Regulating Maize(Zea Mays L.)Root Architecture And Improving Utilization Of Water And Nitrogen Efficiency By Placing A Reduced Amount Of Nitrogen Fertilizer At A Moderate Depth

The utilization of nitrogen（N）fertilizer has played a major role in enhancing maize production and meeting the demand of global population growth.Under the condition of a deep fertilizer placement machine in combination with a planter in close planting and high-yield of summer maize,it is very important to adjust the application mode of nitrogen fertilizer and realize the synergetic improvement of crop yield and fertilizer utilization efficiency.This study was conducted in 2017-2018 at Mazhuang Town（35°58′10N,117°03′30E）,the greenhouse（36°11’N,117°06’E）and State Key Laboratory of Crop Science,Tai’an,Shandong Province.Summer maize variety Denghai 605 was used as experimental material.In the study,four N application rates:225,191.25,157.5 and 0 kg ha^-1（denoted N1,N2,N3,and N0,respectively）applied at four depths:5,10,15,and 20 cm（denoted D5,D10,D15,and D20,respectively）were designed in field experiments and soil column experiment.The objectives of this study were to investigate effects of different N rates and N placement depths on the architecture and function of roots,and to clarify the physiological and ecological mechanism of high nitrogen and water use efficiency in high-yield summer maize by studying the relationships of the spatial and temporal distribution of roots and soil water and nitrogen.Then provide theoretical basis for the high yield and efficiency of maize production under deep fertilizer placement machine in combination with planter.The main results were as follows:1.Effects of different N rates and N placement depths on the architecture and function of roots.With decreasing N application amount,the morphological indicators such as root dry weight,root length density,root surface area,and active root absorption area gradually decreased.Moderate deep N placement might stimulate the growth and development of deep root system.Compared to the conventional N application method(225 kg N ha^-1 at a depth of5 cm),15%reduction in the N fertilizer rate combined with deep N placement at a depth of 15cm could promote deeper and more root growth,improved the distribution of root absorption area and active absorption area in deep soil,and increased nitrogen absorption efficiency.At the same time,it increased root cortical aerenchyma,cortical cell size,and decreased the number of cortical cell files,reduced the cost of carbon metabolism and nitrogen metabolism of roots,and thereby reduced the demand and occupation of carbon and nitrogen during the establishment and growth of root system.This was beneficial to allocate more carbon and nitrogen to the above ground for growth and yield formation.2.Effects of different N rates and N placement depths on photosynthesis and senescence characteristics of summer maize leavesWith decreasing N application amount,the leaves area per plant,leaves area duration,chlorophyll content,activity of leaves antioxidant enzyme and soluble protein content all decreased,and MDA content increased significantly.With an increasing N application depth,the leaves area per plant,leaves area duration and chlorophyll content initially increased,then reached a peak,and finally decreased.Relative to D5 treatment,D15 treatment significantly enhanced activity of leaves protective enzyme and soluble protein content,and decreased MDA content.Deploying moderate deep N placement at a reduced N application rate could slow down the adverse effects of nitrogen reduction on these photosynthetic characteristics of summer maize.Compared to the conventional N application method,the leaves area per plant,leaves area duration,and chlorophyll content at maturity stage were increased by 3.6%,3.8%and 20.6%by placing a 15%reduction in the N fertilizer rate at a depth of 15 cm.The activiy of SOD,POD and CAT,and soluble protein content at milk stage for placing a 15%reduction in the N fertilizer rate at a depth of 15 cm were 9.3%,19.7%,6.3%and 13.8%higher than these for the conventional N application method.However,MDA content at milk stage for placing a15%reduction in the N fertilizer rate at a depth of 15 cm were 12.9%lower than that for the conventional N application method.This was conducive to ensuring a strong supply of photosynthetic products in the later stages of growth,and improving the antioxidant capacity of the leaves and delaying the senescence of the leaves.3.Effects of different N rates and N placement depths on dry matter accumulation and grain yield of summer maizeWith decreasing N application amount,the biomass and grain yield of summer maize gradually decreased.With an increasing N application depth,the biomass and grain yield of summer maize initially increased,then reached a peak,and finally decreased.D15 treatment had the maximum values.Deploying deep N placement at a reduced N application rate could reduce the decrease of biomass and grain yield due to the decreased amount of nitrogen application,and even increased the yield to a certain extent.Compared to the conventional N application method,the banded placement of 15%less N fertilizer at a depth of 15 cm significantly increased the rate and amount of dry matter accumulation after silking.It promoted more dry matter transport to the grain,increased the harvest index and then achieved a 3.8%higher grain yield.4.Effects of different N rates and N placement depths on the coordination of the spatial distribution of roots and soil water and nitrogenModerate deep deep N placement not only promoted root growth to deep soil,but also significantly increased the soil nitrogen content in the lower layer and reduced the nitrogen depletion area in deep soil layer.This was beneficial to enhance the coordination between the spatial distribution of roots and soil moisture and nitrogen,and achieving root-soil coupling,thereby enhancing the root system to absorb water and nitrogen in the deeper soil layers.Compared to the conventional N application method,15%reduction in the N fertilizer rate combined with deep N placement at a depth of 15 cm had an improved root structure and function,which promoted the absorption and utilization of deep water and nitrogen by roots.It enhanced the N recovery efficiency by 66.7%,the N agronomic efficiency by 38.5%,the partial factor productivity of N by 22.1%,and the water use efficiency by 4.8%.