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 mu g g(-1) compost, respectively. On average, 1.3% of total P was stored as microbial biomass P. The addition of KH2PO4 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 KH2PO4 were incorporated on average into the microbial biomass throughout the incubation. Approximately, 4.7% of the 1 mg and 5.8% of the 2 mg 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%) < NaHCO3-P-i (11.9%) < NaHCO3-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%) < NaHCO3-P-o (4.9%) < residual P (14.8%) < HCl-P (15.2%) < NaHCO3-P-i (57.1 %). At the end of the 56-day incubation, the largest and highly soluble fraction of NaHCO3-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. (C) 2008 Elsevier Ltd. All rights reserved.