Heavy metal availability, microbial biomass and respiration, bacterial diversity and enzyme activity were studied in soils from long-term field experiments contaminated with Mn–Zn- or Cd–Ni-rich sludge, incorporated into soils at two different rates. Soils that never receivedsludge were used as controls. Microbial biomass C content (BC) and soil respiration (CO2–C) were slightly reduced in soils amended withMn–Zn at the higher incorporation rate whereas in soils receiving Cd–Ni-rich sludge BC and respiration were unaffected. Metabolic quotientvalues (qCO2) calculated by the BC-to-CO2-C ratio were not significantly different, regardless of the sludge type whereas the microbialbiomass C-to-total organic C (BC-to-TOC) ratios were significantly reduced in the soils receiving the higher rates of both sludge types.Phosphomonoesterase, b-glucosidase and arylsulfatase activities and hydrolase-to-BC ratios, were significantly reduced in soils amendedwith Ni–Cd-sludge at both rates, whereas the Mn–Zn-sludge only reduced the arylsulfatase activity at the higher rate. Protease activity wasgenerally higher in all the sludge-amended soils as compared to control soils whereas urease activity was unaffected by sludge amendments.The structure of the bacterial community, as determined by denaturing gradient gel electrophoresis (DGGE), was different in the sludgeamendedsoils as compared to the respective controls. The most important changes were observed in the soils amended with high-levelNi–Cd sludge. Because some of the adverse effects were observed at moderate contamination levels, our results indicate that the presence ofcertain heavy metal combinations can be a serious limitation for sludge disposal.
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