Nitrogen fixing potential of indigenous bradyrhizobia nodulating soybean varieties grown in Embu and Thara Kanithi Counties in Eastern Kenya
Wambui, Simon Mburu
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Nitrogen deficiency in most African soils is the main factor limiting crop productivity. Grain legumes such as soybean (Glycine max (L.) Merril) can form a symbiotic association with soil root nodule bacteria (rhizobia) and the process can provide sufficient nitrogen for crop production. Continuous cultivation without nutrient replenishment, leaching and poor soil fertility are the major limitations for soybean production by smallholder farmers in Embu and Tharaka-Nithi Counties. The objectives of this study were to assess the soybean varieties grown by smallholder farmers from Embu and Tharaka-Nithi Counties, to determine the diversity of indigenous bradyrhizobia nodulating soybean varieties and to determine symbiotic effectiveness of bradyrhizobia isolates on biological nitrogen fixation. DNA fingerprinting was carried out using 16S rRNA gene for polymerase chain reaction (PCR) products based on restriction digest using HaeIII, Msp 1 and Eco RI enzymes. The symbiotic effectiveness of isolates was carried out both in greenhouse and field conditions. A split plot arrangement in a randomized complete block design (RCBD) replicated three times was used. During the study, four soybean varieties namely Gazelle, Nyala, Namsoy, and Mausoy were recorded where Gazelle was most abundant with 85 % of smallholder farmers surveyed. Thirty-nine (39) isolates were obtained from trapping experiments and placed into nine groups based on their morphological characteristics. Analysis of molecular variance (AMOYA) on genetic data based on restriction digest revealed significant (p < 0.015) variation within populations and not among the zones or populations. Based on principal coordinate analysis (PCA), there was sympatric speciation of indigenous rhizobia isolates. EUMZ rhizobia isolates had the highest genetic diversity estimates (H = 0.419) compared to other agroecological zones according to Shannon's Information Index I (H). Authentication experiment showed that most of the isolates were bradyrhizobia due to their ability to infect soybeans. The test isolates had varied ability to infect and fix nitrogen. The best performing indigenous isolates (R19 ad R14) in greenhouse outperformed commercial bradyrhizobia (Bio-fix) in terms of symbiotic effectiveness with 119.17 %,142.35 %, and 101.01 %, respectively, when compared to nitrogen controls. Results from the field experiment indicated that indigenous isolates were competitive where they improved significantly soybean shoot biomass (p < 0.001). Different zones were found to have a significant influence on yield after use of inoculants (p = 0.00 I). The indigenous isolate RI9 scored highest grain yield of 823 kg ha' when compared to other treatments. The effective indigenous bradyrhizobia therefore showed the potential of being sources of inocula for soybean smallholder farmers.