Isolation of Peanut (<i>Arachis hypogaea</i> L.) Nodulating Rhizobia and Assessment of Their Phosphate Solubilizing Activity

Abstract

Phosphate-solubilizing bacteria can enhance the dissolution of insoluble phosphorus in the soil, promoting the availability of soluble phosphorus. Thus, their application can reduce the use of chemical fertilizers and ease sustainable agriculture. The study aimed at isolating and assessing the phosphate solubilizing activity of peanut rhizobia isolated from root nodules of peanut in three Cameroonian soils (Douala, Bafoussam and Ebolowa). Rhizobia were trapped by seeding peanut in plastic pots containing different soils. 45 days later, at the flowering time, root nodules were harvested. From these nodules, rhizobia were isolated, purified on Yeast Extract Mannitol Agar (YEMA) medium and authenticated by inoculating them on sterile sand containing peanut plants and watered with a nutrient solution without nitrogen. The phosphate solubilizing activity of the rhizobia isolates was then assessed in Modified Mineral Salt Medium (MMSM) containing bromocresol green and where the soluble phosphate was replaced by insoluble inorganic phosphates from Cameroon, Algeria, Senegal and tricalcium phosphate in both solid and broth media. The results were analysed statistically by one-way ANOVA using SPSS. A collection of twenty-five isolates was constituted among which 04 from Douala, 12 from Bafoussam and 09 from Ebolowa. All isolates were distinct morphologically. There was significant solubilization of inorganic phosphate in both solid and broth media (p<0.05). Isolated bacteria were characterized as being phosphate solubilizers with values ranging from 1.75 (AhBf1 on Algeria rock phosphate) to 18.9 mg/L (AhDa3 on Algeria rock phosphate). The Algeria rock phosphate was the most solubilized by peanut nodulating bacteria, followed by the tricalcium phosphate, the Senegal rock phosphate and the Cameroon rock phosphate. The solubilizing activity of inorganic phosphates in the broth medium was associated with a global decrease in the pH of the culture medium. The peanut nodulating rhizobia isolates that show the best solubilizing capacity could alleviate the problem of phosphorus availability in agricultural soils.