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In order to define the optimum application rate of nitrogen fertilizer under the condition of straw returning in Yanghuang Irrigation Area of Ningxia,，the treatment with no straw returning (N0) as the control,，the effects of different nitrogen application rates (0kg/hm2 (N0)，150kg/hm2 (N1),，300kg/hm2(N2) and 450kg/hm2(N3)) on soil organic carbon and nitrogen,，microbial biomass carbon and nitrogen content，soil enzyme activity,，and maize growth were investigated．The results showed that the straw returning with an optimum application rate of nitrogen was useful for increasing soil organic carbon and total nitrogen at 0~40cm layer,，and the contents of soil organic carbon，total nitrogen with N2 and N3 were significantly increased,，compared with N0．The ration of carbon to nitrogen was decreased with the increase of application rate of nitrogen under the condition of straw returning．The treatment with straw returning and nitrogen application significantly increase the soil microbial biomass carbon and nitrogen compared with straw returning and no nitrogen fertilizer treatment,，and the ratio of microbial biomass carbon to microbial biomass nitrogen was decreased with the increase of nitrogen application rate．The improvement effects of soil microbial biomass carbon with N2 and soil microbial biomass nitrogen with N3 were the most significant．Straw returning with nitrogen application could significantly improve the activities of soil urease，catalane,，alkaliphosphata,，and sucrase，and the improvement effect of soil enzyme activities with N2 and N3 were the best．The effect of N2 on maize growth at middleandlate stages were great,，while it had no effects on maize growth at early stage．The maize grain yield and water use efficiency enhancement were the highest under N2．The 2year mean maize grain yields with N2 were significantly increased by 220%,，compared with N0．The 2year results indicated that straw returning with application of nitrogen can increase the soil organic carbon and total nitrogen contents，regulate the ration of carbon to nitrogen,，increase the soil microbial biomass carbon and nitrogen and enzyme activity,，thus improving maize growth and grain yield in Yanghuang Irrigation Area of Ningxia．The straw returning with application of pure nitrogen of 300kg/hm2 was the best．