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Microbial Respiration and Chemical Speciation in Metal-Amended Soils¹

ABSTRACT

Soil microbial respiration was measured on five soils after treatment with metal salt additions (0.0–50 mmol kg^–1 metal to dry wt soil) to evaluate the effect of metal toxicity. After 45 d incubation at 20°C, most lower level metal treatments (0.05 and 0.5 mmol kg^–1) had little effect on soil microbial respiration, whereas the higher levels (5 and 50 mmol kg^–1) sharply inhibited respiration, suggesting a general threshold for metal toxicity between the 0.5 and 5 mmol kg^–1 metal.

GEOCHEM, an equilibrium thermodynamic computer model, was used to simulate the chemical speciation of Cu and Cd in two soils. Values for the simulated concentration of the "free" Cd²⁺ species at which 5–10% inhibition of soil respiration occurred (10 µM) were comparable with those measured in pure bacterial culture fluids. Because of adsorption and complexation by the organic soil, more total Cd was required to achieve an equivalent Cd²⁺ concentration value in the simulated soil solution. The model of simulated Cd species distribution was supported by the fact that equal Cd amendments to both soils resulted in less growth inhibition in the organic soil. When Cd contamination of the two soils was near the toxic threshold level, relatively small increases in soil acidity could markedly increase Cd²⁺ concentration in the soil solution.

Simulated Cu species distribution for the organic soil correlated with inhibition of respiration. Dissolved Cu was predicted to inhibit microbial respiration when the free ion acidity reached a value of 0.01–0.1 µM in the soil solution.

Key Words: soil microorganisms • soil solution speciation • trace and heavy metals • GEOCHEM • Cu and Cd toxicity

¹ Corvallis Environmental Research Laboratory, USEPA, Corvallis, OR 97333; Oregon Agricultural Experiment Station, Oregon State University, Corvallis, OR 97331. Technical paper no. 6898.

² Microbial Ecologist, Corvallis Environ. Res. Lab., USEPA, Corvallis, Oreg., and Assistant Professor and Professor, respectively, Dep. of Soil Sci., Oregon State Univ., Corvallis.

Received for publication September 27, 1982.

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