Influence of different fractions of heavy metals on microbial ecophysiological indicators and enzyme activities in century old municipal solid waste amended soil

The study was carried out on an municipal solid waste dumping site, more than hundred year old, located on the outskirts of Kolkata metropolitan city in India to determine the concentrations of different forms of selected metals (Zn, Cu, Pb, Cr, and Ni), their effect on microbial ecophysiological parameters (microbial biomass, microbial metabolic quotient, microbial respiration quotient), fluorescein diacetate hydrolyzing activity and enzyme activities in solid waste amended soils. A sequential extraction technique was used to quantify water soluble, exchangeable, carbonate bound, Fe/Mn oxide bound, organic bound, and residual fractions of metals. Metal concentrations in the two most labile fractions (i.e. water soluble and exchangeable fractions) were generally low. The concentrations of different forms of metals, microbial ecophysiological parameters and enzyme activities were found to be significantly higher in solid waste amended soils compared to the normal background soil around the study area. Significant positive correlations were observed between the microbial parameters and organic carbon content of the waste amended soil. The contents of microbial biomass C, fluorescein diacetate and enzyme activities did not necessarily decrease with increasing heavy metal content, reflecting the importance of other environmental factors, e.g. differences in organic C content. The ratios of different microbial parameters with organic C were significantly negatively correlated with metal concentrations while inhibition increased with increased bioavailability of metals. Although the waste amended soils had significantly higher microbial biomass and activities than the background soil, due to higher organic matter content, the ratios of microbial parameters/organic carbon indicated that inhibition of microbial growth and activities had occurred due to metal stress. This indicates that the use of municipal solid wastes in agriculture would lead to destruction of soil quality in the long run.