Metal-Microbe Interaction in Municipal Solid Waste Compost Amended Soil and its Effect on Maize (Zea May L.) Growth

Abstract

The use of municipal solid waste compost (MSWC) can be very helpful in agricultural soils because of its associated useful characteristics. It improves soil physico-chemical properties, soil microbial biomass (SMB), mitigates soil diseases and enhanced soil organic matter (SOM) etc. However, the threat related to toxic heavy metal presence in MSWC can limit its extensive use as a soil amendment in agriculture. Higher heavy metals concentrations can decrease SMB, soil microbial ecology and activities of enzymes etc. The microbial processes affect transformation of metals which influence the availability of these metals in soil. The microbiological processes can either solubilize metals which may result in their potential toxicity and bioavailability or immobilize them. A lot of work has been done on the effect of toxic heavy metals on soil microorganisms, however, the research work is mainly experimented in acidic soils. The data regarding metal–microbe interaction in alkaline calcareous soils amended with MSWC is very limited. A study was planned to evaluate the effect of heavy metals present in MSWC on SMB and how soil microorganisms (Enterobacter sp. MN-17 (ST1) and Bacillus sp. MN-54 (ST2)) transform cadmium (Cd) and lead (Pb). The effect of MSWC on heavy metals availability, and growth of maize (Zea mays L.) was studied. The role of biochar and metal tolerant bacterial strain (MN-17) was also assessed to reduce metal availability in MSWC amended soils. Results showed that the increase in MSWC rate from 5 to 20 t ha-1 significantly increased the SMB (C and N) and soil dehydrogenase activity (DHA). MSWC application at 20 t ha-1 showed highest values of SMB C (21 %), SMB N (111 %) and DHA (25%) over control. No negative impacts were observed on the soil quality indicators by applying higher rates of MSWC during the whole incubation period. The addition of metal tolerant strains (MN-17 and MN-54) showed significant difference in reducing the exchangeable fraction of Cd and Pb in MSWC amended soil. Soil microbial biomass C and N and soil dehydrogenase enzyme concentrations were also increased by the addition of metal tolerant strains in MSWC. It was noticed that the metal tolerant bacterial strains can affect the mobility of metals through microbial transformation in soil and can reduce the exchangeable fraction of heavy metals in MSWC amended soil. Furthermore, roots of maize plants showed the highest metal concentrations which might act as a barrier for Pb, resulting in reduced concentrations of Pb in shoot of maize. However, the uptake of Cd remained higher from roots to shoots in maize vii plant. MSWC showed a positive impact on the plant growth and physiological parameters. Soil quality parameters (SMB and DHA) were also enhanced in soil where maize plants were grown upon the application of MSWC. In addition, poultry manure biochar and metal tolerant strain MN-17 and their combination remained more effective in the immobilization of Cd (66%) and Pb (61%) in MSWC amended soil. The mixing of biochar and metal tolerant strain MN-17 alone and their combination with MSWC showed better growth and physiological parameters as compared to the plants grown in MSWC amended soil. Soil microbial biomass C and N and soil dehydrogenase enzyme concentrations were also increased by the addition of biochar and MN-17 in MSWC amended soil. With incorporation of MSWC in soil, no detrimental effects of heavy metals on maize plant growth and soil microbial biomass were found. However, due to the high bioavailability potential of Cd and Pb, the repeated applications of MSWC would carry a risk of Cd and Pb build-up in soil.