Microbial biomass dynamics following addition of metal-enriched sewage sludges to a sandy loam

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The short-and long-term effects of additions of sewage sludge enriched, or not, with the single metals Zn, Cu, Ni or Cd, on soil microbial biomas and microbial activities were studied following incubation under laboratory conditions (25°C, 50% WHC). The unenriched (low-metal) and metal-enriched (high-metal) sludges were added separately at four rates so that total soil concentrations of each metal were between 1–4 times the European Union (EU) current permitted limits. The main aim was to determine the minimum scil concentrations at which individual metals added in sewage sludge caused a decline in the amount or activity of the soil microbial biomass. Sludge addition increased biomass C by about 30% at the lowest rate of application (40 t ha −1) and about 4.5-fold at the highest rate (160 t ha −1) after 4 wk of incubation, irrespective of whether the sludges were metal-enriched or not. However, during the longer 64-wk incubation, the amounts of microbial biomass declined exponentially. larger applications of high-metal sludges caused final biomasses which were smaller than those both in soils given no sludge and low-metal sludge. None of the metals at about the currently permitted EU soil limits showed any adverse effects on microbial biomass. However, Zn, Cu or Cd individually at about twice the EU limits decreased biomass C by about 20%, whereas Ni at 4 times the limit decreased the biomass by about 15%. The amounts of metals extracted with 0.5 m K 2SO 4 increased both with increasing sludge application rates and with prolonged incubation, but different proportions of each metal were extracted. The extractability decreased in the order Cd > Zn > Ni > Cu. The amounts of inorganic N in the control soil increased with prolonged incubation. However, inorganic N in all the soils treated with sludge declined to practically zero after 4 wk, presumably due to immobilization or denitrification. After this initial decline, soil inorganic N increased steadily until the incubation ended. Averaged over the four sludge application rates, the accumulation of inorganic N was greatest in the soils which received low-metal sludge, closely followed by Cd-sludged soils and was least in the Zn-sludged soils. This suggests that the relative toxicity of the heavy metals towards N mineralization after 64 wk was Zn>Ni>Cu⪢Cd. There were close positive correlations between biomass C and dehydrogenase activity in all the soils that received sludge, although dehydrogenase activity was about one-fourth in the soils that received high-Cu sludge. This was attributed to an artefact in the methodology rather than a true biological effect.