Screening of Soil for Assessment of Toxicity of Heavy Metals to Organisms

Regular use of compost prepared from municipal solid waste is associated with the entry of heavy metals into the soil, which poses considerable risks to different components of the environment. Total metal content does not generally reflect the availability of metals for the expression of environmental risk because of rapid and strong interactions of the metals with different constituents of the soil. Hence, the present study was conducted to determine screening levels of Cd, Cr, Cu, Ni, Pb and Zn for a susceptible soil by following a widely recommended soil test procedure involving the extraction of these heavy metals with a dilute calcium chloride solution. Separate sets of pot-culture experiments were carried out for each of these heavy metals in graded dose levels (Cd at 0.02-20 mg kg^-1, Cr at 0.4-200 mg kg^-1, Cu at 1.6-800 mg kg^-1, Ni at 0.5- 250 mg kg^-1, Pb at 0.4^-1 50 mg kg^-1 and Zn at 4.6- 1000 mg kg^-1) added to an acidic, light-textured alluvial soil. Soil test screening levels were determined through three different approaches, namely, phytotoxicity, food contamination and soil microbial activity diminution. Except Pb, all other heavy metals significantly reduced the above-ground biomass growth of spinach. Activities of soil enzymes were adversely affected with increasing soil test values of the heavy metals. Screening levels of the heavy metals determined through food contamination and soil microbial activity diminution were much lower than those determined through phytotoxicity. The lowest values of these soil test screening levels of the heavy metals determined by three different approaches were considered to be protective for all target organisms and were found to be: 0.003 mg kg^-1 Cd, 0.052 mg kg^-1 Cr, 0.637 mg kg^-1 Cu, 0.022 mg kg^-1 Ni, 0.008 mg kg^-1 Pb and 3.800 mg kg^-1 Zn.