[Simulation of nitrogen mineralization and immobilization of crop straw during its initial decomposition in soil]

The nitrogen (N) transformation of crop straw during its initial decomposition in soil was simulated by Model-maker 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 their 15N 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.