Effects Of Nitrogen Management On Yield And Nitrogen Use Efficiency In Ratoon Rice

Rice ratooning has the potential to improve rice yield per unit area with higher economic profits and environmental benefits.The wide adoption of rice ratooning technology in suitable regions is an effective approach to promote sustainable rice production and ensure national food security in China.However,relatively low and unstable grain yield of ratoon crop（RC）had limited the large-scale practice of this technology.Nitrogen（N）applications at grain-filling stage of main crop（MC）for promoting bud initiation（N_B）and after harvest of MC for promoting tiller growth（N_T）are the most important crop managements for determining RC yield,and consequently their impacts on the yield formation of RC had been intensively studied.However,limited information is available on the effects of N_Band N_Tapplications on the grain yield and N use efficiency of ratoon rice under different N managements of MC（N_M）.Therefore,this study was conducted at Qichun County,Hubei Province in 2021 and 2022 using a widely planted cultivar Liangyou 6326 for ratoon rice production.In 2021,experiment was arranged in a randomized complete block design with four N_Ttreatments including treatment without N_Tapplication and 75 kg N ha^-1was applied with single,or two,or five splits with four replications.In 2022,experiment was arranged in a split-split plot design with two N_Mtreatments(0 and 180 kg N ha^-1)as main plot,two N_Btreatments as subplot(0 and 100 kg N ha^-1),and four N_Ttreatments as sub-subplot including treatment without N_Tapplication and 75 kg N ha^-1was applied with single or two splits,or 37.5 kg N ha^-1was applied with single split with four replications.This study aimed to examine the effects of different N managements on the yield and N use efficiency of MC and RC and the related mechanism by detecting grain yield and yield components,ratooning ability,SPAD value,dry matter accumulation,and N uptake.The main results are as follows:（1）Grain yield of MC were 9.56 t ha^-1and 10.63 t ha^-1in 2021 and 2022,respectively.In 2022,there is a significant difference in MC yield between N_Mtreatments.Compared with treatment without N_Mapplication,N_Mapplication significantly increased MC yield by 34.6%due to the increases in panicles m^-2and total dry weight.The increase in total dry weight at maturity from N_Mapplication was attributed to a higher leaf area index at heading.In addition,N_Bhad no significant impact on the grain yield of MC.（2）N_Mhad no significant impacts on RC yield,but N_Band N_Tsignificantly affected RC yield.Compared with treatment without N_Band N_Tapplication（CK）,applications of N_Band N_Tsignificantly increased RC yield by 11.6%-26.1%and 15.8%-33.5%,respectively,and different mechanisms in yield improvement were observed between them.The increase in RC yield was attributed to increased only panicles m^-2for N_B,but spikelets panicle^-1for N_T.Furthermore,there were no significant differences in RC yield among split N_Ttreatments under 75.0 kg N ha^-1.However,the RC yield of 37.5 kg N ha^-1was 8.8%-9.8%lower than that of 75.0 kg N ha^-1in different N_Ttreatments,mainly due to the decrease in spikelets panicle^-1.（3）Compared with treatment without N_Mapplication,N_Msignificantly increased total dry weight but decreased harvest index of RC,consequently N_Mapplication had no significant effect on RC yield.Furthermore,N_Band N_Tapplication significantly increased total dry weight of RC by 11.7%and 13.7%,respectively,compared with CK.More importantly,different mechanisms in the increase in total dry weight were observed between N_Band N_T,which was attributed to increased dry matter accumulation at pre-heading for N_Bbut at post-heading for N_T.This discrepancy was further explained by the fact that both N_Band N_Tapplications significantly increased leaf area index at heading,but only N_Tsignificantly improved the SPAD value of leaf after heading of RC.（4）N_Mexhibited profound effects on total N uptake and N harvest index in RC,and partial factor productivity of fertilizer N for ratoon rice system.Compared with treatment without N_Mapplication,N_Mapplication significantly increased total N uptake in RC by14.5%,but decreased N harvest index and partial factor productivity of fertilizer N by7.2%and 60.0%,respectively.Similarly,applications of N_Band N_Thad also significantly increased total N uptake of RC.Total N uptake of RC significantly increased by 19.5%in N_Band 43.4%in N_Tcompared with CK.However,partial factor productivity of fertilizer N significantly decreased 45.7%in N_B,whereas it was increased by 21.6%in N_Tcompared with CK,suggesting that N_Tapplication achieved a higher N use efficiency than N_Bin ratoon rice system.In conclusion,N_Mexhibited small effect on RC yield and excessive N_Minput largely reduced partial factor productivity of fertilizer N in ratoon rice.Furthermore,both N_Band N_Tapplication significantly increased RC yield,but N_Twas more effective than N_B,and N_Bapplication largely reduced partial factor productivity of fertilizer N.These results indicated that it is urgent need to reduce the amount of N_Mand N_Bfor achieving higher N use efficiency with lower N rate in ratoon rice system.In addition,split N_Tapplication did not increase RC yield under 75 kg N ha^-1,suggesting that single N_Tapplication is enough for achieving high RC yield in the study regions.