EFFECTS OF HEAVY-METALS FROM PAST APPLICATIONS OF SEWAGE-SLUDGE ON MICROBIAL BIOMASS AND ORGANIC-MATTER ACCUMULATION IN A SANDY LOAM AND SILTY LOAM UK SOIL

Amounts of microbial biomass were measured in soils from two different U.K. field experiments, one on a sandy loam (15% clay) at Luddington (Wick series) and the other on a silty loam soil (21% clay) at Lee Valley (Hamble series), where sewage sludges, mainly enriched with single metals, were applied 22 yr ago. No single metal (Zn, Cu, Ni and Cd) at or below current EC permitted total soil metal concentrations, or limits, decreased the amounts of soil microbial biomass. However, Cu at about two and a half times permitted metal limits decreased the amounts of biomass by about 40% at both sites and caused an increased accumulation of organic C and total N of about 30% in the sandy loam and about 13% in the silty loam soil. Zinc, at about the same concentration, decreased the biomass by about 40% in the sandy loam and 30% in the silty loam soil while soil organic matter accumulation increased by only 9-14%. Cadmium, at about twice current EC limits did not affect the amount of biomass or soil organic matter in the silty loam soil. Similarly, neither were affected by Ni at 2-3 times current metal limits. The amount of microbial biomass C as a percentage of total soil organic C was much lower (< 1.0%) in soils contaminated with Zn and Cu at about two-and-a-half times current permitted limits than in soils containing less metal. This also suggested that the metals were causing decreased microbial biomass at these metal concentrations.