Comparison of different microbial biomass and activity measurement methods in metal-contaminated soils

The aims of this study were: (1) to compare different microbial methods of detecting the effects of heavy metals on the functioning of the soil ecosystem; and (2) to evaluate the effect of incubation on microbial biomass and microbial activity in soils that were not pre-incubated after sampling in order to determine their suitability for measuring the effects of heavy metals on the soil microbial ecosystem. The microbial biomass methods (included: biomass C, N and ninhydrin-N by fumigation-extraction (FE); substrate-induced respiration (SIR); soil ATP content and microbial activity as evolved CO2-C and arginine ammonification. All were tested in soils from the Woburn Market Garden Experiment. Due to past sludge application the soils contained, Zn, Cu or Ni at around current European Union upper limits and Cd at up to three times the limit. The amount of microbial biomass in metal-contaminated soils was about half of that found in soils from the experiment that received uncontaminated organic manure or inorganic fertilizer. The amount of biomass measured by FE and soil ATP content in incubated soils showed little change over 20 days incubation. However, SIR measurements were statistically affected over the first few days of incubation. The rates of arginine ammonification were higher in this order: farmyard manure (FYM) > inorganic fertilizer > sewage-sludge throughout the incubation. However, the evolved CO2-C rates were not significantly different among the treatments. Discriminant analysis confirmed smaller amounts of biomass in the metal-contaminated soils than in the other treatments. Linked properties, such as relationships between biomass and soil organic matter, or biomass-specific respiration rates, may provide “internal control” which may help overcome problems of establishing suitable control, or comparative measurements, when moving from experimental to natural environments.