A Statistical Comparison of the Two-Source δ15N and 15N Isotope Dilution Methods for Estimating Plant N2-Fixation using Trifolium pratense and Lolium perenne

Abstract

In field studies, ¹⁵N-enriched and ¹⁵N-natural abundance methods may yield similar mean estimates for N₂-fixation, but with no correlation of the individual estimates. This study was designed as a glasshouse-based microcosm and aimed to remove the landform and weather variables found in previous field studies, while retaining other sources of variability such as mineralisation of organic N and whatever slight variations of environment may exist in the glasshouse.

The results showed little or no correspondence among the estimates of N₂-fixation at the three ¹⁵N-enrichment levels; however, estimates were consistent within each ¹⁵N-enrichment level. There was also no correlation between the results of acetylene reduction and those of any of the isotopic enrichment treatments.

If the isotopic ¹⁵N-enrichment levels measured the same processes with varying accuracies, then the general patterns of the replicated results from each enrichment level should resemble each other against a generally uniform background. They did not. Hence, we conclude that the observed differences are due to unknown factors in addition to the varying noise-to-signal ratio at different enrichment levels.