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

² Department of Biology and Center for the Biology of Natural Systems, Washington University, St. Louis, Missouri 63130, ³ USDA-SEA, Agricultural Research, University of Illinois, Urbana, Illinois 61801

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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