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Field experiments were performed in Austria, Brazil, Chile, Colombia, Guatemala, Mexico and Peru as part of an F A O / I A E A Co-ordinated Research Programme to investigate the nitrogen fixing potential of cultivars and breeding lines of common bean (Phaseolus vulgaris L.). Each experiment included approximately 20 bean genotypes which were compared using the 15N isotope dilution method. Great differences in nitrogen fixation were observed between and within experiments, with average values of 35% N derived from atmosphere (% Ndfa) and highest values of 70% Ndfa being observed. These values which were larger than had been reported previously for common bean, were observed only when environmental factors were favorable. Therefore, common bean lines are available, which can support high biological nitrogen fixation. These can be used either directly as cultivars for production or in breeding programmes to enhance nitrogen fixation in other cultivars.

Two field experiments were performed to evaluate the nitrogen fixation potential of twenty common bean cultivars and breeding lines during summer and winter seasons of 1986 and 1988, respectively. The ~SN isotope dilution method was used to quantify N 2 fixation. The cultivars and breeding lines were variable in terms of their N 2 fixation. The cv. Caballero was very efficient, with more than 50% N derived from the atmosphere and 60-80 kg N ha -~ fixed in both seasons. Other cultivars were less efficient, since the poorest ones derived less than 30% of their nitrogen from the atmosphere and fixed less than 20 kg N ha -1. After additional testing the best cultivars may be used directly by the farmers for cultivation. The experiments have provided information about which genotypes may be used to breed for enhanced fixation in common bean.

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