| Planting density and the amount of nitrogen fertilizer are important cultivation measures that affect the formation of maize yield and lodging.The suitable planting density and amount of nitrogen fertilizer vary according to ecological conditions.The hilly area of central Sichuan is the main maize producing area in Sichuan Province,and the phenomenon of low planting density and excessive nitrogen fertilizer is common.Therefore,it is necessary and possible to increase the density and reduce nitrogen.To this end,this study uses the maize variety Zhenghong 6 which endure high-density planting and is suitable for mechanized harvesting.Taking planting density（plant/hm2）as the main area,set medium density of 52500（D1）,medium high density 67500（D2）,high density 82500（D3）,nitrogen fertilizer level（kg/hm2）as the secondary area,and conventional nitrogen application rate of 240（N240）,reduction 25%180（N180）,reduction 50%120（N120）and no nitrogen application 0（N0）.Through a three-year split-field test,the effects of nitrogen and density interaction on summer maize population structure,lodging resistance,nitrogen uptake and utilization,yield and its composition in the central Sichuan hilly area were studied,to provide theoretical and practical basis for the densification and nitrogen reduction,high yield,lodging resistance and mechanized and efficient production of maize in this area.The main results are as follows:1.There is an extremely significant positive correlation between density （X1）nitrogen fertilizer（X2）and extinction coefficient（K）:K=0.0045+5.67×10-8X1+12.3×10-6X2(R2=0.8607**),with the increase of density and nitrogen fertilizer,the extinction coefficient of the maize group increases,and the light transmittance within the group decreases.Proper reduction of nitrogen under dense conditions can maintain a more suitable extinction coefficient,which not only ensures a certain light transmittance within the group,but also avoids ground a large amount of light leakage,so as to obtain the maximum light interception rate.2.With the increase of density,the plant height,stem thickness,compression and crushing strength of the base nodes all decreased,the ratio of length to thickness of the base nodes and the coefficient of height of the ears showed an upward trend,and the lodging and folding rate increased.With the decrease of nitrogen application,the plant height and stem thickness decreased,and the compression and crushing strength between the base nodes increased slightly and then decreased.The values were higher under the treatment of nitrogen reduction 25% （N180）.The ratio of length to thickness of the base nodes and the coefficient of height of the ears,lodging and folding ratio decreased slightly and then increased,the values were lowest under the treatment of nitrogen reduction 25%（N180）,the lodging rate under this nitrogen treatment was 11.2%lower than that under total nitrogen treatment.3.Increase planting density,maize stalk base internode material fullness,mechanical tissue and cortical thickness decreased,cellulose synthase activity and relative expression of Zm Ces A-8 gene,Ctechol-O-methyltrasfer（COMT）activity and the relative expression of Zm COMTgene decreased,the number of large and small vascular bundles decreased,the area of single largle and small vascular bundles and total aera of large and small vasculae bundles,cellulose ad lignin content were reduced.As the amount of nitrogen fertilizer decreases,the number of large vascular bundles and total vascular bundles,the area of a single large vascular bundle and its total area,and the total area of vascular bundles all decrease or increase slightly and then decrease.The difference between N180 and N240 is small,especially under medium-density and medium-high-density conditions.The mechanical tissue and cortical thickness,the number of small vascular bundles,cellulose synthase activity and the relative expression of Zm Ces A-8gene,COMT activity and Zm COMT relative gene expression,single vascular area and total area,cellulose and lignin content increased slightly and then decreased,with N180 being the highest.This effect of nitrogen fertilizer has a tendency to weaken with increasing planting density.4.With the increase of planting density（X1,plant/hm2）and nitrogen application （X2,kg/hm2）,the nitrogen accumulation of maize population（Y,kg/hm2） increased,and the regression equation of the two was Y=189.9648+0.0021X1+0.5129X2(R2=0.9386**),the effect of nitrogen on maize nitrogen accumulation at low and medium density is small at high density.Densification reduces the apparent loss and loss rate of soil nitrogen,improves the apparent utilization rate of nitrogen fertilizers,dry matter productivity and partial productivity of nitrogen （at 2018,high partial density resulted in a decrease in partial nitrogen productivity due to severe lodging）dueing to increased absorption and accumulation of nitrogen by maize.Reducing the application of nitrogen fertilizer will also reduce the apparent loss and rate of soil nitrogen,increase the apparent utilization rate of nitrogen fertilizer,dry matter productivity and partial productivity of nitrogen.That is,increasing nitrogen density can increase the utilization rate of nitrogen fertilizer and reduce the loss of nitrogen.The average utilization rate of nitrogen fertilizer in D2N180,dry matter productivity and partial productivity of nitrogen in 2018-2019 are increased by 3.9%,4.5%and 4.5%compared with D1N240.5.Although a reasonable increase in density will make the ear traits worse,it will increase maize grain yield and increase its economic benefit due to the increase of effective ears.However,excessive dense planting may reduce yield due to serious lodging.Excessive reduction of nitrogen fertilizer will affect maize growth,the ears are shortened and the yield is reduced,but the reduction of nitrogen by 25% （N180）is not significant than that of total nitrogen（N240）.The average yield in 3 years is only 3.30%decreased,and the economic benefit is increased by 2.1% （medium density and medium high density conditions averaged）due to the reduction of nitrogen fertilizer costs.In the three-year test,the output and economic benefits of D2N180 are≥D1N240,that is,proper densification can greatly offset the production loss caused by the appropriate amount of nitrogen reduction,so it is an effective measure to reduce nitrogen and increase efficiency.Under this test condition,the planting density of 67500 plants/hm2+pure nitrogen 180kg/hm2（D2N180 treatment）is suitable. |
