Thermodynamic characteristics of urease in four Shaanxi soils (Hapstagnic Anthrosds,Cal-Ustic Luvisols and Eum-Orthic Anthrosols) were systematically studied based on the relationship between kinetic parameters of soil urease and temperature,and classical enzymological theory.Results show that enzyme-catalyzed reaction could be divided into two steps,namely,formation of enzyme-substance complex (ES) and dissociation of the ES,of which the thermodynamic functions could thus be calculated,separately,and more information obtained of the mechanism of the enzyme-catalyzed reaction.The four soil samples differed significantly in temperature coefficient Q₁₀,activation energy Ea,enthalpy of activation ΔH,activation entropy ΔS and activation free energy ΔG,in the two steps.Q₁₀ of the soil urease ranged between 0.83 to 2.48,lower than the Q₁₀ in normal chemical reactions.The values of Ea,ΔH₁,ΔH₂ and ΔS of the soil urease in both steps were all positive except for in some individual treatments,suggesting more confusion and less ordering of the reaction,which means that more external energy is needed to complete the enzyme catalyzed reaction.The ΔG₁ in the step of ES formation and the ΔG and ΔH of the enzyme reaction were all negative,indicating that ES formation and the entire enzyme catalyzed reaction is a spontaneous,heatreleasing process.The ΔG₂ of ES dissociation was positive with no difference between samples,which means that type and fertility of the soils had minor influence on this reaction.It was also verified that the ES dissociation was a ratelimiting step of the reaction.This work provides a better understanding of the subtle differences in mechanisms of the enzyme-catalyzed reactions between different soils.