Abstract:Based on analysis of the variation of 210Pbex inventory in soil of cultivated land in response to changes in soil erosion rate, mistakes in calculation of soil erosion rate of cultivated land using the 210Pbex method are discussed. A model is established to fit response of 210Pbex inventory to variation of soil erosion rate in soil of cultivated land, based on the principle of mass balance, demonstrating that the long-commonly-held assumption that what is determined with the 137Cs method is the mean soil erosion rate of the past 40 a or 50 a and with the 210Pbex method the mean of a longer past of 100 a or 200 a is incorrect. The model is used to fit response of 210Pbex inventory to the variation of soil erosion rate from 0.06 cm a-1 to 0.6 cm a-1 and from 0.6 cm a-1 to 0.06 cm a-1. The results indicate that 210Pbex inventory responds quickly, which rises or reduce exponentially with time, but negatively with soil erosion rate. Consequently, 210Pbex can replace 137Cs in determining soil erosion rate in cultivated land not only 100 years old, but also just a few years old due to change in land use.