Manipulating Root Growth For Efficient N Acquisition Of Field-grown Maize Under Diffierent Planting Density Through Localized N Supply And Method

Increasing planting density is one of the major agronomic practices to achieve high grain yield in maize.However,high plant populations can lead to poor root growth which can limit nutrient acquisition.Therefore,constructing an efficient root system is essential for improving crop productivity and nutrient use efficiency simultaneously.In the previous study,we found that,under high plant density,the adaptive response of maize root system architecture is“intra-row contraction and inter-row extension”.We hypothesize that nutrient uptake efficiency can be improved by increasing root growth in inter-row direction.In this study,we will jointly use the techniques of inter-tillage and localized nitrogen supply to enhance inter-row root growth and increase nitrogen fertilizer use efficiency by exploiting the potential of nutrient-tropism root growth.Two-year field experiment was conducted in 2021-2022,Kenwo1,a density tolerant cultivar was used.Split-plot was designed,main plot was planting density（60,000 plants per hectare and 80,000 plants per hectare,D₆ and D₈ were abbreviated）).Sub-plot included two nitrogen（N）fertilizer application pattern（100%N was applied as base-N,and base-N:side-dress N=2:3,N₁ and N₂ were abbreviated）.Sub-sub-plot was the horizontal distance between the inter-tillage position and the seedling belt,which included 10 cm(H₁₀)and 15 cm(H₁₅).Main results were as follows:（1）High planting density increased leaf area index（LAI）by6.8%to 27.2%across all growth stages,decreased SPAD by 4.8%to 10.1%.Split-N application with optimized horizontal distance between inter-tillage position and the seedling belt could improve both LAI and SPAD.（2）High planting density limit root growth of maize per plant was inhibited under denser planting conditions.The root length,root surface area,root biomass,specific root length and root injury flow decreased by 0.8%～30.6%in V12 and R1 stages,and increased by 1.6%～17.8%in specific root length.The root growth of maize under denser planting conditions can be improved by optimizing the middle-tillage distance and nitrogen fertilizer operation mode:The two-year average root length,root biomass,root surface area,root volume,specific root length and number of nodes in R1 stage treated by D₈N₂H₁₅ were all superior to those treated by D₆N₁H₁₀,indicating that the lateral deep application of nitrogen fertilizer could be achieved under densification condition by adjusting the distance between middle tillage and seedling belt,and the root growth of maize could be significantly improved.（3）High planting density changed root vertical distribution in 0-60cm soil,reduced root length and root biomass in surface soil,and promoted root insertion.By adjusting the middle-tillage distance and applying nitrogen fertilizer separately,the root length ratio in the 0-20 cm depth soil layer was significantly reduced,the root length ratio in the 20-40 cm depth soil layer was significantly increased,the root length density in the 0-50 cm soil layer was significantly increased,and the root root crowding in the surface soil was reduced and the root growth was improved under the denser planting condition.（4）High planting density significantly increased nitrogen accumulation in maize population,nitrogen transport efficiency in vegetative organs,nitrogen harvest index,nitrogen uptake efficiency and nitrogen use efficiency.At the same time,the agronomic efficiency and partial productivity of nitrogen fertilizer were significantly improved.Under the condition of nitrogen application,the effect of lateral nitrogen application could be achieved by adjusting the horizontal distance between middle tillage and seedling belt,which significantly promoted the nitrogen uptake and utilization efficiency of plants.Compared with D₆N₁H₁₀ treatment,the population nitrogen accumulation,nitrogen transport efficiency of vegetative organs,nitrogen harvest index,nitrogen uptake efficiency and nitrogen use efficiency of maize under D₈N₂H₁₅ treatment increased by 6.8%to 30.8%.（5）Under High planting density,the grain yield and economic benefit of maize were significantly improved by fractional nitrogen application and middle-tillage position optimization.Compared with household habits(D₆N₁H₁₀),D₈N₂H₁₅ had the highest grain yield and the best economic benefit,reaching 15 578.2kg/hm² in 2021 and 14 132.1kg/hm² in 2022.