Isolation and characterisation of Chilembwe and Sinda rock phosphate solubilising soil microorganisms

The availability of native phosphorus in most Zambian agricultural soils is relatively low. This necessitates the use of organic and inorganic fertilizers for crop production. There are several deposits of rock phosphate in Zambia that can be used as alternative mineral sources of phosphorus; however the agronomic value of these sources of phosphorus is low and therefore need to be enhanced. Plant roots associated with certain microorganisms can enhance the dissolution of these mineral forms of phosphorus. This study was conducted to isolate and characterize soil microorganisms capable of solubilizing Chilembwe and Sinda rock phosphates readily available in Zambia. Single isolates were obtained by direct plating and enrichment cultures with succinate, cellulose and glucose as the carbon sources. Isolates were differentiated using colony morphology (form, margin, elevation and colour) and a subset of 20 was chosen for identification using the 16S rRNA gene sequences. After being cultured in nutrient broth and defined medium with tricalcium phosphate as the sole phosphate source, ten isolates were characterized for acid phosphatase enzyme activity. In total 86 isolates were obtained from different rock phosphate-sugar combinations. Isolates had varying morphological characteristics and the 20 strains identified showed a large diversity of organisms belonging to the genera Enterobacter, Burkholderia, Arthrobacter, Bacillus, Beijerinckia and two genera not previously reported to have P-solubilizing capabilities, Dyella and Curtobacterium. Both the observed growth and genera to which isolates belonged were dependent on the type of rock phosphate. Almost all isolates tested showed acid phosphatase activity that were similar to previously reported levels, ranging from 0.62 to 4.8 µg p-nitrophenol per 108 cells per hr. This work will be useful as the basis for the development of appropriate seed inocula that can enhance Sinda and Chilembwe rock phosphate dissolution and increased plant available P for crop production.