Residual effects of zinc, copper and nickel in sewage sludge on microbial biomass in a sandy loam

Relationships between total metals, CaCl2-extractable metals and soil microbial biomass were investigated in a sandy loam soil (Cuckney series) at Gleadthorpe Experimental Husbandry Farm, U.K. The metals occurred because sewage sludges, enriched either with different rates of the single metals Zn, Cu or Ni, or with combinations of the metals (Zn and Cu or Zn and Ni) at different rates, were applied in 1982 and again, in some cases, in 1986. The observed increases in total soil metal concentrations were generally in good agreement with the intended soil metal additions. However, the proportional amounts of metals extracted by CaCl2 differed between metals. Calcium chloride extracted a maximum of about 42% of total Zn, 9% of total Cu and 26% of total Ni in the sludged soil, but very much less (3% of total Zn, 1% of total Cu and 2% of total Ni) in control soils (i.e. the soils that never received sewage sludge). Neither Zn, Cu or Ni present singly in soils at below current EC permitted total soil metal concentrations decreased the amounts of soil microbial biomass. However, Cu at about 4.9 times and Zn at about 2.3 times permitted limits decreased the amounts of soil microbial biomass by 51 and 36%, respectively, when present separately, compared to the control soil. The soils which contained either Cu or Zn separately at about 1.4 times permitted limits contained about 12% less biomass C than the control soil. In contrast, Cu and Zn in combination at about 1.4 and 1.2 times permitted limits, respectively, decreased the biomass by about 29%, and soils containing Cu and Zn in combination at 1.8 and 1.4 times the limits contained 53% less biomass than the control soil. Thus a combination of Zn and Cu decreased the amount of biomass at lower soil metal concentrations than were required when either metal was present singly, suggesting the effects were additive. Biomass C as a percentage of total soil organic C in soils contaminated singly with higher rates of Zn or Cu or with both metals in combination was less than half that in the soil which received no sludge, uncontaminated sludge or sludge contaminated with lower rates of metals. Thus, this statistic provides a sensitive indicator of the effects of heavy metals on microbial biomass.