Isolation and molecular characterization of domestic Bradyrhizobium species from soybean roots in the savannah soil of Nigeria

Abstract. The identification of effective indigenous strains of Bradyrhizobia could lead to the development of efficient and affordable inoculants for improving soil fertility. This can also promote nitrogen fixation in smallholder farming systems of Nigeria and as well make the use of nitrogen fertilizers unnecessary. This study was conducted to characterize and evaluate the nodulating properties of indigenous Bradyrhizobium species in soybean plant. A total of 18 strains were isolated using Bradyrhizobium japonicum selective medium (BJSM) from the root nodules of plants harvested from five sites on Ahmadu Bello University farm, Zaria Kaduna State, Nigeria. These isolates were evaluated for nodulating potential in a screen house using soybean (TGx 1448-2E) as a test crop. The total number of nodules, percentage effective nodules, nodule fresh and dry weights, shoot fresh and dry weight and plant nitrogen content were assessed. The nodules produced by the isolates showed high percentage effectiveness with isolate A4 having significantly higher nodule dry weight (78.00 mg) than the rest of the test isolates. The soybean inoculant, BIOFIX used in this study performed poorly having a nodule dry weight of 6.0 mg. There was positive and highly significant correlation between the nodule dry weight, shoot dry weight and plant nitrogen content (r=0.740, 0.641, 0.616, respectively) at p<0.001. Sequence analyses were carried out on the high performing isolates from each site of sampling and the result obtained using the NCBI Database showed similarity of these isolates with reference strains belonging to the genus Bradyrhizobium (A4- Bradyrhizobium japonicum RV9, B2- Bradyrhizobium guangdongense CCBAU 51649, C1- Bradyrhizobium sp. UFLA05-149, D3- Bradyrhizobium sp. B918 and E3- Bradyrhizobium sp.UFLA05-149). A 16srRNA phylogenetic tree constructed with the sequences obtained grouped the isolates without any close reference strain. However, isolate B2 showed close affiliation with Bradyrhizobium guagdongense with 95% sequence identity. Based on the effectiveness of these five strains of Bradyrhizobium, it is suggested that they can be used as potential candidate for inoculants production.