Nodulation And Nitrogen Fixation Of Indigenous Rhizobia In Soils Cultivated And Uncultivated With African Yam Bean (Sphenostylis Stenocarpa)

ABSTRACT The association between leguminous plants and rhizobia helps to meet the limiting nitrogen in the soil by producing about 40 to 60 million metric tons (Mt) per hectare per year of nitrogen. However, the focus of legume-Rhizobium symbiosis has been on biomass productivity, nodulation and nitrogen fixation of the major and domesticated food legumes whereas little research has been conducted on the indigenous legumes which hold promise in retaining agricultural diversity and achieving food security. Among these are the underutilized, neglected and unknown ones which include African yam bean (Sphenostylis stenocarpa). Further research into this crop is necessary to explore its biological nitrogen fixation and nutritional capabilities. This study was carried out to investigate the nodulation and nitrogen fixing capacity of the indigenous rhizobia in African yam bean (AYB) from some Ghanaian soils (Adenta series, Bekwai series, Nzima series and soils from Akorviefe, Dzolokpuita and Dzologbogame also known as Kuli series) that have been cultivated and uncultivated with African yam bean. Secondly, the study was to assess the response of AYB to inoculation and nitrogen fertilization. Results from Most probable number estimates showed that Bekwai series did not record any African yam bean nodulating rhizobia (0 cells g-1 soil), whereas the other five soils recorded variable numbers of rhizobia. Soils from Akorviefe (Kuli series) recorded the highest African yam bean nodulating rhizobia (290 cells g -1 soil), followed by soil from Dzolokpuita (Kuli series, 140 cells g -1 soil), Adenta series (72 cells g -1 soil) with soil from Dzologbogame (Kuli series) and Nzima series recording the least number (52 cells g -1 soil). The results of symbiotic effectiveness studies showed that there were no highly effective rhizobia present in the soils tested but moderate and ineffective rhizobia strains were observed. Soils not cultivated with African yam bean (Adenta, Toje, Akuse and Haatso) recorded 87% ineffective and 13% vi moderately effective rhizobia whereas soils cultivated with African yam bean (soils from Akorviefe, Dzolokpuita and Dzologbogame [Kuli series]) recorded 97% ineffective and 3% moderately effective rhizobia. Nitrogen application at the rate of 70 kg N ha-1 had a negative effect on nodule numbers, nodule dry weight and N2 fixation whereas positive effect was observed for total N accumulation and shoot biomass in all the soils. In contrast, inoculum application had a positive effect on nodule numbers, nodule dry weight, N2 fixation, N accumulation and shoot biomass in all the soils used. Cross inoculation results showed that 56%, 33% and 11% of African yam bean rhizobia isolates were able to nodulate cowpea, pigeon pea and soybean respectively. However, 80% of soybean, 40% of cowpea and 0% of pigeon pea rhizobia isolates were able to nodulate African yam bean plant. It can therefore be concluded from this study that nodulation and nitrogen fixation are positively influenced by the indigenous Africa yam bean rhizobia in the various soils.