Determination of Biodiversity and Symbiotic Efficiency of Native Rhizobia Isolated from Different Regions in Kenya
Jalloh, Abdul A.
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Crop production has continued to decline in sub-Saharan Africa due to soil infertility and increased cost of farm inputs. To enhance food security, farmers have adopted the use of both inorganic and organic fertilizers on their farms. The production and use of inorganic chemical fertilizers are not only expensive for family farming systems but also contribute significantly to environmental pollution. Biological nitrogen fixation using rhizobia has proven to be a cost-effective and environmentally friendly alternative to inorganic nitrogen fertilizers. Rhizobia interact with legumes symbiotically, improving soil fertility and legume plants productivity. The present study aimed at determining the morphological and genetic diversity, cross-inoculation ability, and symbiotic efficiency of native rhizobia isolates in sterile and non-sterile soils. Greenhouse experiments were set to trap rhizobia from soil from smallholder farms from Kitui, Embu, and Tharaka Nithi Counties using cowpea as the trap host. The cowpea crops were harvested after one month, and a total of 311 nodule isolates were purified from the crop nodules. Based on morphological characteristics, the nodule isolates were clustered into 42 different groups. The effect of the soil on cowpeas nodulation was determined using redundancy analysis where soil characteristics including P, Zn, Mn, and total organic carbon correlated positively with cowpea nodulation and nodule dry weight. Soil pH and Ca correlated negatively with nodule number and weight. In addition, 53 glycerol stocks nodule isolates (archived samples) from previous studies were revived. Revived cultures were clustered based on morphological characteristics into 11 different groups. The isolates were tested for symbiotic efficiency using common bean, cowpea, green gram and soyabean seeds planted in sterile and non-sterile soils in the greenhouse. Un-inoculated plants were used as controls, while the treatments included the revived native isolates and commercial rhizobia inocula. The experiments were laid out in a completely randomized design. Plants grown in both sterilized and non-sterilized soils and inoculated with the different isolates varied significantly (p<0.05) in the shoot, root, and nodule dry weights. Some of the isolates, including; IsAS14, IsAS11, IsAS10, IsAMR6, IsAMR22, IsAMR23, IsAMR27, and IsAGR5 significantly (P<0.05) outperformed commercial isolates in influencing the plants growth parameters. Forty-six revived nodule isolates had the ability to infect, induce nodule formation and influence the growth of the non-original host. The best performing native rhizobia isolate IsAS14, IsAMR3, IsAMR27, IsAMR18 and IsAMR22 outperformed the commercial inocula in terms of symbiotic efficiency of 104.97 %, 136.86 %, 136.99 %, 138.88 % and 155.05 %. DNA from representative isolates was extracted using ZYMO research DNATM extraction Kit. The 16S rRNA region was then amplified using universal primers and sequenced using the same primers. Based on the sequencing of 16S rDNA of representative revived nodule isolates, most of the isolates were rhizobia but clustered with different species with the most dominant cluster being isolates related to Rhizobium leguminosarum and Rhizobium etli. Nodule number and nodule dry weight were dependent on soil physico-chemical characteristics. Rhizobia isolates also had the ability to form symbiotic interaction with non-original host legume crops. These rhizobia isolates can be used as bio-inoculants to improve the production of the different legumes, and to enhance food security.