Study Of Angle Effect On Spectral Reflectance Of Wheat (Triticum Aestivum L.) Canopy And Plant Nitrogen Monitoring

Nitrogen(N)is a critically important element which affects crop growth and yield formation.Crop nitrogen staus is an important indicator evaluating the growth status of crop.The traditional nitrogen nutrition diagnosis was time-consuming and poor timeliness.It is very important to get the information about wheat growth and nutrient status by real-time,rapid,nondestructive testing technology in order to improve the scientific management of nitrogen fertilization and increase the utilization rate of nitrogen and efficiency.Hyperspectral remote sensing technology has real-time monitoring capability,wide monitoring range,high precision,so it can significantly enhance the detection means and ability of the crop physiological and biochemical parameters and improve the precision and accuracy of crop growth monitoring.The purpose of this study was to analyze the dynamic relationship between wheat canopy hyperspectral reflectance and nitrogen nutrition under different experimental conditions and establish the spectral parameters and the corresponding monitoring model of nitrogen nutrition to provide effective technical support for real-time monitoring and accurate diagnosis of nitrogen nutrition.The results are as follows:1 Analysis of the directional characteristics of wheat canopy spectraThe reflection characteristic of the vegetation canopy is not only affected by the vegetation canopy geometry and spectral characteristics,but also affected by the direction of the incident light and the reflection direction to a great extent,the differences between the two kinds of angle obviously cause the canopy reflectance difference.As is usually expressed with bidirectional reflectance factor(BRF)?BRF changed with the incidence and observation angle changes.The results showed that BRF increased with increasing of solar zenith angle(SZA)regardless of the principal plane(SPP)or perpendicular plane(PP)and BRF in the backscattering direction was higher than those collected in the forward scattering direction under all the investigated SZAs.As was more obvious in the visible bands than that in the near infrared bands.BRF changed with view zenith angles(VZA)changing.Hot spot effect usually appeared near the same VZA in the SPP.It should be in the near infrared band and small SZAs to reduce the uncertainty of observation results.It should be in the SPP when we select suitable observation angle,SZA and estimate the structural parameters.The spectral information provided the basis for further monitoring the nitrogen nutrition status.2 Dynamic changes of nitrogen of wheat based on multi angle spectrumWe firstly compared the spectral reflectance variation patterns of wheat canopy in different growth stages at different conditions using hyperspectral analysis technology and typical bands in this study.The results showed that reflectance increased with growth stages under N-H and N-L in the visible region,and reflectance of N-L was higher than that of N-H.Reflectance presented increasing then decreasing in the near infrared region with growth stages,and reflectance was lower of N-L than that of N-H.The "hotspot" effect was taken more into consideration when the Sun was exactly behind the sensor in the retro-solar direction,where objects create their own shadows.In addition,different planting density affected two kinds of wheat canopy spectral characteristics.Wheat canopy reflectance showed decreasing trend with the planting density increasing in visible region and increasing trend in the near infrared region.The wheat canopy reflectance of different plant species was different in different wave bands in SPP and PP.Reflectance of compact wheat was higher than that of loosely wheat in the SPP.However,difference of reflectance for both types in the near infrared region.In the visible region,canopy reflectance of loosely wheat significantly increased with VZA increasing in the back scatter direction and slowly increased in the forward scatter direction,and reflectance was higher in the back scatter direction than that in the forward scatter direction.Reflectance of compact wheat significantly increased with VZA increasing in the back scatter direction and decreased in the forward scatter direction.The reflectance also presented similar phenomenon in the PP.Another,the reflectance presented symmetric in the back scatter direction and the forward scatter direction.So it can be realized for identification of varieties according to the changes of the spectral characteristics.3 Remotely detecting canopy leaf nitrogen concentration in winter wheat based on moisture insensitive spectral index in-situ hyperspectral dataThe quantitative relationship between the leaves nitrogen content(LNC)in wheat and canopy hyperspectral reflectance was studied with spectral analysis technology at a single vertical angle based on analysis of spectral characteristics of wheat canopy in different conditions.The data have shown that the proposed combined index(water resistance nitrogen index,WRNI),ratio of normalized difference red edge index and floating-position water band index was both very sensitive to LNC and very resistant to variations of leaf water.Then we optimized the bands of NDRE/FWBI to become an integrated narrow-band vegetable index in the form of[(R735-R720)*R690]/[Rmin(930-980)*(R735+R720)]to trace the dynamic changes in LNC of winter wheat.Our novel index and 15 selected common indices