Genetic variability in dinitrogen fixation between cowpea [Vigna unguiculata (L.) Walp] cultivars determined using the nitrogen-15 isotope dilution technique

N fixed in 16 cultivars of cowpea [Vigna unguiculata (L.) Walp] inoculated with effective Bradyrhizobium strains collected from the West African MIRCE N culture collection was measured by N-15 isotope dilution technique. In al l plant parts, significant differences in the percentage of N derived from the atmosphere (%Ndfa) and the amount of Ndfa occurred between the cultivar s. Ndoute variety exhibited the highest %Ndfa (74.33% in shoots; 60.90% in roots) and accumulated more fixed N (960 mg N plant(-1) and 38 mg N plant(- 1) in shoots and roots, respectively). Therefore this cultivar should be se lected as the highest N-fixing cowpea cultivar. It also should be used in a breeding programme to contribute to the development of cultivars that coul d stimulate an intensive use of cowpea in many different cropping systems i n Africa with a view to maintaining soil fertility.