Abstract:

Unmanned Aerial Vehicle Remote Sensing (UAVRS) has the features of real time, flexibility, and low cost. It has been extensively used in monitoring crop nutrition and precision agricultural management. Litchi is a tropical fruit with the largest planting area, the most distinctive variety characteristics, the most obvious regional advantages, and the longest planting history in South China. Its yields are low and unstable. Nutrition diagnosis and fertilization technology as the main factors restricting the yields and quality of litchi have been research hotspots of agricultural precision management. Using litchi in the flower bud differentiation stage as an example, litchi orchards in Huizhou City, Guangdong Province, were monitored using a UAV with a Parrot Sequoia multispectral camera, and the spatial distribution differences of nutrient content in the canopy leaves were analyzed. Eighteen spectral variables were selected for the study, and the spectral variables and leaf nutrient mass fraction were analyzed by correlation analysis. A regression equation between the vegetation index and nutrient mass fraction was established by screening the correlation relationship between leaf nutrient mass fraction and vegetation index with significant and high correlations. The stability of an Residual Prediction Deviation (RPD) comprehensive evaluation model with determination coefficient, cross-validation determination coefficient, and prediction residual, as well as the relationship between the leaf nutrient mass fraction and vegetation index in different canopy layers of litchi were all discussed. The system and its influence on a UAV multi-spectral remote sensing monitoring model were examined. The results revealed the following. 1) The leaf nitrogen and potassium contents in different canopy layers of litchi increased significantly with a decrease in canopy height, particularly the spatial distribution of potassium. 2) The nitrogen mass fraction of the upper and middle leaves of the canopy was significantly correlated with the Carotenoid Reflectance Index (CRI) index as calculated by orthophoto data. The correlation between the nitrogen mass fraction in the upper leaves and CRI was significant (r=0.86, p<0.01). The potassium mass fraction in the middle and lower leaves of the canopy was significantly correlated with the multispectral parameters, namely, Normalized Green (NG) and Normalized Near Infrared (NNIR), of orthophoto data derived from the UAV. Of these parameters, the middle potassium mass fraction was significantly correlated with the spectral variable NNIR (r=-0.80, p<0.01) and the lower leaf mass fraction had the highest correlation with the NG index (r=-0.83, p<0.01). This indicated that the UAV multispectral data had the potential to estimate the changes in nitrogen and potassium mass fraction in litchi leaves of different layers. 3) The vegetation index CRI could effectively retrieve the nitrogen quality fraction of litchi upper and middle layers. The upper layer nitrogen model R², and RPD were 0.74, 0.57, and 1.50, respectively. The middle layer nitrogen model R², R^2_CV and RPD were 0.64, 0.44, and 1.40, respectively. NNIR, NG, and other spectral parameters could better retrieve the potassium quality fraction of litchi upper and lower layers, where the upper layer nitrogen model R², R^2_CV and RPD were 0.64, 0.44, and 1.40, respectively. The potassium model R², R^2_CV and RPD in the middle layer were 0.79, 0.56 and 1.50, respectively. The potassium model R², R^2_CV and RPD in the lower layer were 0.69, 0.55, and 1.70, respectively. The model accuracy of the nutrient mass fraction in the full canopy was also higher (R²> 0.70,R^2_CV > 0.50, and RPD > 1.4), indicating that the spatial variation of the nutrient mass fraction in the litchi canopy leaves was based on vertical remote sensing data. The accuracy of the model for estimating the leaf nutrient content was affected. The application of UAV multi-spectral remote sensing image data can monitor litchi nutrient content very well and provide information support for precise fertilization management of litchi orchards, which has important research significance and application value.

Key words: litchi, nutrient content, multispectral, vertical diversification, UAV