Physiological And Ecological Response Of Spring Maize To Nitrogen Fertilizer And Chemical Regulation Under High Density Planting And Its Impact On Greenhouse Gas Emissions In Heilongjiang Province

Enhancing planting density is an effective method to raise maize yield.Nevertheless,maize is prone to lodging under high planting density,and the shade between plants has severely accelerated leaf senescence,which inhibit dry matter accumulation and grain filling process,and harmful to yield formation.How to coordinate the contradiction between high yield and lodging of maize under high planting density,and achieve high yield and high efficiency production of maize is an urgent problem for us to solve.Therefore,coordinating the contradiction between lodging resistance and high yield by using nitrogen fertilizer and chemical regulation and exploring the impact on nitrogen utilization and greenhouse gas emission is of great significance for high yield and high efficiency production of maize.The experiment was conduct from 2021 to 2022 at Minzhu area experimental station of Academy of Agricultural Sciences,in Harbin,Heilongjiang Province.This study used maize varieties JNK728 and SD5 as experimental material,with four nitrogen levels(0 kg·hm^-2,120 kg·hm^-2,240 kg·hm^-2 and 360 kg·hm^-2)and chemical regulation treatments（PGR:30%amine ester·ethephon,and CK:water）at high planting density(90000plants hm^-2),and explored the influences of chemical regulation and nitrogen rates on maize yield,lodging resistance,canopy photosynthesis,root morphology and bleeding sap,grain development,N use efficiency,and greenhouse gas emission,which providing a theoretical foundation for maize production with high yield and high efficiency.The main results are as below.1.Effect of nitrogen application levels and chemical regulation on leaf photosynthesis and canopy indicators under high planting densityApplying nitrogen fertilizer could enhance leaf photosynthetic capacity under high planting density,significantly increased the SPAD value,Pn,F_v/F_m,Ru BP carboxylase,and PEP carboxylase activities of leaf.But there was no significant difference between N240 and N360treatments.Chemical regulation could further improve leaf photosynthetic performance and enhance photosynthetic capacity.With the increase of nitrogen application,the population LAI increased and the canopy light transmittance decreased.Chemical regulation significantly reduced LAI in early growing stage,increased LAI in later growing stage,and improved light transmittance of the canopy population,which was conducive to improve the light interception and photosynthetic characteristics of the maize population during growth,and create favorable conditions for dry matter accumulation and yield improvement.2.Effect of nitrogen application levels and chemical regulation on stem lodging resistance characteristics under high planting densityIn terms of plant and internode morphology,the application of nitrogen fertilizer increased plant height,ear height,ear ratio,center of gravity height and the 3rd basal internode length,which increased lodging risk of maize under high density.Chemical regulation could reduce internode length,plant height,and ear height,resulting in a reduction in center of gravity height and lodging risk.The stem diameter and dry weight per unit length increased with the increase of nitrogen application,and reached the maximum under N240 treatment.Excessive nitrogen application actually decreased dry weight per unit length,and reduced internode plumpness,which lead to a decrease in lodging resistance.Chemical regulation could increase stem diameter and dry weight per unit length,thus enhancing stem plumpness.In terms of stem mechanical characteristics,with the increase of nitrogen application,internode bending strength,puncture strength and lodging resistance index increased first and then decreased under high planting density,which reached a higher level under N120 and N240treatments.Chemical regulation could further improve the internode strength and lodging resistance index under different nitrogen fertilizer levels,thus reducing the lodging rate and improving the lodging resistance.In terms of stem physicochemical characteristics,internode soluble sugar,cellulose,hemicellulose,lignin contents and lignin synthesis related enzymes activities were consistent with the change trend of stem bending strength and puncture strength,and reached a higher level under N120 and N240 treatments.Moreover,chemical regulation further increased internode soluble sugar,cellulose,hemicellulose,lignin contents and lignin synthesis related enzymes activities under different nitrogen fertilizer levels,which was