Estimating characteristic coefficient of vertical leaf nitrogen profile within wheat canopy from spectral reflectance

Abstract

The characteristic coefficient of vertical leaf nitrogen (N) profile is a canopy parameter that indicates the attenuation steepness of leaf N from the top of canopy downward. It is sensitive not only to crop production, grain yield and quality, and light- and N-use efficiency but also to N deficiency in the crop. We introduce this coefficient by exploring a robust method to estimate it from canopy spectral reflectance. We analyze comprehensively various approaches based on diverse datasets of winter wheat. We test and compare the accuracy and stability of models by using the adjusted and weighted coefficient of determination (wRadj2), the mean absolute error (MAE), and the mean and coefficient of variation (CV) over multiple seasons. The analysis focuses mainly on the coefficient of mass-based leaf N profile (Km); nevertheless, a comparison with the coefficient of area-based leaf N profile is presented. The results indicate that the most robust model to estimate Km of winter wheat is Km=(1.8037RGVI-0.9702+0.0786exp(0.6315/DASF))/2, where RGVI is the red and green ratio vegetation index, and DASF is the directional area scattering factor. The mean wRadj2 is 0.663 with CV = 8.2% and the mean MAE is 0.117 with CV = 12.8% over three seasons including various situations. This makes it possible to assess Km at large areas and follow its dynamics over multiple periods in a timely and nondestructive manner.