Regulation Effects Of Increasing Planting Density And Decreasing Fertilizer Rate On Yield And Efficient Utilization Of Water And Nitrogen Of Spring Maize And Its Mechanisms Under Ridge-Furrow Rainwater Harvesting Planting System

The realization of high yield,high efficiency,and environmental friendliness of crop production is of great significance to the sustainable development of global agriculture.Optimizing planting density and fertilization rate is an important measure to promote the full utilization of limited precipitation,crop yield and efficiency,and sustainable agricultural development.However,under the condition of improved water conditions by ridge-furrow rainwater harvesting planting system,whether different rainfall years can fully tap the production potential and efficient coordination function of the production system through centralized reduced fertilization and increasing planting density,and realize the sustainable high yield and efficient utilization of water and nitrogen of spring maize needs further research.Based on this,this study carried out a six-year localization experiment（2015-2020）in the dry-farming area of southern Ningxia.By setting four different planting densities(5.25×10⁴ plants ha^-1,D1;6.75×10⁴ plants ha^-1,D2;8.25×10⁴ plants ha^-1,D3;9.75×10⁴ plants ha^-1,D4)and three fertilization rates(285 N+143 P₂O₅ kg ha^-1,CF,conventional fertilization;230 N+115 P₂O₅ kg ha^-1,RF,reduced fertilization;no fertilization,CK.Among them,CF was the control.),to study the effects of planting density and fertilization rate on the changes of soil available water storage,dry matter accumulation,yield formation and yield formation of maize,and nitrogen redistribution and grain nitrogen accumulation under different precipitation years.The mechanism of yield formation and efficient utilization of water and nitrogen under the interaction of planting density and fertilization under different precipitation years were discussed.The effects of different planting densities and fertilization rates on yield,yield stability,economic benefits,water and nitrogen efficient utilization,and water and nitrogen balance in dryland maize fields were comprehensively evaluated.The study laid a foundation for the construction of high-yield,high-efficiency,and green production and cultivation mode that coordinated water,fertilizer and planting density under ridge-furrow rainwater harvesting planting system.The main results are as follows:（1）Increasing planting density under ridge-furrow rainwater harvesting planting system promoted the utilization of deep soil moistureThe precipitation during the growth period of maize in dry years and normal years could not meet the water consumption required for crop growth,which led to the continuous transport of soil moisture from the middle and deep layers to the shallow layer in the late growth stage so that the water consumption of high-density planting gradually transitioned from the shallow layer to the deep layer,which eventually led to the decrease of soil water storage in 60-120 cm and 120-200 cm soil layers in the late growth stage of maize.Among them,high-density planting reduced soil water storage in 60-120cm and 120-200 cm soil layers by 2.1%-7.2%and 2.4%-6.5%in dry years,while that of normal years decreased by 6.4%-12.9%and 9.1%-13.2%,respectively.In addition,the variation of soil available water storage（ASWS）characteristics during the growth period was significantly affected by rainfall years.Among them,in the adjustment stage（sowing to the silking stage）,the ASWS of 120-200 cm soil layer in dry years showed a supplementary state and decreased with the increase of density.ASWS in normal years showed a consumption state that increased first and then decreased with the increase in density.ASWS in wet years showed a supplementary state and increased with the density increase.At the same time,in the consumption stage,the ASWS of 120-200 cm soil layer in dry years first increased and then decreased with the increase of density.ASWS in normal years and wet years increased with the increase in density.Therefore,high-density planting makes more efficient use of available water in deep soil during maize consumption periods in different rainfall years.However,continuous dry-dry years would destroy the annual water balance of dryland ecosystems.In contrast,continuous dry-wet years allow the deep soil moisture to be supplemented entirely before the next season of maize sowing to maintain soil water balance.