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
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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.