Estimation of Vertical Leaf Nitrogen Distribution Within a Rice Canopy Based on Hyperspectral Data

Accurate estimations of the vertical leaf nitrogen (N) distribution within a rice canopy is helpful for understanding the nutrient supply and demand of various functional leaf layers of rice and for improving the predictions of rice productivity. A two-year field experiment using different rice varieties, N rates, and planting densities was performed to investigate the vertical distribution of the leaf nitrogen concentration (LNC, %) within the rice canopy, the relationship between the LNC in different leaf layers (LNC(Li), i = 1, 2, 3, 4), and the relationship between the LNC(Li) and the LNC at the canopy level (LNC(Canopy)). A vertical distribution model of the LNC was constructed based on the relative canopy height. Furthermore, the relationship between different vegetation indices (VIs) and the LNC(Canopy), the LNC(Li), and the LNC vertical distribution model parameters were studied. We also compared the following three methods for estimating the LNC in different leaf layers in rice canopy: (1) estimating the LNC(Canopy) by VIs and then estimating the LNC(Li) based on the relationship between the LNC(Li) and LNC(Canopy); (2) estimating the LNC in any leaf layer of the rice canopy by VIs, inputting the result into the LNC vertical distribution model to obtain the parameters of the model, and then estimating the LNC(Li) using the LNC vertical distribution model; (3) estimating the model parameters by using VIs directly and then estimating the LNC(Li) by the LNC vertical distribution model. The results showed that the LNC in the bottom of rice canopy was more susceptible to different N rates, and changes in the LNC with the relative canopy height could be simulated by an exponential model. Vegetation indices could estimate the LNC at the top of rice canopy. R(705)/(R(717)+R(491)) (R(2) = 0.763) and the renormalized difference vegetation index (RDVI) (1340, 730) (R(2) = 0.747) were able to estimate the parameter “a” of the LNC vertical distribution model in indica rice and japonica rice, respectively. In addition, method (2) was the best choice for estimating the LNC(Li) (R(2) = 0.768, 0.700, 0.623, and 0.549 for LNC(L1), LNC(L2), LNC(L3), and LNC(L4), respectively). These results provide technical support for the rapid, accurate, and non-destructive identification of the vertical distribution of nitrogen in rice canopies.