Field evaluation of N2-fixation and N-utilization by phaseolus bean varieties determined by 15N isotope dilution

Differences in N2-fixation by Phaseolus vulgaris bean cultivars were successfully evaluated in the field using 15N isotope dilution technique with a non-fixing test crop of a different species (wheat). The Phaseolus cultivars could have been similarly ranked for N2-fixation capacity from either seed yield or total nitrogen yield, but the isotope method provided a direct measure of N2-fixation and made it possible to estimate the proportion of fixed to total nitrogen in the crop and in plant parts. Amounts of nitrogen fixed varied between 24.59 kg N/ha for the 60-day cultivar Goiano precoce to 64.91 kg N/ha for the 90-day cultivar Carioca. The per cent of plant nitrogen due to fixation was 57-68% for the 90-day cultivars and 37% for Goiano precoce (60-day cultivar). Fertilizer utilization was 17-30% of a 20 kg N/ha fertilizer application. 100 kg N/ha fertilizer application decreased N2-fixation without suppressing it totally. Differences in yield between the highest yielding (Carioca) and the lowest (Moruna) 90-day cultivars were also due apparently to varietal differences in efficiency of conversion of nitrogen to economic matter i.e. seed, as well as to differences in capacity of genotypes for N2-fixation. (orig.)