Estimates of N2 Fixation Based on Differences in the Natural Abundance of 15N in Nodulating and Nonnodulating Isolines of Soybeans 1

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RESUMO

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 15N abundance between the two isolines. For plants grown in a greenhouse in nutrient-poor soil, both estimates showed a high level of N2 fixation; from 58 to 89% N fixed by differences in N yield and from 51 to 95% by differences in 15N abundance. Decreasing contributions of fixed N were estimated by both methods with increasing levels of added NO3−. Results of field experiments carried out over two years on an unamended highly fertile midwestern soil showed a modest level of N2 fixation by both methods (7.3 to 51% by differences in N yield, and 5.4 to 46% by differences in 15N abundance). When the soil was amended with ground corn cobs, both methods showed higher contributions of fixed N. The two methods of estimating N2 fixation gave similar results. Both appear to be semiquantitative and the standard errors of the estimates were about the same (6% on the average).

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