| Nitrogen(N)is the basic nutrient element for crop growth and yield formation.How to acquire and diagnose the crop growth of wheat in a rapid,real-time,and non-destructive manner,and then accurately manage N fertilizer is a hot pot in the research of smart agriculture.Based on the different wheat cultivars as the research material,the field experiments with different nitrogen rates were carried out in different years and ecological regions.The objective of this study was:(1)to systemically compare the relationship between wheat canopy spectral characteristics and its growth parameters and then develop the quantitative monitoring models for leaf N accumulation and aerial nitrogen accumulation;(2)to analyze the dynamic characteristics in dry matter and nitrogen concentration of different organs(i.e.,leaf,spike and plant)and develop the models of critical nitrogen concentration(Nc)based on dry matter of different organs;(3)to develop the models of nitrogen nutrition index(NNI)and accumulative nitrogen deficit(Nand)based on the critical nitrogen concentration;(4)to establish the nitrogen regulation model of wheat based on spectral index method and nitrogen nutrition index method.The results of this study provide a theoretical basis and technical knowledge for evaluating N status to help guide wheat fertilization strategies.Crop N status is an important index to evaluate crop growth and improve the yield.Remote sensing is a rapid,accurately and non-destructive approach for monitoring crop nitrogen status.In this study,the quantitative relationship between leaf N uptake of wheat and vegetation indices was investigated based onthe comprehensive analysis of the spectral characteristics of wheat canopy vertical angle observation.The results showed that the optimized common vegetation indices DIDA and SDr/SDb were closely related to leaf N uptake,which resulted in coefficients of determination(R2)of 0.816 and 0.807 and root mean square error(RMSE)of 1.707 and 1.767,respectively.The novel spectral index,defined as shifting red-edge absorption area(s REA),was constructed on basis of the red-edge characteristics and area-based algorithm.This index could improve the predictive ability for leaf N uptake estimation(highest R2=0.831;lowest RMSE=1.556).Fitting independent data to the equations showed that s REA was more effective forassessing leaf N uptake than common vegetation indices,in which R2,RMSE and average relative error(RE)were 0.814,1.905 and 16.2%,respectively.As a result,the new vegetation index,s REA was superior for evaluating leaf N status on a regional scale in heterogeneous fields under different climatic conditions.The relationships of aerial N uptake with vegetative indices under different view zenith angles(VZAs)was compared,systematically analyzed,and then the VZAs sensitive to the aerial N uptake were extracted to establish a quantitative monitoring model of aerial N uptake.The results showed that the overall performance of the vegetation indices in the backward observation angles were better than that in the forward observation angles.Regardless of the backward or forward observation angle,the R2 value between aerial N uptake and the 17 common vegetation indices increased with decreasing VZAs,and reached the maximum at the-10°.Among the published VIs,DIDA and DDn are most closely related to aerial N uptake.The novel vegetation index,defined as the modified right-side peak area index(m RPA),was constructed in accordance with exploration of the spectral area calculation and red-edge feature.It could effectively improve predictive ability for aerial N uptake estimation and reduce the influence of different experimental factors effect.The performance of m RPA was relatively similar from-20 ° to 10 °,which permitted us to establish an unified monitoring model under this observation range to improve the field operation applicability and simplicity.Fitting independent datasets to the models,the results showed that m RPA obstained the highest accuracy for assessing the aerial N uptake.By coupling the m RPA model with the Stanford equation,a nitrogen regulation model of wheat based on spectral index method was constructed,which provided the theoretical basis and technical support for the application of remote sensing technology in the precision N fertilizer management.The study analyzed the changes of dry matter and nitrogen concentration in different wheat organs under different growth stages,N and irrigation treatments.And then the models of critical nitrogen concentration(Nc),nitrogen nutrition index(NNI)and accumulative nitrogen deficit(Nand)were constructed,according to the method of the critical nitrogen concentration dilution model proposed by Justes.The results showed that the relationship between the critical nitrogen concentration and dry matter of different organs conformed to the power function(N=a DW-b).When the dry matter was the same,the critical nitrogen concentration of irrigation treatment was higher than that of the non-irrigation treatment.To Nc model of each organ,the coefficients of determination(R2)of irrigation treatment were also higher than that of the non-irrigation treatment.The NNIincreased with the increasing N levels,which could distinguish the crop N abundance and deficiency.There was a significant negative linear correlation between NNI and Nand,in which the highest R2 value was found in plants(0.775),followed by leaves(0.747)and stems(0.675).Coupling Nand model with fertilizer contribution rate and nitrogen utilization rate,a model of N regulation based on NNI method was constructed,which could accurately quantify the N fertilizer supply amount during wheat growth stages and achieve the goal of saving nitrogen and increasing production efficiency.Scientific nitrogen fertilizer management has a significant effect on increasing crop yield while reducing environmental risks.The relationships between soil N supply and N demand in winter wheat was analyzed on basis of the dynamic characteristic of aboveground N uptake,soil nitrate N(NO3--N)and root weight density.The results showed that aboveground N uptake and soil nitrate N(NO3--N)increased with increasing N,while NO3--N decreased with increasing soil layer and gradually moved down with growth stage.Root weight density showed the same spatial-temporal characteristics as NO3--N,following a unimodal trend with increasing N,with a peak at 90 kg hm-2.The root was mainly distributed in the 0-60 cm layer(accounting for more than 80%);the root weight of 20-60 cm layer accounted for 30% of the total root system.A significant correlation(p<0.001)was observed between increasing aboveground N uptake and increasing NO3--N under N application,with the best relationships occurring in the 20-60 cm layer during jointing-anthesis(R2=0.402-0.431)and the 20-80 cm layer at maturity(R2=0.474).In conclusion,with the comprehensive consideration of wheat yield,NO3--N residue and nitrogen use efficiency,an N application rate of 180-270 kg hm-2 is recommended under sufficient irrigation in the Huang Huai wheat production area. |
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