The Effects Of Soil Fertility On Agronomic Performance And Nitrogen Use Efficiency Of Super Rice Varieties

Super rice is of the great importance to the yield improvement in rice production due to the high yield potential and high lodging resistance.It was widely recognized that amply nitrogen（N）fertilizer application and indigenous N supply（INS）were required to realize the yield potential of super rice varieties.However,there are 70%of the paddy rice fields in China with low INS and low land productivity.Meanwhile,Chinese government has issued several policies to reduce the N fertilizer application to develop an environment-friendly agricultural production system.However,the information on the response in grain yield and nitrogen use efficiency of super rice varieties to INS under low N application rate is limited.The topsoil of paddy field is an important source of nutrients for rice plant growth and development.Removing the topsoil of rice field has been reported to reduce INS to create a low soil fertility environment.Therefore,field experiments were conducted in Wuxue City of Hubei Province during 2016-2018.The experiments were arranged in a split plot design during 2016-2017 with soil fertility[low soil fertility（LF,desurfacing soil at about half of the soil depth）and high soil fertility（HF,control）treatment]as main plot and rice varieties[4 super rice varieties（CY1000,YY2640,TYHZ,and YLY6）and 4 ordinary rice varieties（HHZ,HY549,LY10,and QY6）]as sub-plot.The objective was to determine the effects of soil fertility on agronomic performance and nitrogen use efficiency of super rice under low N fertilizer input condition.In 2018,a split-split plot design was arranged with N treatments(0 and 100 kg N ha^-1)as main plot,soil fertilities（LF and HF）as sub-plot,and rice varieties（YY2640,YLY6,HHZ,and QY6）as sub-sub-plot.The objective was to compare the contribution of INS to grain yield and agronomic N use efficiency between super rice and ordinary rice varieties under different soil fertility.The main results of this study are as follows:（1）Compared with HF,grain yield was reduced by 15.9%in LF,which was mainly due to decreased panicles m^-2 and dry matter accumulation.Higher yield of super rice varieties than ordinary rice varieties was observed in two soil fertility treatments.Higher yield of super rice was primarily attributed to higher spikelets m^-2,dry matter accumulation,and harvest index.The yield advantage of super rice varieties over ordinary rice varieties was reduced with the decrease in soil fertility and N application rate.Compared with ordinary rice varieties,the pre-heading dry matter translocation and the efficiency of dry matter translocation of super rice varieties was decreased by 3.5%and 18.7%,respectively.However,the spikelets-to-leaf area ratio and dry matter production during ripening of super rice varieties was increased by 18.5%and 14.5%,respectively,relative to ordinary rice varieties,which contributed largely to the yield advantage of super rice.（2）The total N uptake in LF was 27.4%lower than that in HF.Specifically,the N uptake from mid-tillering to panicle initiation in LF was reduced by 57.8%compared to that in HF.The substantial decrease in N accumulation during this period resulted in decreased panicles m^-2,spikelets m^-2,biomass production,and grain yield in LF compared to HF.Higher yield of super rice varieties was mainly due to higher N accumulation and N use efficiency for grain production as compared with ordinary rice varieties.It was noted that the pre-heading nitrogen translocation and efficiency of nitrogen translocation of YLY6（super rice variety）were much higher than those of ordinary rice varieties,resulting in substantially higher grain yield and N use efficiency for grain production in YLY6.（3）The contribution of INS to total N uptake was 60.3%in LF and 64.2%in HF,while the N from N fertilizer contributed less than 40.0%to total N uptake in the two soil fertility treatments.The contribution of INS to grain yield was 65.1%and 81.7%in LF and HF,respectively.The INS contributed substantially higher to total crop N uptake as compared with N fertilizer,and the contribution of INS increased with the increase of soil fertility.There was no significant difference in crop N uptake from INS between super rice and ordinary rice varieties,which was consistent in the two soil fertility treatments.The average crop N uptake from N fertilizer of super rice varieties was 25.0%higher in HF but 18.0%lower in LF than ordinary rice varieties.Additionally,the agronomic N use efficiency of super rice varieties was 13.2%and 47.8%higher than that of ordinary rice varieties in LF and HF,respectively.This was mainly attributed to higher N accumulation during ripening of super rice than ordinary rice varieties.Overall,grain yield of super rice varieties was superior to ordinary rice varieties even under low soil fertility,although the yield difference between super rice and ordinary rice varieties in LF was lower than that in HF.This study suggested that super rice varieties are vital to improving rice yield and N use efficiency in rice production.Furthermore,improving INS would be beneficial for increasing crop productivity and N use efficiency of super rice varieties in low soil fertility environment with the goal of promoting the sustainable development of rice production.