Effects Of Nitrogen Management On Grain Yield And Nitrogen Use Of Double Cropping Rice System With All Rice Straw Returned To The Field

China is a big producer and consumer of nitrogen fertilizer, the application of nitrogen fertilizer has played a tremendous role in promoting China’s food security. However, in addition to crop uptake and soil residues, there also has a variety of ways of loss when a large amount of nitrogen fertilizer is applied into the soil, which can not only reduce the nitrogen fertilizer efficiency, but also lead to a series of environmental problems, such as water eutrophication. Studies have shown that seasonal average nitrogen recovery was 32% in China, especially in high-yielding field, a large amount of nitrogen fertilizer application is often associated with lower utilization rate of nitrogen fertilizer, which causing huge losses of nitrogen fertilizer. But reduction of nitrogen fertilizer application may lead to a significant reduction in crop yield. Along with the increase of crop yield, the yield of straw also increased rapidly. And the annual output of crop straws in China is over 8×108t. Crop straw contains a large number of mineral nutrients, therefore straw returning not only can alleviate the pressure of straw itself of to the environment, but also can improve soil fertility. However, considerable change of effective nitrogen supply dynamic occurs in the soil with all crop straw returning, which may affect crop yield and nitrogen efficiency. In this study, a long-term field experiment was conducted to investigate the effects of nitrogen management on yield and nitrogen utilization in double cropping rice system with full rice straw returning, in order to formulate nitrogen application strategy for early and late rice. The results obtained showed as follows:(1) The effects of nitrogen management on grain yield of double cropping rice system with all rice straw returned to the field was studied. The results showed that, under full returning of rice straw, recommended N (SRN) treatments reduced grain yield of early and late rice, but no significant yield difference between SRN and SFN treatments was found (p>0.05). Meanwhile, grain yield of late rice with SRN treatments was more stable than that of early rice with the same treatments. In conclusion, with full returning of rice straw, moderate nitrogen fertilizer reduction and higher topdressing ratio can improve the grain yield of early rice and panicle, grains and grain yield of late rice without significant yield loss in double rice cropping system. Microbial inoculant may help to stabilize rice yield.(2) Under full returning of rice straw, recommended N (SRN) treatments reduced nitrogen accumulation of early and late rice, but the difference of nitrogen accumulation between SRN and SFN treatments was not significant(p>0.05). However, with full returning of rice straw, moderate nitrogen fertilizer reduction and higher topdressing ratio can improve both nitrogen accumulation and nitrogen recovery of early and late rice. Microbial inoculant may help to stabilize rice nitrogen recovery.(3) Under full returning of rice straw, recommended N (SRN) treatments reduced the content of soil organic matter, available P, available K, total N, nitrate N and fixed ammonium. Moderate nitrogen fertilizer reduction and lower topdressing ratio can improve the content of soil available P, available K and ammonium N, and higher topdressing ratio can elevate the content of soil organic matter and total N. Microbial inoculant may help to improve soil fertility except nitrate N and fixed ammonium. The fixation and release of fixed ammonium in early growth stage of rice had important effects on rice yield and nitrogen use efficiency.(4) Under full returning of rice straw, recommended N (SRN) treatments reduced the content of soil urease and microbial biomass carbon and nitrogen. Moderate nitrogen fertilizer reduction and lower topdressing ratio can improve soil urease and microbial biomass carbon of early growth stage of rice, and reduce soil urease and microbial biomass carbon of middle and late growth stage of rice. Microbial inoculant may help to improve the content of soil urease and microbial biomass carbon and nitrogen. Also this study found that during the rice growing period, soil microbial activity was closely related to grain yield of rice.