Effect of Cadmium and Lead Contamination with and without Wheat Straw on Microbial Activity in a Swell-shrink Soil

A study was undertaken to assess the toxicity of Cd and Pb contamination on soil microbial and enzyme activities in a swell-shrink soil (Vertic Ustopept) amended with or without wheat straw (0, 0.5 and 1%, w/w) incubated at 30°C for sixty days. Increasing levels of Cd and Pb contamination caused a significant reduction in soil microbial biomass ranging from 14.4 to 38.3 and 7.9 to 41.3 per cent, respectively. Addition of wheat straw at 0.5% increased the per cent C mineralization inhibition from 18.7 to 59 per cent which was caused by increasing levels of Cd. Further decrease in C mineralication in inhibition was observed from 12 to 58 per cent when 1 per cent wheat straw was added to soil. A significant negative correlation was recorded between DTPA-extractable Cd with soil respiration, dehydrogenase, soil microbial biomass carbon and alkaline phosphatase and Pb with soil respiration and alkaline phosphatase activity. The mean recovery of Cd by DTPA extractant was maximum (about 43.7 per cent) at initial stage, which decreased with time of incubation. Increase in level of wheat straw to the soil decreased the average recovery of Cd from 44 to 40.6 per cent and that of Pb from 41.4 to 27.5 per cent. It was observed that Cd proved to be two-fold more toxic in reducing the soil microbial activities when compared to the same levels of Pb.