Thirty one selected bean lines were evaluated in the field for ability to support N 2 fixation when intercropped with maize which received 0, 30 and 60 kg N ha-I as ammonium sulphate. The amount of fixed N 2 was estimated using the natural variation of ~5N 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. I~N data revealed a high potential of the bean genotypes to fix N 2, 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.

Common bean (Phaseolus vulgaris L.) is able to fix 20-60 kg N ha-1 under tropical environments in Brazil, but these amounts are inadequate to meet the N requirement for economically attractive seed yields. When the plant is supplemented with N fertilizer, N 2 fixation by Rhizobium can be suppressed even at low rates of N. Using the ~SN enriched method, two field experiments were conducted to compare the effect of foliar and soil applications of N-urea on N z fixation traits and seed yield. All treatments received a similar fertilization including 10 kg N ha -I at sowing. Increasing rates of N (10, 30 and 50 kg N ha-~) were applied for both methods. Foliar application significantly enhanced nodulation, N 2 fixation (acetylene reduction activity) and yield at low N level (10 kg N ha-l). Foliar nitrogen was less suppressive to nodulation, even at higher N levels, than soil N treatments. In the site where established Rhizobium was in low numbers, inoculation contributed substantially to increased N 2 fixation traits and yield. Both foliar and soil methods inhibited nodulation at high N rates and did not significantly increase bean yield, when comparing low (10 kg N ha-l) and high (50 kg N ha-~) rates applied after emergence. In both experiments, up to 30 kg N ha-1 of biologically fixed N 2 were obtained when low rates of N were applied onto the leaves.

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