Variability in nitrogen fixation of common bean (Phaseolus vulgaris L.) intercropped with maize

Thirty one selected bean lines were evaluated in the field for ability to support N₂ fixation when intercropped with maize which received 0, 30 and 60 kg N ha^–1 as ammonium sulphate. The amount of fixed N₂ was estimated using the natural variation of ¹⁵N and wheat as the standard non-fixing crop. Nitrogen as low as 15 kg N ha^–1 at sowing suppressed nodule weight and activity (acetylene reduction activity) but not nodule number, suggesting that the main effect of mineral N was on nodule development and function. ¹⁵N data revealed a high potential of the bean genotypes to fix N₂, with the most promising ones averaging between 50–60% of seed N coming from fixation. Bean lines CNF-480, Puebla-152, Mexico-309, Negro Argel, CNF-178, Venezuela-350 and WBR22-3, WBR22-50 and WBR22-55 were ranked as good fixers.