Abstract:In order to explore the effects of combined application of straw and nitrogen fertilizer on microbial activity and soil aggregates in sandy fluvo aquic soil, the effects of the different amounts of nitrogen fertilizer and combined application ratios of straw on organic matter, soil microbial biomass carbon, nitrogen content, soil enzyme activity and soil aggregate in sandy fluvo aquic soil were studied by using the method of indoor culture and controlling urea amounts under the condition of quantitative straw. The results showed that compared with CK, straw combined with proper amount of nitrogen fertilizer could significantly improve soil organic matter content (29.04%~41.90%), microbial biomass carbon content (53.34%~90.33%), nitrogen content (25.41%~38.67%), soil urease activity (35.29%~44.12%), polyphenol oxidase activity (36.67%~53.33%) and the proportion of large aggregates (> 0.25 mm)(8.81%~65.22%), and enhance the stability of aggregates (25.89%~91.96%)(P < 0.05). When the amount of urea was 0.213 g/kg, it was most conducive to increase soil microbial carbon content, the proportion of large particle aggregates (> 0.25 mm) and improve the stability of aggregates. When the amount of urea was 0.086 g/kg, the soil organic carbon and soil microbial nitrogen contents were significantly affected. Correlation analysis showed that soil organic carbon, microbial biomass carbon, nitrogen content enzyme activities were closely related to WMD and GMD, showing a positive correlation. In conclusion, straw returning combined with nitrogen fertilizer could improve soil nutrient content and soil physical properties in sandy fluvo aquic soil. The research results could provide a theoretical basis for optimizing straw returning technology and improving the fertility of sandy fluvo aquic soil.