TIME-COURSE OF NITROGEN-FIXATION IN 2 BAMBARA GROUNDNUT (VIGNA-SUBTERRANEA L VERDC) CULTIVARS

The A-value method, involving the application of a higher N-15 rate to a reference non-N-2-fixing plant, was used to assess the magnitude of N-2 fixation in two bambara groundnut cultivars at four growth stages (vegetative, 0-47 days after planting (DAP); early pod-filling, 47-99 DAP; mid-pod-filling, 99-120 DAP; physiological maturity, 120-148 DAP). The cultivars were Ex-Ada, a bunchy type, and CS-88-11, a slightly spreading type. They were grown on a loamy sand. Uninoculated Ex-Ada and CS-88-11 were used as reference plants to measure the N-2 fixed in the inoculated bambara groundnuts. In this greenhouse study, soil was the major source of N in bambara groundnuts during vegetative growth, and during this period it accounted for over 80% of the N accumulated in the plants. However, N-2 fixation became the major source of plant N during reproductive growth. There were significant differences between the two cultivars in the ability to fix N-2, and at physiological maturity, almost 75% of the N in CS-88-11 was derived from the atmosphere compared to 55% in Ex-Ada. Also, the total N fixed in CS-88-11 at physiological maturity was almost double that in Ex-Ada. Our data indicate that the higher NZ fixation in CS-88-11 was due to two factors, a higher intensity of N-2 fixation and a longer active period of Nz fixation. The results also suggest that bambara groundnut genotypes could be selected for higher N-2 fixation in farming systems.