| Maize yield has increased dramatically over the recent past,while nitrogen use efficiency(NUE)lags far behind this increase due to excessive nitrogen(N)application.Since the newer hybrids are more responsive to fertilizer N,therefore,maize crop is usually applied with higher N application to get high yield,thus causing leaching problems and low nitrogen recoveries at the same time.Each maize hybrid has unique genetic and agronomic characters which should be exploited by optimizing the hybrid–specific agronomic managements,to characterize the performance in given agronomic conditions and to extract its yield potential.In this context,a two-year field experiment was conducted to evaluate the N uptake and utilization performance of two popular high–yielding Chinese maize hybrids(ZD958,XY335)under different N rates(0N,120 N,240N,450N)and planting densities(67,500 and 90,000 plants ha–1).Summer maize was sown in 2017 and 2018,under RCBD(Randomized Complete Block Design)split–split plot design,in north of China.Specific objectives of this study were,a)to quantify the temporal dynamics of N uptake and remobilization patterns,b)to evaluate the effects of N rates and planting density pressure on dry matter,grain yield and yield components,c)to study the interaction effect of high N and high population density on light penetration within crop canopy,gas exchange parameters and other photosynthesis performance,d)to estimate the Nmin removal in different soil depths to quantify the nitrogen uptake for different maize populations,and e)to examine N demand and allocation under different planting densities.The main findings are listed as:1)Results revealed that increasing planting density had helped in improving maize grain yield(GY),but more nitrogen application did not enhance yield significantly.Maximum grain yield of 16.91 Mg ha–1 was exhibited by XY335 at 240 N under high plant density(90,000 plants ha–1),and it was followed by 14.66 Mg ha–1 for ZD958 at 240 N under high plant density.Compared to low plant density,the increase in yield for ZD958 and XY335 was 24 and 31% higher due to high density at 240 N.From results,it is evident that XY335 is more responsive to high N compared to ZD958.However,treatment of 450 N did not improved significantly the agronomic performance of each hybrid compared to 240 N,as the GY of 16.09 and 13.86 Mg ha–1 were recorded for XY335 and ZD958 at 450 N under high density,respectively.Dry matter accumulation(DMA)during pre–silking in both maize hybrids accounted for 41% in ZD958 and 37% in XY335 on average.From the total post–silking DMA,which was 59–63% in both maize hybrids,approximately 74% of post–silking accumulation occurred during the fast grain–filling phase(30 DAS).High plant density decreased the total dry matter(per plant)by 7%,and average decrease of 2.26% was recorded for post–silking DMA,compared to low plant density.2)Stress tolerance against planting density was found greater in XY335 for almost all the agronomic parameters studied(i.e.DMA,N uptake,hundred seed weight,DMA rate,kernel number per ear),compared to ZD958 which was relatively affected more due to the density pressure.High plant density had caused 9% decrease in grain yield per plant(GYP)on average,and similarly an overall decrease of 7% was observed for total DMA per plant under high density.Total N uptake was also negatively affected(9%)under high plant density on average.High density decreased the pre–silking N uptake per plant by 29% in control(0N)plots compared to low density,while the decrease in pre–silking N uptake per plant due to high density was 6% for XY335 and 11% for ZD958 at 240 N compared to low density.Similarly,the total N uptake per plant was reduced by 24% due to high density compared to low density for 0N plot,however,there was only 5% on average reduction in total N uptake per plant for N rate treatments(120N,240 N,450N)due to high density compared to low density,showing that high N helped to mitigate high density stress.Decrease in pre–silking organ N content was ~20% each for leaf,sheath and husk,and 15.6% for stem due to high density compared to low density,and at maturity the N content for leaf,stem,sheath,husk and grain was reduced by 17.5,11.5,2.0,25.5 and 7.2% due to high density.3)Higher grain yield per plant(GYP)for XY335 under high and low plant density was explained by greater kernel weight and kernel number per plant,compared to ZD958.Results revealed that high grain yield(GY)in XY335 was due to the greater post–silking DMA,and higher DMA was coupled with higher post–silking N uptake rates as well,compared to ZD958.Nitrogen use efficiency(NUE)was 21% higher for XY335 than ZD958.For NUE components,N uptake efficiency for XY335 was 24% higher than ZD958,however,the N utilization efficiency was 10% greater in ZD958 compared to XY335.ZD958 had also greater N translocation efficiency 17% higher than XY335,and further the proportion of translocated N in final kernel N was 32% higher in ZD958,than XY335.Consistently,ZD958 had greater leaf remobilization efficiency than XY335.Pre–silking organ N content in ZD958 for leaf,sheath and husk was 9,25 and 17% higher than XY335(respectively),while stem N was higher(9.6%)in XY335.Similarly,at maturity the organ N content was also higher in leaf(3.6%),sheath(13.8)and husk(14.9%)for ZD958,than XY335,but the stem and grain N content was 5.8 and 12% higher in XY335.Irrespective of high remobilization rates in ZD958,on average the final grain N content(g plant–1)was 22% higher in XY335 compared to ZD958.4)N supply during late vegetative stages(V12,VT)had improved the gas exchange parameters(photosynthesis rate,stomatal conductance and transpiration rate)for both hybrids during late reproductive stages(R3,R5),with 450 N and 240 N resulted in higher values.In general,the greater photosynthesis rate was recorded for upper leaves and ear leaf for XY335 but the significance between hybrids was relatively low.Regarding plant density effects,in most cases the photosynthesis rate was accelerated for both hybrids under high density in 2018.Better photosynthesis for high N plots is attributed to maximum light penetration under high density within the canopy,and due to better leaf SPAD readings.This implies that plants under competition tends to accelerate carbohydrate metabolism due to higher DMA rates during late reproductive stages under high density,which is first report about photosynthesis and nitrogen balance for high planting density population.5)High N rates of 240 N and 450 N significantly increased the photosynthesis for maize hybrids under low and high plant densities.Due to higher post–silking DMA rate under high plant density,coupled with the increased photosynthate production and better light penetration due to high N application,had altogether resulted in maximum plant DMA and eventually higher grain yields.As the photosynthesis rate for 240 N and 450 N were almost similar,therefore the grain yield differences were also non–significant on average.The fact was supported by the N surplus(Nmin)in different soil depths,where the maximum N surplus was recorded under 450 N treatment(0-20cm=19,20-40cm=12 mg NO3 kg–1)compared to all other N rates.Since,the low N rate(0N)had significantly low LAI and SPAD values,due to which the photosynthesis was minimized during post–silking period,thus affected the kernel weight and final grain yields.In conclusion,XY335 and ZD958 resulted in comparatively greater growth rates and N uptake due to the better photosynthetic capacity and dry matter production during the post–silking period under each N level and plant density.The key point for high planting density and nitrogen application to get higher yield is balance of nitrogen allocation and photosynthesis.Based on results,it can be recommended that N rate of 240 N is suitable for XY335 to attain the yield targets more than 16 Mg ha–1 under density of 90,000 plants ha–1,and the same N rate and density is optimum for ZD958 for a target of 15 Mg ha–1.Crop responses to planting density pressure revealed that XY335 had improved agronomic performance under high plant density,due to its greater density stress tolerance compared to ZD958,which actually resulted in overall yield improvements.Finally,the balance between pre– and post–silking N uptake could ensure higher assimilate allocation to kernels,and the elevated grain filling rate beyond 30 days after silking contributes to higher grain yields under high plant density and high N rates.Application of excessive nitrogen cannot enhance yield significantly,which because of lower photosynthetic nitrogen use efficiency and due to the accumulation of leached nitrogen in soil. |
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