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(M/MF170) Microbial responses to metals added to sewage sludge. Scullion, John1, Khan, Masil2, 1 2 ABSTRACT- Metals in sewage sludge can affect soil micro-organisms and their activity if present at elevated concentrations. A series of 7 week laboratory incubations of soil-sludge mixes was carried out and effects on microbial indices of varying sludge concentrations of different metals, applied singly or in combination, measured. The soil used had a relatively high organic content and was a slightly acidic clay loam. Metals added singly included Cd, Cu, Ni, Pb and Zn at rates within and several times current EC limits. In later experiments, Cu, Ni and Zn were added in various combinations at rates slightly above and below these limits. At the rates applied, Cd and Pb had limited effects on microbial indices. However, elevated inputs of Cu, Ni and Zn increased respiration and soil mineral N, whilst generally reducing microbial biomass, leading to higher microbial metabolic quotients. There was evidence of a shift in population composition from bacterial to fungal biomass with higher metal inputs, especially for Zn and Cu. In incubations with more than one metal added to sludges, responses to combinations of different metals were generally additive. However, there were some examples of both positive and negative interaction effects. Under the experimental conditions, microbial composition and processes were influenced by inputs of metals close to current EC guidelines for sludge application to agricultural soils. Key words: sewage sludge, metals, microbial activity |