（2）Increasing planting density has a compensation effect on yield reduction of maize with reduced fertilizationUnder normal and wet years,increasing the density by at least 1.50×10⁴ plant ha^-1 could compensate for the fertilizer reduction effect,and the compensation effect showed a trend of increasing first and then decreasing with the increase of density,and the highest point was D3 density.The main reason is that increasing density can compensate for the yield reduction effect of reduced fertilization by simultaneously increasing the population growth rate at 30-60 days and 60-90 days after sowing,prolong the duration of rapid dry matter growth,and increase the maximum dry matter growth rate,and the compensation effects are 0.2%-2.0%and 8.8%-19.0%,respectively.However,increasing density does not have a compensation effect in dry years.The multiple stepwise regression analysis showed that mean increase rate（Vmean）,harvest index（HI）,rainfall from sowing to 30 days after sowing（P30）,and grains number per spike（KSN）were the critical factors for reducing fertilizer and increasing yield.The degree of influence of each factor on yield was Vmean>HI>KSN>P30.Vmean and KSN interacted with maize grain yield,and the direct path coefficients of April rainfall before sowing（PA）and rainfall from 60 days to 90 days after sowing（P90）rainfall on yield were 0.71 and 0.54,respectively.Therefore,PA and P90 precipitation can be used to predict maize grain yield.（3）The interaction of planting density and fertilizer application rate affected the nitrogen transport and nitrogen use efficiency of maize40.22%-49.41%of grain nitrogen was redistributed from vegetative organs at the post-silking stage,and grain nitrogen in planting density depended more on nitrogen redistribution than fertilizer amount.Among them,with the increase in fertilizer application,less redistributed nitrogen was needed to meet the demand for grain nitrogen,decreasing grain nitrogen’s dependence on nitrogen redistribution.With the increase of planting density,the dry matter accumulation rate increased first and then decreased,so the relative contribution of nitrogen redistribution in stems and leaves to grain nitrogen accumulation decreased first and then increased with the increase of planting density.Therefore,the nitrogen absorption efficiency decreased with the increase of nitrogen application rate,but increased first and then decreased with the increase of planting density.The multiple stepwise regression analysis showed that nitrogen use efficiency（NUE）,leaf N reallocation amount（Pre-Al）,the contribution of N reallocation in plant to grain N（Pre-Gt）,N utilization efficiency（Nupt E）,and plant N accumulation at mature stage(N_R6)were the key factors affecting grain yield,and the effect of each factor on yield was NUE>Nupt E>N_R6>Pre-Al>Pre-Gt.NUE and Pre-Al interacted with maize grain yield.（4）The comprehensive benefits of planting density and fertilization amount were evaluated by Z-score analysis methodThe Z-score index was used to calculate the scores of each treatment based on the cumulative effect over six years.Reduced fertilization increased the total score,while with the increase in planting density,the total score showed a trend of increasing first and then decreasing.Specifically,under CF and RF conditions,the total scores of D2,D3,and D4 planting densities were 5.34 and 7.00 points,9.89 and11.40 points,and 8.49 and 9.96 points higher than D1 planting density,respectively.In addition,according to the scores of each index,the yield stability,yield,and NI scores of CF·D3 were the highest,which were 1.33,1.17,and 1.17 points,respectively.The WUE_GYand NUE scores of RF·D3 were the highest,1.01 and 1.57 points,respectively.The annual water balance and nitrogen surplus scores of CF·D1 were the highest,1.28 and 1.55 points,respectively.Considering comprehensively,increasing planting density can maintain soil water and nitrogen balance and improve water and nitrogen use efficiency,economic benefits,and grain yield.Increasing density based on reduced fertilization could make full use of resources and maximize yield,water,and nitrogen efficiency.Therefore,reduced fertilization combined with high-density planting(230 N+115P₂O₅ kg ha^-1 and 8.25×10⁴ plants ha^-1)is recommended as a sustainable agricultural measure for the efficient utilization of water and fertilizer with stable yield and high yield under ridge-furrow rainwater harvesting planting system.