Study On The Nitrogen Management Strategies For Double Cropping Rice System With All Rice Straw Returned To The Field

Rice is the main food crop which is widely cultivated and hugely demended in China.As an important role in rice production,the double cropping rice system in southern China ensures country's long-term grain security and meets the needs of population growth.Farmers usually increase nitrogen fertilizer blindly to increase rice yield,leading to serious waste of fertilizer resources and environmental concerns.However,reducing nitrogen fertilizer may cause the risk of rice yield decline.Therefore,nitrogen management will become an important measure in rice production.With the increase of crop yield,the yield of straw also increase constantly.Straw yield of China accounts for about 30%of the total straw yield of the world at present.Such a huge amount of straw has caused great pressure on the environment that the rational utilization of straw resources has become an urgent requirement in environmental protection.Straw rich in many nutrients needed for crop growth returned to farmland as a fertilizer can not only solve the problem of straw resources,but also improve soil fertility and reduce the amount of chemical fertilizers.A large amount of straw returned to field will cause significant effect on soil fertility.Therefore,it is of great significance to develop appropriate nitrogen management strategies to stabilize rice yield and improve the utilization of nitrogen.In this study,based on the long-term field experiment of double cropping rice system with full rice straw returning,the differences of rice yield and nitrogen utilization under different nitrogen management strategies(including N rate and its distribution between basal and topdressing,and the application of microbial inoculant)was analysed to elucidate the long-term effect of rice straw returning and nitrogen management on rice yield and N utilization.The results obtained showed as follows:(1)After 3-4 years successive full returning of rice straw,compared to treatment S with no N applied,treatment N(SFN?SRN)elevated the content of soil organic matter and urease activity;While these fertility factors declined in reduced N treatemnts.Microbial inoculant helped to increase the content of soil organic matter,available P,available K and urease activity.(2)Under full returning of rice straw,compared with treatment S,treatment N(SFN?SRN)increaed the content of soil total N,exchangeable ammonium,nitrate N,fixed ammonium and MBN(microbial biomass nitrogen);Reducing the amount of nitrogen fertilizer decreased the contents of these forms of soil N.Microbial inoculant helped to improve the content of soil fixed ammonium and MBN and decrease the content of soil total N,exchangeable ammonium and nitrate N.In recommended N(SRN)treatments,the N treatments with higher topdressing ratio improved the content of soil total N,nitrate N,fixed ammonium and MBN but decreased the content of soil exchangeable ammonium.The content of soil fixed ammonium and MBN both showed the decreasing trend during late rice growth.(3)Compared with treatment S,treatment N(SFN?SRN)improved the grain yield of double cropping of rice;Reducing the amount of nitrogen fertilizer did not significantly decrease the grain yield of double cropping of rice indistinctively except treatment SRNs8:2;In recommended N(SRN)treatments,the treatment of higher topdressing ratio nitrogen fertilizer improved the number of spikelets per panicle and seed-setting rate,which led to increase of grain yield.Microbial inoculant helped to improve the grain yield of double cropping of rice.In four year of field experiment from 2013 to 2016,the grain yield of early rice decreased first and then increased continuously for two years;The grain yield of late rice was increased for three consecutive years and decreased slightly in 2016.Nitrogen recovery of early rice increased first and then decreased and nitrogen recovery of late rice showed a decreasing trend in the early year,and then tended to be stable.(4)Compared with treatment S,treatment N(SFN?SRN)improved the nitrogen accumulation of double cropping of rice.Reducing the amount of nitrogen fertilizer decreased the nitrogen accumulation of early rice significantly.In recommended N(SRN)treatments,the treatments with higher topdressing ratio improved the nitrogen accumulation of double cropping of rice.Compared with treatment S,moderate nitrogen fertilizer reduction improved nitrogen use efficiency for grain production(NUEg)and nitrogen harvest index(NHI)of double cropping of rice;Moderate nitrogen fertilizer reduction and higher topdressing ratio improved both agronomic efficiency of nitrogen fertilizer and nitrogen recovery compared with treatment SFN.Microbial inoculant helped to improve nitrogen accumulation,agronomic efficiency of nitrogen fertilizer and nitrogen recovery and decrease NUEg and NHI.(5)In this study,there was a negative corelation between N accumulation of late rice and changing rate of fixed ammonium during heading to maturing stage(r=-0.414*);while N accumulation was negativle corelated with both changing rate and amount of fixed ammonium during whole growth period of late rice(r=-0.512*,r=-0.505*,respectively).N recovery of late rice was negatively corelated with changing rate and amount of MBN during transplanting to heading stage(r=-0.655**,r=-0.596**,respectively),and negatively corelated with total changing amount of MBN during whole growth period of late rice(r=-0.539**).These results indicated that soil fixed ammonia and microbial nitrogen can regulate soil N supply during rice growth.In summary,reduced N with proper topdressing ratio(SRN4:6 in this study)can achive relatively high yield and N use efficiency in double rice system with full straw returned,and such effect could be enhanced by applying microbial inoculant.