Kinetics of soil dehydrogenase in response to exogenous Cd toxicity

Highlights: • pH explained 30–45% of the dehydrogenase activity (DHA), V_max, and K_m variations across soils. • Different inhibition mechanism of Cd to DHA varied soil types. • Soil properties and inhibition constant affect the toxicity of Cd. • Reaction constant (k) could indicate sensitively the toxicity of Cd to DHA. - Abstract: Soil dehydrogenase plays a role in the biological oxidation of soil organic matter and can be considered a good measure of the change of microbial oxidative activity under environmental pollutions. However, the kinetic characteristic of soil dehydrogenase under heavy metal stresses has not been investigated thoroughly. In this study, we characterized the kinetic characteristic of soil dehydrogenase in 14 soil types, and investigated how kinetic parameters changed under spiked with different concentrations of cadmium (Cd). The results showed that the K_m and V_max values of soil dehydrogenase was among 1.4–7.3 mM and 15.9–235.2 μM h⁻¹ in uncontaminated soils, respectively. In latosolic red soil and brown soil, the inhibitory kinetic mechanism of Cd to soil dehydrogenase was anticompetitive inhibition with inhibition constants (K_i) of 12 and 4.7 mM, respectively; in other soils belonged to linear mixed inhibition, the values of K_i were between 0.7–4.2 mM. Soil total organic carbon and K_i were the major factors affecting the toxicity of Cd to dehydrogenase activity. In addition, the velocity constant (k) was more sensitive to Cd contamination compared to V_max and K_m, which was established as an early indicator of gross changes in soil microbial oxidative activity caused by Cd contamination.