The present study was conducted to evaluate the changes in microbial biomass indices (C, N, and especially P) and in P fractions in compost amended with inorganic P fertilizers. In the non-amended control, the average contents of microbial biomass C, N, and P were 1744, 193, and 63 microg g(-1) compost, respectively. On average, 1.3% of total P was stored as microbial biomass P. The addition of KH(2)PO(4) and TSP (triple super phosphate) led to immediate significant increases in microbial biomass C, N, and P. Approximately, 4.6% of the added TSP and 5.8% of the added KH(2)PO(4) were incorporated on average into the microbial biomass throughout the incubation. Approximately, 4.7% of the 1mg and 5.8% of the 2mg addition rate were incorporated on average into the microbial biomass. In the amendment treatments, the average contents of microbial biomass C, N, and P declined by 44%, 64%, and 49%, respectively. Initially, the average size of the P fractions in the non-amended compost increased in the order (% of total P in brackets) resin P (0.7%)<NaOH-extractable P(i) (inorganic P, 3.0%)<NaOH-P(o) (organic P, 6.9%)<NaHCO(3)-P(i) (11.9%)<NaHCO(3)-P(o) (17.0%)<residual P (24.6%)<HCl-P (35.7%). Initially, the relative contributions of the P fractions in the amended compost treatments increased in the order: NaOH-P(i) (1.0%)<resin P (2.5%)<NaOH-P(o) (4.5%)<NaHCO(3)-P(o) (4.9%)<residual P (14.8%)<HCl-P (15.2%)<NaHCO(3)-P(i) (57.1%). At the end of the 56-day incubation, the largest and highly soluble fraction of NaHCO(3)-extractable P(i) had decreased in place of the less soluble fractions NaOH-extractable P(i) and P(o), but especially HCl-P, but not in place of the insoluble fraction of residual P. The microbial biomass is able to rapidly store significant amounts of easily soluble P and to prevent it from adsorption or other fixation processes.