Changes in the microbial biomass C(MB-C) and P(MB-P),Olsen-P,and fixed inorganic P fractions in red-earth soil under flood-drought cultivation were invest igated after amendment of organic materials(glucose and rice straw) and incubation at 25℃ and 100% humidity.Results showed that soil MB-C increased by 215%,74% and 163%,respect ively,by 3 days for the treatments of adding glucose(G5,at 5 g kg^-1 soil) and straw(S5,at 5 g kg^-1 soil;S10,at 10 g kg^-1 soil).Thereafter,soil MB-C in Treatments G5 and S10 underwent a decreasing process for a period of 3~14 days and 3~7 days,respectively,and then leveled off,but in Treatment S5,it remained relatively constant till the end of the incubation.However,soil MB-P in the former increased considerably during the early phase(by 3 or 7 days),and followed by a stable trend throughout the remaining incubation period.Soil MB-P in Treatment S5 remained relat ively constant during the entire incubation period,and by the end was about 2 times greater than that in the control(without amendment) as soil MB-P in it decreased significantly during the period.A significant decrease(3.5 mg kg^-1 soil) in Olsen-P occurred in Treatment G5 during the first 3 days.Differences between the content of Olsen-P in Treatments S5 and S10 and that in the control were limited(generally under 1 mg kg^-1 soil).It was found that,by the end of the 43 d incubation,the contents of A-l and Fe-bonded P,and total fixed inorganic P in the soil of Treatment G5 were significantly lower than those in the control.It was concluded that the amendment of organic materials enhanced the microbial biomass and MB-P in the soil under flood-drought cultivation,and consequently act ivated some fixed inorganic P(mainly Fe-bonded P) for utilization.A further analysis of the data obtained suggests that a considerable part(over 30% ) of the P used by the microbial biomass was transformed into organic P pool,and that P released from decomposing straws could preferentially be used by microbial biomass and generally filled up its need.Thus,the influence of straw incorporation on soil P availability may be limited.