MEASUREMENTS OF N2 FIXATION BY COMMON BEAN IN CENTRAL BRAZIL AS AFFECTED BY DIFFERENT REFERENCE CROPS

An experiment was conducted at EMBRAPA/CNPAF, Goiania, Goias, Brazil, on a typic haplustox soil to evaluate growth and N2 fixation-related p arameters of Phaseolus vulgaris L. Bean lines, which had been selected for N2 fixation at CNPAF, including production cultivars, germplasm b ank entries, and parents and progenies of a cross made to improve this characteristic. Wheat (Triticum aestivum L.) and dwarf sorghum (Sorgh um bicolor (L.) Moench) were evaluated as non-N2-fixing reference crop s for difference method (DM) and N-15 isotope dilution technique (IDT) estimates of N2 fixation. IDT estimates ranges from 4 to 18 kg N2 fix ed ha-1. High variability associated with low levels of N2 fixation pr ecluded definitive identification of the best N2 fixing bean lines. Du e to differences in growth cycle and in patterns and amounts of soil N uptake during the season, neither of the reference crops tested appea rs to be an adequate control for either DM or IDT estimates of N2 fixa tion. However, ranking of lines for effectiveness in N2 fixation could be performed without the use of any reference crops.