Heat from fire disturbance induces chemical oxidation of soil organic matter that alters the conversion of carbon (C) and nitrogen (N), which seriously influences soil structure and functions. However, the extent of oxidation depends on the fire intensity and duration, and heat penetration. In the present study, a burned area of Pinus massoniana secondary forest located in the Gaojian State Forest Farm, Zhuzhou, Hunan Province, was selected to conduct an experiment of the effects of fire disturbance on microbial activity and N mineralization of forest soil. The experiment was conducted in a completely randomized design based on two kinds of soil, three different temperatures, and three soil water potentials, with each treatment replicated three times. A variance analysis was performed on the data. The results showed the concentration of inorganic N and initial soil organic matter content were positively correlated with fire intensity. Shortly after fire disturbance, soil C and N concentration were high, but microbial biomass C and potentially mineralized nitrogen (PMN) were low, and temperature and soil water potential had no significant effect on the basic respiration. PMN loss was observed in the unheated control soils of fire disturbance compared with soils not exposed to fire. When soil temperature reached 160℃, it resulted in only a modest increase in PMN concentrations in the soil not previously exposed to fire. If soil temperature exceeded 160℃, the PMN content fluctuated, and then increased rapidly. A soil temperature of 350℃ resulted in the destruction of 90% non-microbial tissues. The soil water potential after heating had a significant effect on N mineralization:the higher the soil water potential, the more the PMN loss. There was a positive correlation between the water content of fire disturbed soil and nitrate N. During the 14 day incubation, microbial activity and C and N mineralization were significantly influenced by soil fire history, initial heat treatment, and soil water potential. The release of soil available C and N from fire disturbance supported the recovery of microbial activity in low water potentials. When soil was heated to 380℃ at -1.5 MPa water potential, soil microbial biomass C was the highest in both soils. A negative correlation between soil water potential and soluble sugar was observed for both soils. The concentration of soluble anthrone reactive carbon (ARC) dropped significantly(P<0.05) in both soils over time, resulting from the microbial consumption of sugars released from the fire disturbance. The interaction between water potential and fire disturbance significantly affected the microbial activity and N conversion, and in low water potential soil, microbial biomass C, soluble sugar, and PMN were high. Newly formed labile N by fire disturbance was protected in soil with low water potential. Labile N remained lower in moist than in dry soils regardless of soil fire history.