Different sources of heavy metals and their long-term effects on soil microbial properties

Abstract.

We analysed soils of Lower Saxony, Germany, variously contaminated with sewage sludge, sediment, and dump material from mining and factory residues, to assess the effects of different sources of heavy metals and different metal species (total, EDTA and NH₄NO₃ extractable) on activity (basal respiration), biomass (microbial C, adenylates) and community structure (ergosterol) of soil microorganisms in relation to important chemical soil properties. The contents of biomass C, adenylates and ergosterol did not necessarily decrease with increasing heavy metal content, reflecting the importance of other environmental factors, e.g. differences in C input. The biomass C/soil C ratio ranged from 0.19 to 2.10% and decreased in the order: sludge>sediment>dump contamination, in accordance with the increasing heavy metal load. The ergosterol/biomass C ratio, which in contrast increased in the orders/sludge<sediment<dump contamination, varied between 0.45% and 3.10% and was on average 1.09%, i.e. it was much higher than in uncontaminated soils, indicating a change in community structure towards fungi. This ergosterol/biomass C ratio was most markedly affected by the concentration of NH₄NO₃-extractable Pb. The adenylate energy charge, ranging from 0.49 to 0.79, was mainly affected by the three Cu species and showed a strong positive correlation with the metabolic quotient qCO₂. The average ATP/biomass C ratio was 7.9 µmol g^–1 biomass C. It was the only ratio that remained unaffected by any of the soil properties or heavy metal species.