Biological nitrogen fixation (BNF) of 17 soybean cultivars was comparatively estimated by the delta 15N natural abundance technique using two non-nodulation soybeans (Clay and Chippewa) as reference plants. A field study was established on the experimental farm of the University of Abomey-Calavi, Benin on a typical "terre de barre" soil classified by Food and Agriculture Organization-United Nations Educational, Scientific and Cultural Organisation as Rhodic Ferralsol. A nitrogen-free pot trial was also carried out using soil substrate sampled from the Atlantic Ocean beach. In the N-free medium, N content of the whole soybean cultivars ranged from 2.6 to 8.1 mg N per plant compared with an average of 1.8 mg N per plant observed with the non-fixing soybeans. Plant delta 15N of the nodulating soybeans ranged from -2.7756 degrees C (Jupiter) to 0.1951 degrees C (Conquista), while the non-nodulating cultivars Chippewa and Clay had 2.67 degrees C and 9.30 degrees C, respectively. Percentage and amount of N derived from air (Ndfa) were significantly different (P < 0.01) among soybean cultivars, and values depended highly on the selected reference plants. When Clay was used as the reference plant, the average percentage Ndfa was 1.4 times higher than when Chippewa was the reference plant. Both reference plants consistently ranked promiscuous soybean cvs. TGx 1894 3F and TGx 1908 8F as the best cultivars and cv. TGx 1888 29F as the least in percentage Ndfa, suggesting that any of the reference plants could be used in delta 15N method for assessing N-2-fixation. The two identified promiscuous soybean cultivars with greatest capacity to fix N could be included in a soybean extension program for West African farming systems
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