Physiological And Ecological Mechanism Of Optimized Nitrogen Application Promoting Nitrogen Utilization And Increasing Rice Yield Under Paddy-upland Rotation

Increase in the crop yield per unit area and rational application of nitrogen fertilizer to increase nitrogen utilization efficiency,is the key to guarantee food security and sustainable development of agriculture.Rice-based cropping systems,especially paddy-upland rotation,play an important role in agricultural production,implementation of which is an effective way to increase crop yield per unit area.In this study,morphogenesis,structure changes,matter accumulation,nitrogen utilization,enzyme activity,gene expression level,and yield formation of rice plants were surveyed,and the ¹⁵N isotope was used in both rice and dry land crop growth seasons,to study（I）the regularity of grain yield formation,nitrogen uptake and utilization of indica hybrid rice;（II）the mechanism of optimized nitrogen management increasing nitrogen utilization efficiency and yield in indica hybrid rice under paddy-upland rotation;and（III）the continuation effects of residual nitrogen in previous crop,which would provide the theoretical basis for the high yield and high efficiency of rice production.The main research results are as follows:1.Increasing in rice yield and nitrogen use efficiency of optimized nitrogen application:Rice yield of optimized nitrogen management was 9.45%-12.41%and 6.28%higher than that of the traditional nitrogen application under normal（180kg/hm²）and low（90kg/hm²）nitrogen application condition,respectively.Nitrogen harvest index,partial factor production,agronomic efficiency,and recovery efficiency for applied nitrogen of optimized nitrogen management were all improved.The recovery efficiency is the most important index to evaluate the utilization of nitrogen,which was 71.7%and 110.2%higher with optimized nitrogen management in the year of 2014 and 2015 respectively,and the difference were significant.2.Mechanism of optimized nitrogen application increasing rice yield:Dry matter production after heading was the main source for grain filling of hybrid indica rice Fyou,the yield contribution rate was higher than 64%.Under the condition of optimized nitrogen management,the appearance of ineffective tillers decreased,the growth of superior tillers improved in the early growth stage,which laid a good foundation for the matter production in the later stage.Meanwhile,compared with the traditional nitrogen application,the leaf area and net photosynthetic rate of the three upper leaves of optimized nitrogen management improved greatly in the late growth stage,and leaves senescence also delayed to some extent,which made the matter productivity in the late growth stage raised by optimized nitrogen management.The diameter of the neck-panicle node,the number of big and small vascular bundles of optimized nitrogen management was 12.2%,19.1%,and10.1%higher that of traditional nitrogen application.The good configuration of the neck-panicle node was conducive to promoting the matters transport to panicle.Sufficient supply of matters promoted the formation of big panicles.Under the optimized nitrogen management condition,the panicle length increased by 5.36%,the number of secondary branches increased by 20.5%and the difference was significant,and the number of spikelets and filled grains were also increased,compared with the traditional nitrogen application.The expression levels of genes related to the branches growth,MOC1 in the secondary branch and spikelets differentiation stage（T3）,LAX1 in the first bud differentiation stage（T1）and the primary branch differentiation stage（T2）,were all significantly higher than in traditional nitrogen application;the expression levels of genes related to the panicle size（SP1）and the number of grains（GS1;1）were significantly higher than that of the traditional nitrogenous treatment in two critical stages of young panicle differentiation.Therefore,under the optimal condition,the panicle type was larger and there were more branches and spikelets on each panicle,and the number of grains per panicle increased by 7.3%-19.6%,which was the main reason for increasing in rice yield under optimized nitrogen application.3.Mechanism of optimized nitrogen application increasing nitrogen use efficiency:The results of ¹⁵N isotope tracer test showed that the recovery efficiency of base and tillering nitrogen was as low as about 22%in both nitrogen applied treatments,most of the nitrogen absorbed by rice plant was from soil nitrogen.Reduction the application rate of nitrogen fertilizer in this stage of optimized nitrogen management improved the absorption of soil nitrogen by plant,which accounted for as much as 79.12%of total nitrogen uptake in the vegetative growth stage.This not only satisfied the growth demand of the plant,but also reduced the nitrogen loss significantly.The peak period of nitrogen uptake by rice plants was from jointing to heading stage,the recovery efficiency of nitrogen applied at jointing stage and 15-20 days after jointing stage was as high as 61.2%and 73.2%,respectively,which was the key stage to improve the nitrogen utilization rate of optimized nitrogen management.AMT1;1 is the key gene to regulate absorption of nitrogen.Under the condition of optimized nitrogen management,nitrogen applied at young panicle differentiation stage increased the expression level of AMT1;1 in the root at the jointing stage and 15-20 days after jointing stage,improved the absorption of nitrogen by plants;the increase in nitrogen level of the plant under optimized nitrogen management condition significantly improved the activity and expression of the key enzymes（GS and GOGAT）associated with nitrogen activation and translocation,compared with the traditional nitrogen application.After heading,relatively few nitrogen could be absorbed by rice plants,which only accounted for about 17%of the total nitrogen uptake,therefore,nitrogen transport was the main nitrogen behavior after heading.Plants with the traditional nitrogen application treatment without nitrogen applied at young panicle differentiation stage made the leaf senescence more faster than the optimized nitrogen application after heading stage,the related enzymes activity（GS,GOGAT）and genes expression level（GS2,NADH-GOGAT1,Fd-GOGAT1）were all higher in traditional nitrogen application treatment.While,the gene of GS1;1 was also closely related to the number of grains,its expression level was still higher in optimized nitrogen management treatment after heading stage,because of the benefit of the optimized nitrogen management for the formation of big panicles.At the middle stage of grain filling（15 days after heading）,the activities of GS and GOGAT with optimized nitrogen management were all higher than that with traditional nitrogen application.Combined with the aging process of leaves indicated that the translocation of nitrogen was delayed under optimized nitrogen management condition,which was mainly occurred in the late stage of grain filling,but the total amount of nitrogen translocation was also higher in optimized nitrogen management treatment.4.Environmental compensation effect and production guidance:The nitrogen supply capacity of soil decreased gradually with the advance of the growth process.Soil nitrogen content decreased after one rice growth season,the more the nitrogen accumulated by rice plants,the more the total nitrogen of soil reduced.It is clear that the environmental compensation effect does exist.Under the paddy-upland rotation condition,the soil nitrogen content changed with the dry land crop species.The amount of residual nitrogen in the soil after the harvest of garlic was 1.6 times as much as that after wheat,which could provide more nitrogen for rice production.As a result,even with the same nitrogen management in the rice growth season,rice yield under garlic-rice rotation was10.29%-25.52%higher than that under wheat-rice rotation,and the total nitrogen absorbed by rice plants with garlic-rice rotation was 11.7%higher than the rice with wheat-rice rotation,the proportion of residual ¹⁵N absorption in rice season was as high as 24.0%-38.9%in garlic-rice rotation,and that in wheat-rice rotation was only 8.3-10.7%,which led to the improvement in nitrogen utilization efficiency in paddy under garlic-rice system.Therefore,under the condition of paddy-upland rotation,consider of the residual nitrogen in the previous crop fully,reduce nitrogen unput in early rice growth stage,and use the environmental compensation effect rationally to achieve the unification of high yield,high efficiency and soil fertility maintenance.In the Chengdu Plain,the nitrogen application rate of base and tillering fertilizer in paddy could be reduced to 90 kg/ha under wheat-rice crop system,while under garlic-rice crop system,the nitrogen application rate of base and tillering fertilizer in paddy could be further reduced on this basis.