Biology and Fertility of Soils

Dissimilarities in soil N uptake between N 2 fixing and reference non-N 2 -fixing plants can lead to inaccurate N 2 fixation estimates by N difference and 15 N enrichment methods.The natural 15 N abundance (ä 15 N) method relies on a stabilized soil 15 N pool and may provide reliable estimates of N 2 fixation.Estimates based on the ä 15 N and differences in N yield of nodulating and non-nodulating isolines of soybean were compared in this study.Five soybeans from maturity groups 00, IV, VI, and VIII and their respective non-nodulating isolines were grown at three elevations differing in ambient temperature and soil N availability.Despite large differences in phenological development and N yield between the non-nodulating isolines, the ä 15 N values measured on seeds were relatively constant within a site.The ä 15 N method consistently produced lower N2 fixation estimates than the N difference method, but only in three of the 15 observations did they differ significantly.The average crop N derived from N 2 fixation across sites and maturity groups was 81 % by N difference compared to 71 % by ä 15 N.The magnitude of difference between the two methods increased with increasing proportions of N derived from N2 fixation.These differences between the two methods were not related to differences in total N across sites or genotypes.The low N2 fixation estimates based on ä 15 N might indicate that the nodulating isolines had assimilated more soil N than the nonnodulating ones.A lower variance indicated that the estimates by N difference using non-nodulating isolines were more precise than those by ä 15 N. Since the differences between the estimates were large only at high N 2 fixation levels (low soil N availability), either method may be used in most situations when a non-nodulating isoline is used as the reference plant.The ä 15 N method may have a comparative advantage over N difference and 15 N enrichment methods in the absence of a suitable non-N 2 -fixing reference plant such as a non-nodulating isoline.