Effect of soil on microbial responses to metal contamination

An experiment was conducted to investigate microbial responses to metal inputs in five soils with varying clay and organic contents; one soil had also a higher pH. These soils were treated with a low metal, sewage sludge control or with this sludge contaminated to achieve Cu = 112, Ni = 58 and Zn = 220 mg kg(-1) in medium and Cu = 182, Ni = 98 and Zn = 325 mg kg(-1) in high metal soils. CO2 evolution rates were measured at 1 week and at 4-5-day intervals thereafter until the end of the incubation (7 weeks). Extractable metals (CaCl2 and water), biomass C, metabolic quotient, ergosterol, bacterial-fungal phospholipid fatty acid (PLFA - 3 weeks only) ratio and mineral N were measured at 3 and 7 weeks. Metal inputs caused a marked increase in metal availability in the slightly acidic sandy loams, a smaller increase in slightly acidic clays and had little effect in the alkaline loam. After an initial increase in CO2 evolution with metal inputs in all soils, the high metal treatment alone caused a significant decrease at later stages, mainly in sandy loams. Although biomass C and metabolic quotient decreased in all soils with higher metal inputs, the effect was more pronounced in the sandy loams. Metal inputs increased ergosterol and decreased bacterial-fungal PLFA ratios in most soils. Larger mineral N contents were found in all high metal soils at 3 weeks but, after 7 weeks, metals caused a significant decrease in sandy loams. CaCl2 and water-extractable Cu, Ni and Zn contents were closely correlated with microbial indices in sandy loam but not in clay soils. Overall, the effect of treatments on microbial and extractable metal indices was greater in loams. Within a single series, higher organic soils showed less pronounced responses to metal inputs, although this trend was not always consistent. (C) 2000 Elsevier Science Ltd. All rights reserved.