An isotope (15N) labelling technique was used to estimate the turnover of soil nitrogen through microbial biomass in red soils. The turnover times of soil nitrogen in microbial biomass were 251, 89,and 63 days, for the long-term cultivated red clayey soil (30 years of tea orchard),mid-term cultivated red clayey soil (4 years citrus orchard) and short-term cultivated red sandy soil (3 years upland),respectively. The annual fluxes of soil nitrogen through microbial biomass were 1.5 to 6 times of the microbial biomass nitrogen itself. The microbial turnover rate of soil nitrogen was closely related to the soil organic matter, and it was higher in sandy soil than in clayey soil. Obviously, microbial biomass turnover of soil nitrogen plays an important role in the availability, retaining and accumulation of soil nitrogen in red soils.