were tested for stability across growth stages,locations,years,treatments,cultivars and plant types in estimating LNC of winter wheat.Among the six well-performance out of 16 previously determined indices,R705/(R717+R491)and mND705 both generated highest coefficients of determination(R2)(0.832,0.818)and lowest root mean square error(RMSE)(0.401,0.417).Compared with the optimized common indices,this novel index WRNI was most closely correlated with LNC,and the corresponding linear equation gave 0.843 of R2 and 0.382 of RMSE across whole 16 datasets,further indicating a superior trace for LNC changes under heterogeneous field treatments.The inversion result showed that WRNI equation fitting accuracy is higher(RMSE,=0.384 R2=0.861,RE= 15.6%).These models can accurately estimate LNC in winter wheat,and the novel index WRNI is promising for detecting LNC on a regional scale in heterogeneous fields under variable climatic conditions.4 Monitoring model of wheat canopy leaf nitrogen concentration based on appropriateobservation angle and moisture insensitive spectral using hyperspectral dataCompared with single remote sensing,multi angle and multi temporal data can obtain more detailed and reliable spectral information of object.In this study,we analyzed different wheat cultivars grown in contrasting climatic and geographic zones in China and Canada and studied the effects of 13 viewing angle on the relationships between various vegetation indices(VIs)and leaf nitrogen concentration(LNC)using hyperspectral data.The sensitive spectral parameters of wheat canopy LNC are mainly red edge index,such as mND705,RI-1DB and NDRE were closely related to leaf nitrogen content.To overcome the influence of environment and VZA on VIs,the optimum combinations of NDRE(790,720)and FWBI(900,min930-980)were developed.That was NDRE/FWBI.Our novel VI exhibited the best performance,with the best prediction accuracy at 0° to-20°(R2 = 0.838,RMSE = 0.360)and relatively good accuracy at 0° to-30°(R2 = 0.835,RMSE = 0.366).As it is possible to monitor plant N status over a wide range of angles using portable monitors,viewing angles of as much as 0° to-30° are common.Consequently,we developed a united model across angles of 0° to-30° to predict LNC in wheat independent of experimental condition.We performed a inversion analysis of angle combinations to determine the relationships between VIs and LNC for mND705.RI-1DB and NDRE/FWBI.The result showed that WANI developed in this study is superior to the common VIs mND705 and RI-1dB.Therefore,the novel index WANI increases the stability of the monitoring model and reduces the dependency on VZA at 0° to-30° viewing angle,which will facilitate its widespread application under different production conditions.5 Estimation of nitrogen accumulation in wheat plants based on hyperspectral multi angle remote sensing dataPlant N accumulation(PAN)is the product of plant dry matter weight and nitrogen concentration,and it is also a comprehensive reflection index of plant nitrogen nutrition level and population growth and development status.Therefore,it is of great significance to evaluate the change of crop nitrogen accumulation rate quickly and accurately in real-time monitoring of plant growth and nitrogen nutrition diagnosis.To develop a method for diagnosing PNA of wheat with the hyperspectral techniques,field experiments were carried over two years in different conditions.The multi angle hyperspectral data of wheat in different growth stages were collected,and then the quantitative relationship between PNA and multiple spectral parameters and Optimal observation angle was analyzed in this study.The results showed that the best observation angle was in the 0°--20°.The mean value of the correlation coefficient was the highest in-20°view zenith angle through the comparison of the correlation between the parameters and PNA at 13 observation angles,Of all the spectral indexes,the correlation of DVI(810,560).DIDA.VIopt?DDn and DD was better.R=0.832?-0.871,0.858?-0.872?0.868?The model estimation accuracy of DIDA was highest,R2=0.87,followed by DDn?VIopt?DD,R2=0.827?0.82?0.813.The lowest was DVI,R2=0.775?The overall results indicated that the PNA of wheat could be reliably estimated with DIDA.As provided a new research idea for the rapid and accurate evaluation of wheat PNA.In short,spectral reflectance characteristics the wheat canopy was comprehensively analyzed in this paper,as was studied from both the vertical angle of single and multi-angles,together with the relation of spectral index and leaf nitrogen content.The estimation equation of LNC was constructed,and then the best spectral monitoring model is established.The results showed that the new hyperspectral index WRNI based on the view of single vertical observation can real-time evaluate LNC of wheat canopy.And multi-angle remote sensing data presented more advantages than conventional observation data when crop structure parameters were extracted.The newly constructed parameters WANI can be called a wide adaptability index because it not only reduced the reliance on observation angles,but also increased the stability of the model.The test was proved that the correlation of the newly constructed vegetation index WANI and LNC under observation angles was best compared with other parameters both in China and Canada,and the accuracy of estimating LNC of wheat canopy were higher under the different conditions.However stability and practicability of the model still need further verified in other crops.