Monitoring Leaf Nitrogen Status with Spectral Reflectance in Wheat.
Yan Zhu1, Yingxue Li1, Wei Feng1, Yongchao Tian1, Xia Yao1, and Weixing Cao2. (1) Dep. of Agronomy, Nanjing Ag. Univ., Nanking, 210095, China, (2) Hi-Tech Key Lab of Information Agriculture of Jiangsu Province, Nanjing Agricultural University, Nanjing, Jiangsu, 210095, China
Non-destructive monitoring of leaf nitrogen (N) status can assist in growth diagnosis, N management and productivity forecast in field crops. The objectives of this study were to determine the relationships of leaf nitrogen concentration on leaf dry weight basis (LNC) and leaf nitrogen accumulation per unit soil area (LNA) to ground-based canopy reflectance spectra, and to derive the regression equations for monitoring N nutrition status in wheat (Triticum aestivumL.). Four field experiments were conducted with different N application rates and wheat cultivars across four growing seasons, and time-course measurements were taken on canopy spectral reflectance, LNC and leaf dry weights under the various treatments. The results showed that LNC and LNA in wheat increased with increasing N fertilization rates. The canopy reflectance differed significantly under varied N rates, and the pattern of response was consistent across the different cultivars. Overall, an integrated regression equation of LNC to normalized difference index (NDI) of 1220 and 710 nm of canopy reflectance spectra could describe the dynamic change pattern of LNC with reflectance spectra in wheat. The ratios of several bands in near infrared (NIR) to visible light were linearly related to LNA, with the ratio index (RI) of the average reflectance over 760, 810, 870, 950 and 1100 nm to 660 nm being the best index for quantitative estimation of LNA in wheat. The average root mean square error (RMSE) values of the predicted LNC and LNA relative to the observed values were no more than 15.1% and 15.2%, respectively, indicating a good fit between the estimated and observed LNC and LNA. Thus it is concluded that the above relationships of leaf N status to spectral indices of canopy reflectance can be potentially used for non-destructive and real-time monitoring of leaf N status in wheat.