This study used three cowpea (Vigna unguiculata L. Walp) varieties, (DschMMBr, Vyuniebe and 58-77) as trap crops to estimate the population of indigenous Bradyrhizobia spp. Soil samples were collected in two sites (Nkoemvone and Nkometou) of Southern Cameroon known to accommodate acid soils with low phosphorus (P) levels, and the population of the indigenous rhizobia was determined using the most probable number (MPN) plant infection technique. The results of the MPN counts indicated that the total Bradyrhizobia population in Nkoemvone was between 1.0 and 5.8 × 105 cells per gram of soil sample while in Nkometou, it was between 5.8 x103 and 1.0 x104 cells per gram of soil sample. Using the cowpea variety DschMMBr as trap crop, the Bradyrhizobium spp. population estimate was 5.8 x103 and 1.0 x 105 cells per gram of soil sample, in Nkometou and Nkoemvone, respectively. When the trap crop was changed to Vyuniebe, the population estimate remained the same in Nkometou but substantially increased (3.1 x 105 cells per gram of soil sample) in Nkoemvone. Using the variety 58-77 in Nkometou, a Bradyrhizobium spp. population size of 1.0 x104 cells per gram of soil sample was estimated whereas this population was 5.8 x 105 in Nkoemvone. Overall, population sizes of rhizobia using the three varieties were higher in Nkoemvone soil than in Nkometou soil. The cowpea variety 58-77 seemed to have high nitrogen fixation potentials as it formed nodules more than the two other varieties in both soils. The levels of Bradyrhizobia populations observed in the two sites were adequate to give satisfactory results on nodulation and nitrogen fixation. This suggests that cowpea production in southern Cameroon does not require inoculation.
Key words: Bradyrhizobia spp., Vigna unguiculata, nodulation, most probable number, low P soils, Southern Cameroon.