Effects of heavy metal toxicity on the biological transformation of phosphorus in soil

Abstract

Experiments were carried out for the isolation, identification and characterization of heavy metal resistant phosphate dissolving bacteria, their tolerance to lead and cadmium using different liquid (NB, PVK and NBRIP) medium and three different soils (silt loam, silty clay loam and sandy loam soil). To investigate the solid-phase speciation of lead and cadmium the three soil samples were also used. In this study, two metals, lead and cadmium were selected based on the fact that these metals are discharged from many of the industries such as tannery, textiles, dying factories and others. A total of 13 bacteria were screened from metal polluted soil from Saver EPZ zone area and uncontaminated agricultural fields from Dhamrai and Pabna. For initial isolation NA, NGKG, MAC, Mannitol salt, Cetrimied and EMB agar media were used for different group of organisms. And for final screening of the phosphate dissolving bacteria were made by using PVK and NBRIP agar medium. From primary screened 13 bacteria, 9 were found as phosphate dissolving bacteria and were used to determine their resistance to heavy metals by amended with different concentrations Pb2+ and Cd2+. On the basis of morphological, physiological, biochemical characteristics and API test, 13 bacteria were authentically identified as different species of Bacillus, Micrococcus, Staphylococcus, Serratia, Pseudomonas and Proteus. NB, PVK and NBRIP liquid media and three soil samples were incubated with bacterial isolates containing different metal concentrations. Phosphate-dissolving bacterial species showed a wide range of tolerance to metals (Pb and Cd). These bacterial species showed tolerance to Pb and Cd concentrations ranging from 15 to 150 μg/ml in nutrient broth medium. In PVK broth medium, bacterial species showed tolerance to Pb and Cd concentrations ranging from 30 to 250 μg/ml and 20 to 200 μg/ml, respectively. In NBRIP broth medium, bacterial species showed tolerance to Pb and Cd concentrations ranging from 50 to 300μg/ml and 20 to 200 μg/ml, respectively. Soils were mixed with compost and oil cake as a source of organic phosphate and rock phosphate as a source of inorganic phosphate. Bacterial tolerance to Pb and Cd at three concentrations (15,000 μg/ml, 20,000 μg/ml, and 30,000 μg/ml) in different soils show different results after different incubation times (24hs, 15days and 30days). Sequential extraction was used to find out the fractions of Pb and Cd from these three soils to assess metal mobility. The sequences of extractions were six operationally defined groups: water soluble (F1), exchangeable (F2), carbonate (F3), oxide (F4), organic (F5), and residual (F6). The distribution of various fractions of Cd in the three soils at different time of incubation indicated that on an average, 58% and 54% of total Cd was associated with the mobile (F1-F2) fractions in silt loam and silty clay loam soils and 23%of total Cd was associated with the mobile (F1-F3) fractions in sandy loam soil. The immobile (F4-F6) fractions were 42 %, 46 % and 77 % in silt loam soil, silty clay loam soil and sandy loam soil respectively. 26 %, 55 % and 23 % of total Pb was associated with the mobile (F1- F3) fractions in silt loam, silty clay loam and sandy loam soil. The immobile (F4-F6) fractions were 74 %, 45 % and 77 % in silt loam, silty clay loam and sandy loam soil respectively. The behavior and bioavailability of metals in soil are affected by many chemical processes. Soil properties have profound influence of the mitigation of metal toxicity. So that organisms can survive at high concentration of Pb and Cd contaminated soil and they can participate the transformation of phosphorus in soil.