Abstract: A study on nitrogen dynamics in soil under grain-cotton double cropping system has been carried out by using isotope ¹⁵N from 1981 to 1984. The results showed that too much nitrogen fertilizer has been used for cotton fields in Hubei province. The amount of nitrogen fertilizers applied in the cotton fields in Hubei province was excessive and it should be controlled. Broadcasting or shallow dressing, methods should be changed to deep layer dressind in order to increase the utilization ratio of N fertilizers and to reduce pollution to the environment. Increasing the proportion of legume crops in the rotation systems could decrease the amount of N fertilizers required and at the same time, could serve as an important method of preventing nitrogen pollution within the field ecosystem.