Effectiveness of rhizobia nodulating recent promiscuous soyabean selections in the moist savanna of Nigeria

The ability of soybean to nodulate with a wide range of indigenous bradyrhi zobia has been used in a breeding programme since 1997 in Nigeria. As far a s is known, these indigenous bradyrhizobia strains have not been tested for compatibility and effectiveness with recent selections from a breeding pro gramme which has proceeded without input from soil microbiologists for the last 20 yr. Twenty bradyrhizobia strains isolated from soyabean and cowpea grown in Ibadan and Zaria soils in Nigeria were examined in a pot experimen t for symbiotic effectiveness on two promiscuous soyabean breeding lines (T GX 1660-19F and TGX 1456-2E) and a cowpea cultivar (IT 849-92). Two bradyrh izobial isolates (R25B and IRj 2180A) had an average symbiotic effectivenes s (SE) of 2.36-fold and 1.62-fold of the uninoculated control when inoculat ed on 1456-2E and 1660-19F, respectively. These strains, however, were less effective on cowpea having a SE of 1.20-fold. Cowpea bradyrhizobia inocula ted on promiscuous soyabeans produced less than 77 nodules plant(-1) compar ed to an average of 120 by the two best bradyrhizobia strains from soyabean . The best isolates (R25B, IRj 2180A and their mixture) and one cowpea brad yrhizobia (IRc 461) were further tested on these lines under field conditio ns at three sites in different agroecological zones in moist savanna (Fasho la, Mokwa and Zaria) in Nigeria. Both soyabean lines nodulated with the loc al rhizobia, but the degree of effectiveness depended on the plant genotype and field sites. Soyabean line 1456-2E showed improved growth and yield in response to N fertilizer application indicating that in this line N-2 fixa tion induced by the indigenous bradyrhizobial community supplied less than optimal amounts of N. The mixture of bradyrhizobial isolates R25B and IRj 2 180A increased grain yield of 1456-2E by 30 and 25% at Zaria and Mokwa but failed to do so at Fashola. Grain yield of 1660-19F was not affected by bra dyrhizobial inoculation and N fertilizer at any of the three sites. Thus, t he need for bradyrhizobia inoculation will be determined by the degree of p romiscuity of soybean lines and the effectiveness of the community of indig enous bradyrhizobia present in the site. (C) 2000 Elsevier Science Ltd. All rights reserved.