Abstract: The nitrogen (N) transformation of crop straw during its initial decomposition in soil was simulated by Modelmaker software.A good fit was obtained between the simulated and measured data of 6 variables,including the amount of soil ammonium N,nitrate N,microbial biomass N and their¹⁵N atom %.The simulation results indicated that the main N form immobilized by soil microbes was ammonium,while the immobilization of nitrate was very small.N mineralization occurred dominantly in crop straw but very small in humus.The N mineralization of crop straw could be better described by first order than zero order kinetics.N re-mineralization occurred later than N immobilization. The assumptions on the absence of N re-mineralization or the simultaneous presence of re-mineralization and immobilization during straw decomposition led to a serious error in calculating the gross N mineralization and immobilization; while no consideration of nitrification and volatilization losses led to a low estimation of gross N mineralization.The occurrence of net N mineralization or net N immobilization depended not only on the C/N of straw,but also on the duration of straw decomposition.

Key words: Flooding, Disturbance, Interspecific relationships, Association, Co variation, Songnen Steppe