Effects of metal (Cd, Cu, Ni, Pb or Zn) enrichment of sewage-sludge on soil micro-organisms and their activities

Sewage-sludge may improve soil fertility but there is concern about effects of sludge metals on soil micro-organisms and microbial processes. In a series of laboratory incubations of soil–sludge mixes, effects of varying sludge metal (Cd, Cu, Ni, Pb and Zn) contents on respiration, biomass C and N, and N mineralization were measured. Individual metals were added to a standard sludge in separate experiments at input rates below and several times current EC limits. Responses varied between metals and indices measured. Cd (up to 70 mg kg−1 in soil) did not affect any microbial index. Higher concentrations of the other metals generally caused a decrease in biomass C and N, the reduction for C often being proportionally less than that for N. In most cases, higher metal concentrations increased respiration rates and microbial metabolic quotient. Soil mineral N was increased by higher inputs of all metals, the effect being least pronounced for Cd and most pronounced for Cu. Response patterns for Zn in the earlier stages of incubations differed from those of other metals, in that intermediate concentrations increased biomass C and decreased metabolic quotient, findings associated with an observed proliferation of fungal hyphae. Results are consistent with microbial responses to metals characterised by a shift within the microbial population from bacteria to fungi, increased mineralization of organic matter and reduced assimilation of mineralised N. Except for Cd, all metals tested caused at least some significant microbial responses at inputs close to EC limits. Use of sludges with higher metal concentrations may lead to short-term changes in soil microbial communities and their activities, with increased loss of C to the atmosphere and N availability.