Rock phosphate solubilizing Streptosporangium isolates from casts of tropical earthworms

Phosphorus immobilization is among the recalcitrant problems hampering crop production in the high rainfall acid soils of Nigeria. Applied inorganic phosphorus quickly becomes immobilized and unavailable to plants. There is, therefore, a need to develop cultural methods that foster fast and efficient recycling of phosphorus and carbon to satisfy plant demands and facilitate sustainable soil fertility management in these soils. Nine Streptosporangium isolates of Eudrilus eugeniae (earthworm) casts were found to be acidogenic, acid tolerant, rock phosphate (RP) solubilizers which could grow on synthetic glucose or carboxymethyl-cellulose (CMC) N-free or NH4Cl-enriched media as the sole carbon source. CMC induced production of extracellular cellulase enzyme and release of reducing sugar in all isolates. Both CMC media were acidified by all isolates. The ability to solubilize RP varied among the isolates and ranged from 800 mu g P g(-1)RP 7 d(-1) to 9500 mu g P g(-1) RP 7 d(-1). A maximum release of 1040 mu g (glucose equivalent) ml(-1) 7 d(-1) was recorded in N-free CMC, while a minimum of 121 mu g ml(-1) was recorded in NH4Cl-enriched CMC medium. These isolates could, therefore, be exploited in the industrial production of cellulolytic enzymes and microbial phosphate fertilizers which would enhance organic residues and plant nutrients recycling in the acid soils of Nigeria. (C) 1997 Elsevier Science Ltd.