Impact Of Different Fertilization Treatments On Hyperspectral Characteristics Of Winter Wheat

Winter wheat is the main crop on the Loess Plateau.Monitoring its growth and scientific fertilization management will help increase wheat yield,improve wheat quality,and prevent soil and water pollution caused by excessive fertilization.Hyperspectral is rich in spectral information.Using hyperspectral technology to quantitatively monitor vegetation growth processes and invert biophysical parameters has become a powerful technical method for studying the geological process of surface vegetation.Based on a 35-year long-term location experiment,the effects of different fertilization combinations,different growth periods,different nitrogen levels,and different heavy metal contents on the spectral characteristics of winter wheat in the Weibei dryland of the Loess Plateau were studied.The quantitative relationship between spectral characteristics provides theoretical basis and technical support for the monitoring of nutrient abundance in winter wheat at different growth stages and scientifically and rationally guiding fertilization programs.The following research results have been achieved:（1）The spectral characteristics of winter wheat showed significant differences due to different fertilization treatments.Under a single fertilization condition,CR₅₀₀,CR₆₇₀,and CR₅₅₀ of a single fertilization were higher than CK,and the spectral reflectance of a single fertilization of N and M was significantly lower than that of CK.In the near-infrared band,the spectral reflectance of single-application P is slightly lower than that of CK,and the spectral reflectance of single-application N and M is significantly higher than that of CK.Under combined fertilization conditions,the absorption depths of red and blue light absorption valleys and green light reflection peak-to-peak reflectances of all nitrogen application combinations were significantly lower than CK;the PM spectral reflection characteristic value was slightly lower than CK.In winter wheat jointing and heading stages,the D_λred and Sred of single P application were lower than those of CK,which showed obvious stress status.The values of D_λred and S_red in the combined treatment of NM and NPM were greater than that in the single fertilization treatment and CK treatment in each growth period.（2）Under the same fertilization condition（NM）,the canopy spectra of winter wheat at the jointing stage,heading stage and filling stage were similar in spectral reflection characteristics in the visible light band,but the characteristics of blue,red light absorption valley and green light reflection peaks at heading stage and filling stage were more different than those at jointing stage.The winter wheat canopy spectral reflectance in the near infrared band from the jointing stage to the heading stage showed a high reflectance characteristic.Among them,the reflectance value at the heading stage was the highest at 0.549,followed by 0.402 at the jointing stage and 0.326 at the filling stage.To the lowest value,it is 0.198;the red edge parameter also changes with the advance of the growth period,and then changes to red and blue.The reflectance characteristics of leaf spectra at different growth stages are similar to the canopy spectra.But compared with the leaves,the absorption depth of the canopy blue and red bands is deeper;the absorption characteristics affected by moisture at the near-infrared bands of 980nm and 1200nm are enhanced,and the absorption valley depth is large;the highly reflective platforms in the near-infrared band are smaller.（3）Leaf nitrogen content gradually decreased with the growth of the growing season,and increased with the increase of nitrogen application level.With the increase of nitrogen application level,the spectral reflectance of winter wheat canopy decreases in the visible light band and increases in the near-infrared band;as the amount of nitrogen application increases,REP moves in the direction of long waves,and D_λred and S_red increase with the increase of nitrogen application level.The best sensitive bands of leaf nitrogen content were760nm at greening stage,670nm at jointing stage,670nm at heading stage,and 1450nm at grain filling stage.The polynomial prediction model could best reflect the quantitative relationship between the spectral reflection characteristics and leaf nitrogen content.According to R₂ and RMSE,the models with high correlation between growth stages were tested,and it was concluded that Dr and SDr are the most closely related and stable spectral characteristic parameters with leaf nitrogen content.（4）The soil copper content in the study area was sensitive to spectral characteristic variables R,FD,Log 1/R,and CR.The spectral bands were mainly concentrated at 600 nm,750 nm,and 930 nm.The soil manganese content at 750 nm was significantly positively correlated with the spectral characteristic variable FD.Spectral vegetation index NDVI,DVI,RVI,OSAVI are negatively correlated with copper and zinc content in winter wheat leaves,and positively correlated with manganese content.The best waveband combinations that are sensitive to changes in copper,zinc,and manganese content are DVI（680,760）（r=-0.548,p<0.05）,NDVI（550,950）（r=-0.658,p<0.01）,and RVI（450,760）（r=-0.487,p<0.05）.