Time-course of dinitrogen fixation of promiscuous soybean cultivars measured by the isotope dilution method

Soybean cultivars capable of nodulating with indigenous Bradyrhizobium spp. have been developed by the International Institute of Tropical Agriculture (IITA) and national programs in Africa in order to avoid artificial inocul ation by resource-poor farmers in Africa. The current selection procedure f or enhanced N-2 fixation is based on an assessment of nodule formation whic h does not directly quantify the proportions of crop N derived from the atm osphere. We have monitored N accumulation patterns and N-2 fixation in nine promiscuous soybean cultivars with different maturity periods, using the N -15 dilution technique. Nodule development generally peaked at the early po dfill stage for all cultivars except Tgx 1519-1D and Tgx 1447-2D in which i t continued to increase. The proportion of crop N derived from fixation (%N DFA) ranged between 51% and 67%, 77% and 84%, and 66% and 73% at full bloom , early podfill, and physiological maturity stages, respectively. Total N a ccumulation increased in all soybean genotypes with increasing plant age. S ignificant correlations (P<0.001) were established between nodule weight an d %NDFA, even though this did not explain the relationship between nodule d evelopment and N-2 fixation in cultivars such as Tgx 1519-1D. Promiscuous s oybean cultivars retained between 10% and 19% of total N accumulated at the final harvest, in belowground biomass. Our results indicated that these so ybean cultivars can derive substantial proportions of plant N from N-2 fixa tion in soils where compatible indigenous bradyrhizobia populations are ade quate and effective. Also, we have substantiated the claims that qualitativ e nodulation parameters currently used to select varieties with a high N-2 fixation capacity need to be validated with other measurements of N-2 fixat ion.