Remote estimation of above ground nitrogen uptake during vegetative growth in winter wheat using hyperspectral red-edge ratio data

Hyperspectral sensing can provide an effective means for fast and non-destructive estimation of crop above ground nitrogen (N) status. In this study, quantitative correlations between above ground N uptake and ground-based canopy hyperspectral reflectance in winter wheat (Triticum aestivum L.) were investigated. Field experiments were conducted over four years at different sites (Xinyang, Zhengzhou and Shangshui) in Henan, China. Different N rates, planting densities, basal/topdressing N ratios and wheat cultivars were tested, and a novel spectral index was developed with improved predictive capacity for above ground N uptake estimation. Linear regression was integrated with previously reported Clred-edge3 and MTCI indices to investigate the dynamic nature of above ground N uptake, which resulted in coefficients (R-2) of 0.761 and 0.760, and square error values (SE) of 4.527 and 4.534, respectively. The optimum combination of SR (759,742) and ND (759,742) were derived from two waveband-based algorithms that corresponded to red-edge ratio spectral indices. R-2 for the novel SR and ND models was 0.794 and 0.788, respectively, confirming the superiority of the SR index. The modified red-edge ratio (mRER) was constructed based on the ratio vegetation index (RVI) that was derived from the third waveband (lambda 3) using the formula (R-759 - 1.8 x R-419)/(R-742 - 1.8 x R-419). This novel index was highly correlated with above ground N uptake and had the highest R-2 (0.813) and lowest root mean square error (4.005) of all models tested. Fitting independent data to the equations resulted in RMSE values of 21.9, 20.4, 18.6 and 16.4% between measured and estimated above ground N uptake values for Clred-edge3, MTCI, SR(759,742) and mRER, respectively, further indicating a superior fit and better performance for mRER. These models can therefore accurately estimate above ground N uptake in winter wheat, and the novel mRER index is superior for estimating above ground N uptake on a regional scale in heterogeneous fields under variable climatic conditions. (C) 2015 Published by Elsevier B.V.