Estimates of N₂ Fixation Based on Differences in the Natural Abundance of ¹⁵N in Nodulating and Nonnodulating Isolines of Soybeans

Estimates of the contribution of biologically fixed N to the total N of nodulating soybeans (Glycine max (L) Merrill, variety Harosoy) grown under a variety of conditions were made from: (a) differences in N yield between nodulating and nonnodulating isolines; and (b) differences in (15)N abundance between the two isolines. For plants grown in a greenhouse in nutrient-poor soil, both estimates showed a high level of N(2) fixation; from 58 to 89% N fixed by differences in N yield and from 51 to 95% by differences in (15)N abundance. Decreasing contributions of fixed N were estimated by both methods with increasing levels of added NO(3) (-). Results of field experiments carried out over two years on an unamended highly fertile midwestern soil showed a modest level of N(2) fixation by both methods (7.3 to 51% by differences in N yield, and 5.4 to 46% by differences in (15)N abundance). When the soil was amended with ground corn cobs, both methods showed higher contributions of fixed N. The two methods of estimating N(2) fixation gave similar results. Both appear to be semiquantitative and the standard errors of the estimates were about the same (6% on the average).