| Excessive use of synthetic nitrogen(N)fertilizer and lower nitrogen use efficiency(NUE)are threatening the wheat production under rice stubble in Huaibei region,Jiangsu,due to its climatic-ecology,production conditions,cropping system,and soil properties are different from rainfed wheat.Baomai 1,Xumai 30 and Xumai 33 were selected as research material in this study,with 100%of rice-straw returning to the field when drought occurred in early spring without irrigation.For the purpose of 15%N reduction based on local N input(300 kg·hm-2)and the target yield 6090 kg.hm-2(average yield in Xuzhou,2012),the coordinated interaction between density,N amount and its application proportions on N efficient utilization and its mechanism were explored.Another experiment was carried out in Yangzhou to investigate the effects of N proportions on N uptake,N resides in soil,and N losses in the wheat-soil system with 15N isotope tracer technique.The appropriate combination of density and N were proposed in term of stable yield,N reduction,N efficient utilization,and high quality,aiming to provide theoretical foundation and technical support on efficient N application in wheat after rice stubble in Jiangsu province.All results were as follow:1.According to the results of 2012-2014,the yield over 6000 kg·hm-2 were achieved in the combination of 225 kg ·hm-2 and the proportion as 7:1:2:0(basal:tillering:jointing:booting fertilizer),5:1:2:2 when drought occurred in seeding time in early spring without irrigation;however,the highest yield were achieved in the combination of 270 kg.hm-2,the proportion of 7:1:2:0 and 5:1:2:2.With the decreasing N fertilizer application,Nitrogen Agronomic Efficiency(NAE),the Partial Factor Productivity(PFP)and Physiological Efficiency(PE)all significantly increased.Higher NAE,PFP were achieved under the conditions of 180 kg·hm-2 N fertilizer and the proportion of 7:1:2:0.The grain yield significantly correlated with spikes number and grains per spike,and 1000-grain weight should be paid more attention when increasing spikes number and grain per spike to make sure synergetic increase in the three yield components.Grain yield was positively related to RE in two years.Significant positive correlation was found among PFP,NAE and PE.2.Regulatory effects of N amount and its application proportions on population quality indexes were studied here as well.The interactive effect of N amount and its proportions on the characteristics of population quality were determined here.As the N amount increased,tillers number,leaf area index(LAI)at booting,LAI on 21 days after anthesis,dry matter accumulation after anthesis,and total setting kernels were all increased.Correlation analysis showed that both grain yield and RE were positively linked to LAI at booting,LAI at anthesis,LAI on 21 days after anthesis,dry matter accumulation after anthesis,and total setting kernels.3.As the application of N fertilizer reduced,both N accumulation amount and N translocation amount after anthesis were decreased,while the rate of N transferred from vegetative organs into grain increased.Consequently,translocation efficiency and its contribution to grain-filling increased with lower nitrogen amount.Under different nitrogen levels,N accumulation amount at maturity,N translocation amount and after anthesis and translation efficiency all increased with more input of N fertilizer at the later stage,with highest values found in the proportion of 0:3:3:4.However,the contribution of translocated N to grain-filling was higher in the proportion of 7:1:2:0 and 5:1:2:2.RE was significantly positively related with N accumulation amount,the contribution rate of N assimilated to grain-filling after anthesis,and N translocation amount from each vegetative organ after anthesis,indicating increasing N accumulation,N assimilation and N translocation after anthesis were conducive to N efficient utilization.4.The regulatory effects of nitrogen levels on the actives of nitrate reductase(NR),glutamine synthetase(GS),glutamate synthase(GOGAT)in flag leaf and glutamic acid pyruvic acid-transaminase(GPT),GS,GOGAT in grain were consistent.The N-metabolic key enzymes activities dramatically decreased as the application of N fertilizer reduced,with the trends as following:180 kg·hm-2<225 kg·hm-2<270 kg·hm-2.All these findings illustrated the negative effects of reducing nitrogen application on N-metabolism.During the grain filling,the N-metabolic related enzymes activities increased as more nitrogen applied at the later stage,showing the consistent trend as 3:1:3:3>5:1:2:2>7:1:2:0,but no significance were observed among the three proportion treatments.Higher nitrogen applied at jointing-booting stage could enhance the N-metabolic related enzymes activities in compensation for negative effects of lower nitrogen on N-metabolism,which was conducive to N accumulation and assimilation after anthesis.5.According to the results in the year 2014-2016,the interaction existed between density and nitrogen.To some extent,the density has compensation effects on nitrogen application.When reasonably increasing density from 150×104·hm-2 to 225×104·hm-2,the nitrogen rate Was reduced by 15%~30%without yield reduction,leading to significant improvement of NAE.The yield was positively correlated with NAE and RE within the certain yield limits,illuminating the yield and NUE could be improved synchronously.Yield,NAE and RE were positively correlated with the leaf area index(LAI)at booting stage,LAI on 21d after anthesis,dry matter accumulation after anthesis and total setting kernels.The highest yield and NAE were received at combinations of 225×104·hm-2,229.5kg·hm-2 and the proportion 5:1:2:2.6.According to the results of 15N isotope tracer assay,N could be absorbed efficiently from jointing and booting fertilization,with lower N residue in soil and N loss.The rate of 15N fertilizer absorbed by plants was significantly lower than other treatments when all fertilizer applied before sowing,with higher N loss.This finding showed that most of the applied N fertilizer was lost with the lowest NUE rather than uptaken by crops when all fertilizer applied before sowing,which negatively affected the N-efficient wheat production.More fertilizer applied at jointing and booting stage instead of seeding time and tillering stage was beneficial for the improvement NUE in wheat under rice stubble.7.When drought occurred in seeding time and early spring without irrigation,the appropriate combination of density and N in wheat under total rice-straw application in term of target yield 6000 kg·hm-2 and>10%N reduction was as follow:the density 165×104·hm-2,N E amount 270 kg·hm-2,the N proportion 7:1:2:0 and 5:1:2:2.Additionally,the appropriate combination of density and N in term of higher yield(? 7500 kg·hm-2),>15%N reduction compared with the combination above,and N efficient utilization(improving NAE approximately 15%compared with the combination above)was as follow:the density 225×104·hm-2,N amount 229 kg·hm-2 and the N proportion 5:1:2:2. |
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