Isolation and characterization of phosphate rock-solubilizing microorganisms from soils and rock phosphate samples of Panda Hill and Minjingu, Tanzania

Abstract

Plants acquire P from soil solution as the phosphate anion. Phosphorus for plant can be obtained from soluble P fertilizers or from insoluble phosphate rocks. It is highly fixed by calcium and by Fe and Al oxides depending on soil pH. Phosphorus solubilizing microorganisms play a major role in P nutrition through solubilisation of insoluble phosphate rocks. The use of phosphate solubilizing microorganisms as inoculants increases P uptake by the plant and crop yield. This study was undertaken in order to isolate microorganisms which are capable of solubilizing rock phosphate. A total of 22 fungal and 39 bacterial isolates were isolated from soil and rock phosphate samples previously collected from Minjingu and Panda Hill phosphate deposits, Tanzania. The isolated strains were assessed for their ability to solubilize insoluble phosphorus from rock phosphate samples. Out of the isolated fungal or bacterial isolates, 10 isolates that gave larger diameters of the halo zones in media containing rock phosphateas sole source of P were selected for further studies. Selected isolates were quantitatively tested in broth containing either Panda Hill or Minjingu phosphate rock. Five isolates from each group that were selected as outstanding performers, based on their relative amounts of soluble P released, were identified using molecular techniques. The most efficient fungal isolates was Aspergillus tamarii which, while it solubilised only 12.774 mg P/kg of Panda Hill rock phosphate, was able to solubilize 80.39 mg P/kg of Minjingu rock phosphate. The most efficient bacterial isolate was Stenotrophomonas maltophilia which solubilised 27.45 mg P/kg of Panda Hill rock phospahte and 24.75 mg P /kg of Minjingu rock phosphate. Other isolates that performed well included Aspergillus flavus and Aspergillus stellifer fungal species and bacteria were Bacillus safensis and Acenotobacter baumanii. The outstanding performers are herein recommended as being potential inoculants for use in increasing crop yields.