A potexperiment was conducted to study fate of ¹⁵N-labelled ammonium sulfate incorporated into soil under the rice-wheat rotation system for 5 seasons of cropping.Residue fertilizer N from previous crops would accumulate as the cropping and fertilization practices continued.Assuming that the cropping system and soil management did not change and the total soil nitrogen content remained constant,the replacement rates of soil nitrogen by fertilizer nitrogen could be calculated based on change in residualnitrogen in the soil.By fitting the data of residualnitrogen in the soil into different mathematic equations,a best fitting equation could be picked out in light of their correlation efficiency (R²).The equation could be used conveniently to figure out the replacement rate,which is really high,accounting for 20% within only 3～5 years.The calculated replacement rate,however,could not be extrapolated into field conditions anywhere and the calculation methods presented in this paper is only applicable to similar cases where the assumptions hold water to a certain extent.The significance of replacement rate was discussed