Diversity, symbiotic efficiency and effect of water hyacinth compost on population of rhizobia nodulating phaseolus vulgaris in Lake Victoria Basin
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Despite having a lot of resources, Lake Victoria Basin has had an increase in population and land degradation and a reduction in agricultural production due to poor soil fertility. This has necessitated the use of inorganic fertilizers to boost food production. Inorganic fertilizers production contributes to emission of greenhouse gases. Use of inorganic fertilizer in Lake Victoria basin may have led to eutrophication of Lake Victoria leading to social, economic and ecological problems. Biofertilization such as rhizobia can reduce the need for inorganic fertilizers. The objectives of this study were to determine the morphological and genetic diversity of Phaseolus vulgaris variety rosecoco nodulating indigenous rhizobia isolates, to determine the symbiotic efficiency of the indigenous rhizobia isolates and to assess the effect of water hyacinth compost, DAP and commercial rhizobia inoculum on indigenous rhizobia populations. Rhizobia were isolated from nodules obtained from common bean plants planted in the water hyacinth compost testing farms (farm trapping) and whole soil trapping experiments in the greenhouse. Genetic diversity was carried out based on restriction digest of PCR amplified 16S rRNA gene. Rhizobia populations were estimated using the most probable number technique using P. vulgaris as the trap host. Symbiotic efficiency assessment of the isolates was carried out in comparison with commercial rhizobia strains and a nitrogen supplemented control. One hundred and fifty eight isolates were obtained from whole-soil trapping experiments and were placed into nine groups based on their morphological characteristics. Four hundred and seventy two isolates were obtained from the field nodules and placed into fifteen groups. Based on Analysis of Molecular variance there was significantly p < 0.05 high genetic variation within and not among populations or regions of rhizobia isolates from Lake Victoria Basin. Principal component analyses revealed sympatric speciation of the rhizobia isolates. Cluster analysis based on genetic data and morphological data were congruent. There was negative correlation between rhizobia Shannon diversity index and soil characteristics (pH, nitrogen, available phosphates, soil organic matter). Application of water hyacinth compost prepared using effective microorganisms (EM), water hyacinth compost prepared using manure and inoculation with commercial rhizobia strain significantly (p = 0.009) increased indigenous rhizobia populations in the soil. Diammonium phosphate application significantly (p = 0.009) decreased rhizobia population in the soil. Soil characteristics, pH, nitrogen, available phosphates, and soil organic matter significantly (p < 0.05) influenced rhizobia populations in the soil. There was a significant (p = 0.001) difference in symbiotic efficiency of representative rhizobia isolates with some of the isolates having symbiotic effectiveness above 100 % in comparison with the nitrogen supplemented control. Some of the isolates had symbiotic effectiveness above the commercial Rhizobia leguminosarum (strain 446) which had symbiotic effectiveness of 110.87 %. Due to the high diversity and symbiotic efficiency of some of the rhizobia isolates from Lake Victoria Basin in the present study this shows that the region is a suitable genepool of rhizobia inoculums which can be used to improve soil fertility and common bean production. Water hyacinth compost and rhizobia inocula enhanced rhizobia populations in the soil; hence they can be used to improve soil fertility in Lake Victoria Basin. Studies should be done to establish the symbiotic efficiency of the best isolates in the field for possible inoculum production. Other molecular markers can be used to establish the true identity of the rhizobia isolates.