Assessment of N2 fixation by three forage peanut ecotypes using 15N natural abundance technique

The percentage of nitrogen derived from N-2 fixation was measured by the N-15 natural abundance method, in three Arachis pintoi (Ap) ecotypes: CIAT 17434, 18744 and 18748, using an improved Bradyrhizobium strain CIAT 3101 (BS), and a mixing of native strains (NS). As reference plants, a non-nodulating Arachis hypogaea, Brachiaria arrecta, B. brizatttha and Cytiodon nlemfuensis were used. The experiment was set up under greenhouse conditions, and five harvests were done. Dry matter yield was highest in Ap ecotypes and BS (98.3 +/- 10.0 g pot(-1)), vs plants with NS (19.9 +/- 1.4 g pot(-1)). Values of delta (delta) N-15 in the ecotypes 17434, 18744 and 18748 were, 8.4, 5.7, 7.7 Too, and 1.2, 1.0 and 3.5 Tov for NS and BS, revealing for the later group more N-2 dependence to grow. All reference plants showed positive effects of the BS on the percentage of N-2 fixed: 92.7 +/- 1.8 vs 6.5 +/- 7.6 with ecotypes inoculated with NS. It was concluded that the N-2 fixed was highly dependent of the Rhizobium strain, and Bradyrhizobiunz CIAT 3101 strain was more efficient to fix N-2. The Arachis pintoi ecotypes CIAT 17434 and 18744 showed the highest N-2 fixing values. A. hypogaea highlighted as reference plant considering its growth pattern similar to A. pintoi.