The turnover of the
microbial biomass plays key roles in the cycling of organic matter and nutrients in soil. Although the
microbial biomass-C is generally accounted for only 1%～4% of total soil organic, the balance between the rate of
microbial processes determines the turnover and consequently the accumulation of soil organic matter and nutrients (e.g. N, P and S). In this study, the procedure for
measuring the turnover time of soil microbial biomass-C has been developed. 14 C- labelled glucose was used to label the biomass-C by incubating soils at 25℃ and 100% humidity. The labelled biomass-C, total biomass-C, and extractable-C were measured by every 20 days during a 100-day incubation period, using the fumigation-extraction method. Data obtained were then used to calculate the turnover rate constant of biomass-C, and hence its turnover time. Under the incubation conditions, the turnover time of biomass-C, calculated from the gross turnover rate constant of labelled biomass-C, varied between 93 and 400 days. This is equivalent, by extrapolation, to 1.0～4.1 years under the field conditions where soils were from. It was demonstrated that the turnover time of soil biomass-C was largely affected by soil texture but little by management. Measuring the turnover times of soil biomass-C provides a good indication of microbial activities in relating to the turnover and accumulation of soil organic matter.