Initial results from long-term field studies at three sites on the effects of heavy metal-amended liquid sludges on soil microbial activity

In a long-term study of the effects on soil fertility and microbial activity of heavy metals contained in sewage sludges, metal-amended liquid sludges each with elevated Zn, Cu or Cd concentrations were applied over a 3-year period (1995-1997) to three sites in England. The experiments were sited adjacent to experimental plots receiving metal-rich sludge cakes enabling comparisons to be made between the effects of heavy metal additions in metal-amended liquid sludges and sludge cakes. The liquid sludge additions were regarded as 'worst case' treatments in terms of likely metal availability, akin to a long-term situation following sewage sludge additions where organic matter levels had declined and stabilised. The aim was to establish individual Zn (50-425 mg kg(-1)), Cu (15-195 mg kg(-1)) and Cd (0.3-4.0 mg kg(-1)) metal dose-response treatments at each site, but with significantly smaller levels of organic matter addition than the corresponding sludge cake experiments. There were no differences (P > 0.05) in soil respiration rates, biomass carbon concentrations or most probable numbers of clover Rhizobium between the treatments at any of the sites at the end of the liquid sludge application programme. Soil heavy metal extractability differed between the metal-amended liquid sludge and metal-rich sludge cake treatments; Zn and Cd extractabilities were higher from the liquid sludge additions, whereas Cu extractability was higher from the sludge cake application. These differences in metal extractability in the treated soil samples reflected the contrasting NH4NO3 extractable metal contents of the metal-amended liquid sludges and sludge cakes that were originally applied.