BC Walpola* and RHAN Hettiarachchi
Department of Soil Science, Faculty of Agriculture, University of Ruhuna, Mapalana, Kamburupitiya, Sri Lanka
Abstract
The capacity to solubilize inorganic phosphates by phosphate-solubilizing bacterial strains isolated from soil samples collected from different agricultural fields in Matara District was assessed. The isolated phosphate solubilizing bacterial strains were then tested for tolerance against four heavy metals such as cadmium (Cd), lead (Pb), zinc (Zn) and copper (Cu) each with three concentrations (100 mg/L, 200 mg/L, and 400 mg/L). The bacterial strains with the highest phosphate solubilization capacity and resistance to heavy metal were selected and the phosphate solubilization potential under different heavy metals was then assessed. The results showed that most of the tested isolates proved to be tolerant to the heavy metals at low concentrations. However, a subsequently significant reduction in tolerance was observed when heavy metal concentration increased. Except three isolates, all the other isolates were proved to be vulnerable to the heavy metals of Cd and Pb at the 400 mg/L concentration. Among the tested 15 isolates, PSB-14 showed the highest tolerance to 100 mg/L, 200 mg/L and 400 mg/L of Cd, Pb, Cu and Zn. The strain was identified as Enterobacter cancerogenous according to the 16 rRNA analysis. The bacterial strain showed very high degree of reductions in phosphate solubilization in the presence of heavy metals Cd and Pb. The order of the toxicity of the metals to strain was found to be Pb > Cd > Cu > Zn. According to the results, it could be concluded that heavy metals Cd and Pb was shown to display a major impact on phosphate solubilization while Cu and Zn had a mild effect.
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Key words: Agricultural soils, Enterobacter cancerogenous, heavy metals, phosphate solubilization, tolerance
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