AbstractThe mineralization of 14Cacetate or 14Cglucose was studied in aseptic subsoil samples at 10°C. At 1 μg/L and 100 μg/L, the mineralization showed first‐order kinetics preceded by a short lag phase. For the sandy subsurface soils, the half‐lives of acetate or glucose varied from 0.5 until 21 d, whereas a peat subsoil and a sandy surface soil produced half‐lives of 11 and 1.6 h, respectively.The EC50 is the toxicant concentration that causes 50 reduction of the percentage substrate mineralized at a certain incubation time and increases with time. The time‐independent toxicant concentration IC50 gives 50 inhibition of the mineralization rate and is equal to the EC50 at short incubation times. The IC50 of pentachlorophenol varied between 0.1 and 880 mg/kg subsoil. The aerobic mineralization of acetate in surface sand showed an IC50 of 28 mg pentachlorophenol/kg. Hydrochloric acid showed IC50 values ranging from pH 6.2 to pH 2.8. For chlorite and cadmium, the IC50 values were 34 and 59 mg/kg, respectively.When soils were incubated with 100 mg acetate/L, exponential growth kinetics were observed. Under these conditions the inhibitory effect of pentachlorophenol was relatively small because the resistant part of the acetate‐mineralizing microflora could grow and replace the inhibited part. The kinetics show why toxicity tests using high substrate concentrations are less sensitive than tests using more natural low substrate concentrations. The model predicts that slow‐growing microorganisms, which are vital to the functioning of the ecosystem, would recover only very slowly from the toxic effects of environme
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