixation, but with no correlatio n of the individual estimates. This study was designed as a glasshouse -based microcosm and aimed to remove the landform and weather variable s found in previous field studies, while retaining other sources of va riability such as mineralisation of organic N and whatever slight vari ations of environment may exist in the glasshouse. The results showed little or no correspondence among the estimates of N-2-fixation at the three N-15-enrichment levels; however, estimates were consistent with in each N-15-enrichment level. There was also no correlation between t he results of acetylene reduction and those of any of the isotopic enr ichment treatments. If the isotopic N-15-enrichment levels measured th e same processes with varying accuracies, then the general patterns of the replicated results from each enrichment level should resemble eac h other against a generally uniform background. They did not. Hence, w e conclude that the observed differences are due to unknown factors in addition to the varying noise-to-signal ratio at different enrichment levels.