Effects of cadmium and sewage-sludge on soil microbial biomass and enzyme activities

The effects of Cd and sewage-sludge on microbial biomass and enzyme activities in three soils were studied during two months incubation at 30±1°C. The sewage-sludge amendment enhanced soil microbial biomass by 8–28%, with the greatest affect in clay-loam and the least in sandy-loam soil. The soil dehydrogenase, alkaline phosphatase and arginine-ammonification activities were enhanced by 18–25, 9–23 and 8–12%, respectively, by sludge. Cadmium addition at 10 μg g−1 caused no significant changes in these parameters. However, 50 μg Cd g−1 soil detrimentally influenced the soil microbial biomass and enzyme activities; more in sandy-loam than in loam or clay-loam soil. Cadmium adsorption in soils was 31–56%, high in clay-loam and low in sandy-loam soils. A significant negative correlation (r = −0.58* to 0.80*, P ≤ 0.05) was observed between DTPA-extractable Cd and microbial biomass. The changes in soil microbial biomass were interpreted as cell death and correlated well with soil dehydrogenase and alkaline phosphatase activities. However, no correlation existed between microbial biomass and arginine-ammonification activity. Sewage-sludge amendment did not mitigate the inhibitory effects of Cd.