beneficial to improve the lodging resistance.In terms of stem anatomical structure,internode cortex thickness,mechanical thickness,mechanical cell layers,the number and area of big and small vascular bundles also showed the same trend as the change in stem strength with the application of nitrogen fertilizer and chemical regulation,and reached a higher level under N120 and N240 nitrogen fertilizer levels and chemical regulation.3.Effect of nitrogen application levels and chemical regulation on root morphology and bleeding sap characteristics under high planting densityWith the nitrogen levels increase,the dry root weight,root surface area,root volume,and root length of maize increased first and then decreased under high planting density,which reached their maximum values under N240 treatment.Chemical regulation could further improve root morphological characteristics under different nitrogen fertilizer levels and promote root growth.The application of nitrogen fertilizer significantly increased the root bleeding sap rate,mineral nutrients and amino acids concentrations in root bleeding sap under high density,which reached the maximum value under N240 treatment.However,increased the nitrogen application excessively would cause a decrease in root bleeding sap instead of an increase.Chemical regulation could significantly increase the root bleeding sap rate,mineral nutrients and amino acids concentrations in root bleeding sap,which was conducive to improve root vitality,and create favorable conditions for increasing maize yield.4.Effect of nitrogen application levels and chemical regulation on grain formation and plant dry matter accumulation under high planting densityN240 nitrogen fertilizer level significantly increased V_max,V_m,and P of grain filling process,and increased soluble sugar and starch content,and AGPase and SSS activities in grains under high planting density,and thus increased grain weight.Chemical regulation could significantly increase V_max and V_m,and enhance AGPase and SSS activities,and promote the synthesis of soluble sugars and starch in grains at different nitrogen levels,and thus increase grain weight.The application of nitrogen fertilizer significantly increased dry matter accumulation per plant,ADMA,CPDMA,and dry matter distribution in different organs at maturity stage.However,excessive nitrogen application actually did not increase dry matter accumulation per plant,ADMA,CPDMA,and dry matter distribution in different organs.Chemical regulation significantly increased ADMA and CPMA,and increased the dry matter distribution in grains at maturity stage.5.Effect of nitrogen application levels and chemical regulation on nitrogen uptake and utilization under high planting densityN240 nitrogen fertilizer level and chemical regulation significantly increased nitrogen accumulation in plant,nitrogen translocation of vegetative organs at post-anthesis and nitrogen accumulation in different organs at maturity stage.The application of nitrogen fertilizer significantly reduced nitrogen uptake efficiency,nitrogen partial factor productivity,nitrogen agronomic efficiency and nitrogen use efficiency.Chemical regulation could promote nitrogen uptake and utilization of maize and improve nitrogen use efficiency.In addition,N240 nitrogen fertilizer level and chemical regulation significantly increased grain GS,GDH and GPT activities,promoted the nitrogen accumulation and utilization in grains,which resulting in an increase in maize yield.6.Effect of nitrogen application levels and chemical regulation on yield and greenhouse gas emission under high planting densityThe application of nitrogen fertilizer significantly increased maize yield by increasing grains number per ear and 1000-grain weight,and the highest yield was obtained at N240 nitrogen fertilizer level,which increased by 20.3%and 16.1%compared with N0 treatment in 2021 and2022,respectively.Chemical regulation significantly increased maize yield and its components under high plating density,which the yield increased by 12.0%and 10.8%compared with water treatment in 2021 and 2022,respectively.The application of nitrogen fertilizer significantly increased N₂O and CO₂ cumulative emissions,GWP,and GHGI during maize growth.Chemical regulation significantly reduced N₂O and CO₂ cumulative emissions,GWP,and GHGI.As far as the relationship between yield,lodging resistance,ca nopy photosynthetic characteristics,root morphology and bleeding characteristics,grain formation,nitrogen utilization,and greenhouse gas emission,N240 nitrogen fertilizer level and chemical regulation realized high yield and high efficiency production by enhancing lodging resistance while increasing yield.