Effects Of Nitrogen Rate On Growth,Grain Yield And Nitrogen Utilization Of Multiple Cropping Proso Millet After Spring-wheat In Irrigation Area Of Ningxia

Crop growth and yield formation are limited by nitrogen,so nitrogen fertilizer application is often used as an effective measure to meet crop demand for nitrogen.In recent years,a series of problems of resource waste and environmental pollution have been caused by excessive nitrogen application.Thus,the rational nitrogen rate has great significance for ensuring increased food production and while realizing the sustainable development of agricultural production.Irrigation Area of Ningxia has made a great contribution to grain production,however,the agricultural production in this region has facing the low utilization of land resources.Therefore,how to achieve efficient multiple cropping in limited photothermal resources has of great significance for the full utilization of resources and the optimization of planting structure in the region.Based on this,the field experiments were conducted by single-factor randomized block design in Yinchuan,Ningxia,four nitrogen rates[0（N0）,90（N1）,120（N2）,150 kg hm^-2（N3）]were set in 2019,and six nitrogen rates[180（N4）,210 kg hm^-2（N5）were added]in 2020.The multiple cropping proso millet after spring-wheat was token as the research object,the effects of different nitrogen rates on growth,yield,leaf physiological characteristics and nitrogen utilization of proso millet were analyzed;main findings are as fallows:（1）Growth of canopy and root of proso millet were improved by nitrogen application,and N2 and N3 treatments had the best.The morphological parameters of canopy and root and dry matter accumulation of proso millet were significantly promoted by nitrogen application,however,the nitrogen rate exceeded 150 kg hm^-2,these indexes declined slightly except the stem diameter and leaf area.Compared with N0 treatment,the plant height of N2 and N3increased by 51.10-62.85%,stem diameter increased by 22.87-30.99%,leaf area increased by76.69-86.48%,total root length increased by 35.20-48.27%,root surface area increased by54.73-66.25%,root volume increased by 109.74-139.61%,and root average diameter increased by 9.55-14.56%on average of each growth stage.The canopy and root growth of proso millet could be better coordinated by N2 treatment.The root-shoot ratio reached the minimum under N2 treatment in most growth stage,and which was 0.119 at jointing,0.087 at heading,0.054 at grain-filling,and 0.052 at maturity,respectively.（2）The yield and its components of proso millet were significantly increased by nitrogen application.Regression analysis showed that when the nitrogen rate was 129.25-162.42 kg hm^-2,the grain yield and its components had the best.The measured grain yield under N2 and N3 treatments were best,and the average of 2979.41 kg hm^-2 and 3084.67 kg hm^-2,respectively,which were 76.22%and 83.21%higher than N0,respectively.In addition,the grain yield was significantly positively correlated with each growth parameters,and the 37.0-76.9%variation of grain yield were explained by its growth,indicating that the yield formation by improving growth were promoted by nitrogen application.（3）The nitrogen assimilation and photosynthetic capacity of proso millet leaves were improved by nitrogen application.In most growth stages,the activities of nitrate reductase,glutamine synthetase,glutamate dehydrogenase and nitrate content in leaves of N2 and N3increased most than N0 treatment,increased by 28.23-37.58%,21.42-49.80%,11.38-17.39%and 16.26-53.99%,respectively;and the activity of glutamate synthase,soluble protein content,chlorophyll a,b and carotenoid content increased most in highest nitrogen rate（N3treatment in 2019,N5 treatment in 2020）.Meanwhile,the net photosynthetic rate,stomatal conductance and transpiration rate of N2 and N3 treatments reached the maximum in most growth stages,and the intercellular CO₂ concentration reached the minimum.（4）Nitrogen concentration of vegetative organs（stem and leaf）of proso millet were increased by nitrogen application in most growth stages,while the reproductive organs were not significantly affected.The aboveground nitrogen accumulation under N2 and N3treatments had the highest at each growth stage,which increased by 153.55%and 153.31%respectively compared with N0 treatment.With the increase in nitrogen rate,the proportion of nitrogen accumulation in stem and leaf of proso millet increased after heading,and the proportion in grain decreased significantly in 2020;that is,more nitrogen remained in vegetative organs at the late growing periods under higher nitrogen rate,resulting in the nitrogen harvest index decreased.Furthermore,the nitrogen fertilizer apparent recovery rate（NRE）,agronomic efficiency（NAE）and partial productivity（NPFP）were better under N2treatment,which were 61.81%,10.77 kg kg^-1 and 24.83 kg kg^-1,respectively.On the whole,under nitrogen application conditions,the growth of proso millet and the nitrogen assimilation and photosynthesis capacity of leaves were significantly improved,and the yield was increased finally.However,when the nitrogen rate exceeded 150 kg hm^-2,the promoting effect of nitrogen fertilizer on growth and yield of proso millet were no longer increased,and the nitrogen use efficiency were significantly decreased.When the nitrogen rate was 120 kg hm^-2,the growth of root and canopy could be better coordinated,and the higher yield and nitrogen use efficiency could be obtained.Therefore,the reasonable nitrogen rate for multiple cropping proso millet in Irrigation Area of Ningxia was 120-150 kg hm^-2.