Effects Of Different Nitrogen Application Rates And Densities On Root Characteristics And Grain Yield Of Maize

The response mechanism of increasing planting density and rationally reducing nitrogen fertilizer application to construct nitrogen-dense optimal combination to regulate nitrogen uptake in root morphology and hormone physiology to increase yield is still unclear.Based on this,the maize variet y Xianyu 335 was selected in this study,and two planting densities that used for the common planting densit y of farmers of 67,500plants/hm²（D1）and high yield cultivation densit y of 82,500 plants/hm²（D2）,and 5 levels of nitrogen fertilizer application,the control group 0kg/hm²（N0）,160kg/hm²（N160）,which is 27%less than the conventional nitrogen application rate,conventional nitrogen application rate of 220kg/hm²（N220）,280 kg/hm²（N280）,which is 27%more than the conventional nitrogen application rate,and the amount of conventional nitrogen application increased by 55%to 340 kg/hm²（N340）,which is 55%higher than the conventional nitrogen rate,through the yield and yield composition,nitrogen accumulation,transport and utilization,root biological characteristics and root system.The indicators of physiological characteristics was measured to explore what level of nitrogen application under high-density planting can achieve high yield and high efficiency of maize,and the relationship between root system characteristics and nitrogen absorption and utilization under the optimal combination of dense nitrogen,and the main conclusions were drawn:1.The optimal combination of dense nitrogen can increase the yield of corn.The optimal combination of high density（D2）and 160-220 kg/hm² nitrogen application rate increased yield by 39.07%-51.82%compared with farmers’general cultivation management（N280D1）.Under the density of D2（82,500 plants/hm²）,the highest yield was obtained under the treatments of N160（12.53 t/hm²）and N220（12.51 t/hm²）in 2021 and 2022,the yield under nitrogen fertilization decreased slightly compared with N220,but there was no significant difference between the two;while the yield under 340kg·hm-2 nitrogen fertilization decreased by 25.62%compared with N160 in the first year,and decreased by28.46%compared with N220 in the second year,The interaction of nitrogen application amount,density and year has a significant impact on yield.In 2021,the yield is D2N160>D1N220,and in the second year it is D2N220>D1N160.The specific amount of nitrogen fertilizer application is also related to precipitation and temperature.2.The optimized combination of dense nitrogen can promote the nitrogen uptake by the roots of a single plant,increase the nitrogen accumulation in the aboveground parts,and enhance the nitrogen use efficiency of the population.When the density increased,the accumulation of nitrogen content per plant decreased,and nitrogen accumulation per plant,dry nitrogen absorption per unit root,and nitrogen absorption per unit root length all increased,and reached the maximum under the N160-N220 treatment,which was different from that of N280 Not significant,but decreased significantly under N340 nitrogen application rate.Compared with the general cultivation management（N280D1）,the optimized combination of dense nitrogen increased the contribution rate of nitrogen transport to grain by 23.81%-51.85%,the nitrogen use efficiency increased by 2.49%-8.66%,and the partial productivity of nitrogen fertilizer increased by 41.23%-77.55%.Agronomic utilization efficiency increased by 38.3%-197%.Nitrogen harvest index,on the contrary,decreased first and then increased with the increase of nitrogen application rate,and was the smallest under N220D1 and N160D2 treatments.3.The good root system construction under the optimized combination of dense nitrogen promotes nitrogen uptake and utilization.Density and nitrogen application rate significantly affects the shape and distribution of root system,and the biological characteristics of root system significantly affect nitrogen uptake.Under the optimal combination of dense nitrogen,the surface area,volume,length,root length density and specific root length of the root system were all the largest,the root opening angle between rows and plants,the maximum width between rows and the maximum width ratio were the largest,while the opening angle ratio and the inter-plant ratio were the largest the width decreases,the root system grows to the inter-row,and the overall structure shrinks,showing inter-row expansion and longitudinal extension;especially 90%of the root system is the largest at the depth of 35-40 cm in the soil,which is significantly higher than that of the N280 treatment.The root system showed shallow rooting under high nitrogen,and the surface area,volume,length and root length density of the root system decreased.Excessive nitrogen application inhibited the growth and distribution of the root system.Root biological characteristics were significantly correlated with root nitrogen uptake,shoot nitrogen accumulation,nitrogen transport contribution to grain and nitrogen partial productivity.4.Appropriately reduce nitrogen fertilizer application under high density to regulate the secretion of plant hormones to build a good root system distribution,improve root nitrogen metabolism,and promote nitrogen absorption and transfer.Different physiological indexes of root system had different effects on root biological characteristics and nitrogen uptake and utilization.Under the condition of high density,excessive nitrogen application reduces the activity of nitrogen metabolism enzymes in roots;the activity of nitrate reductase in the optimal combination of nitrogen density has no significant difference from that of N280D1,and the activities of glutamate synthase and glutamine synthase are the largest,which increases by 26.67%respectively and 47.25%,and were significantly correlated with root growth distribution and nitrogen accumulation and transport.Nitrogen fertilization at high density promoted the formation of gibberellin and inhibited the synthesis of cytokinin and auxin.The secretion of auxin content in the nitrogen-optimized combination was greater than that of N280D1,but there was no significant difference;gibberellin and cytokinin were significantly increased by 9.66%and 21.54%compared with N280D1.Gibberellin was significantly negatively correlated with root nitrogen uptake,while cytokinin was opposite.Lower gibberellin and higher levels of cytokinin in the optimized combination of dense nitrogen regulate the inter-row growth and longitudinal extension of root system,and cytokinin and auxin promote nitrogen metabolism in root system.When the planting density was 82500plant·hm^-2,160-220kg·hm^-2nitrogen fertilizer was recommended,the yield of the combination increased by 39.1%-51.8%and the nitrogen side productivity increased by 41.2%-77.6%.Under high-density planting,a small amount of nitrogen application increases the soil nitrate content,promotes the secretion of gibberellin in the root system and regulates the growth and distribution of the root system between rows,the root surface area,volume,length,root length density and specific root length are the largest,especially at 35-40 cm The distribution of roots in the soil layer increases,thereby promoting the absorption of nitrogen in the soil profile,and the nitrate absorbed by the roots stimulates the activity of nitrate reductase to enhance the positive feedback to promote nitrogen absorption in the root system,and at the same time enhance the activity of glutamine synthetase/glutamic acid synthase;on the other hand,the nitrogen-dense optimized combination maintains higher auxin and cytokinin secretion after flowering,improves nitrogen metabolism,and transfers nitrogen absorbed by the root system to the shoot,thereby promoting nitrogen transfer from vegetative organs to the grain and improving yield.