The ¹⁴C tracing technology and isothermally sealed incubation method were used to study effect of inorganic N on mineralization of straw ¹⁴C in a combined fertilization system of mineral N and plant residue for one year.Results indicated that in non calcareous soil,application of inorganic N accelerated mineralization of straw C,but relatively strengthened the immobilization process of indigenous soil C.It might be likely that the increase in the amount of straw C mineralized compensated for the increase in the amount of indigenous C immobilized,hence,inorganic N might have only little ef ect on mineralization of total organic C in soil.The annual mineralization rate of straw ¹⁴C was greater in submerged soil than in upland soils,and the increase in the annual mineralization rate of straw ¹⁴C by inorganic N was similar either in submerged soil or in upland soils.In calcareous soil,inorganic N retarded mineralization of both straw ¹⁴C and soil native C.Promotion of organic C decomposition by inorganic N was not found in this study.This paper put forward some new views about the effect of N fertilizer on transformation of organic C,and about the role of N fertilizer in the internal cycling of soil organic C in the combined fertilization system of mineral N and plant residue.