Abstract: Cadmium (Cd) is ubiquitous in the human environment and has toxic effect on soil microbial biomass or its activity, including microbial biomass carbon (C_mic), dehydrogenase activity (DHA) and basal respiration (BR), etc., C_mic, DHA, BR were used as bioindicators of the toxic effect of Cd in soil. This study was conducted to determine the effects of Cd on soil microbial biomass and its activity in a paddy soil. The inhibition of microbial biomass and its activity by different Cd concentrations was described by the kinetic model (M1) and the sigmoid dose-response model (M2) in order to calculate three ecological doses of Cd: ED₅₀, ED₁₀ and ED₅. Results showed that M2 was better fit than M1 for describing the ecological toxicity dose effect of cadmium on soil microbial biomass and its activity in a paddy soil. M2 for ED values (mg/kg soil) of C_mic, DHA, BR best fitted the measured paddy soil bioindicators. M2 showed that all ED values (mg/kg) increased in turn with increased incubation time. ED₅₀, ED₁₀ and ED₅ of C_mic with M2 were increased in turn from 403.2, 141.1, 100.4 to 1000.7, 230.9, 144.8, respectively, after 10 d to 60 d of incubation. ED₅₀, ED₁₀ and ED₅ of DHA with M2 increased in turn from 67.6, 6.2, 1.5 to 101.1, 50.9, 41.0, respectively, after 10 d to 60 d of incubation. ED₅₀, ED₁₀ and ED₅ of BR with M2 increased in turn from 149.7, 6.5, 1.8 to 156.5, 50.8, 35.5, respectively, after 10 d to 60 d of incubation. So the ecological dose increased in turn with increased incubation time for M2 showed that toxicity of cadmium to soil microbial biomass and its activity was decreased with increased incubation time.

Key words: Cadmium, Soil microbial biomass, Basal respiration, Dehydrogenase activity, Ecological dose, Paddy soil

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