Adjusted nitrogen application using non-destructive monitoring model of citrus leaf functional nitrogen content

The concentration and distribution of functional nitrogen (N) in citrus leaves can be significant indicators for the formation and transportation of fruit assimilation. A non-destructive monitoring model can be used for the functional nitrogen concentration in the leaves. The N application can also be adjusted to quantify the citrus nitrogen using hyperspectral technology. The five-year 'Chunjian' orange was taken as the experimental material in the Changshou District of Chongqing in China. The control treatments of nitrogen application with the different gradients were set: N0, N1, N2, and N3 (Nitrogen application qualities were 0, 50, 100, and 200 g/plant, respectively). The adjusted nitrogen treatments were named Nr1, Nr2, and Nr3, according to the non-destructive monitoring model for the functional nitrogen concentration in the citrus leaf. In the first year of the experiment, the leaves of the spring shoot (the second to fourth leaves from the top to the bottom) were collected at the fruit expansion and color-changed period, respectively. Sixteen leaves were randomly selected from each tree, according to the four directions of "south, east, north, west", where the spectral values were determined simultaneously. A non-destructive monitoring model was then established for the functional nitrogen concentration in the citrus fruit leaves at the fruit expansion and color-changed period by the hyperspectral technique. In the second year, the leaf functional nitrogen concentration (LFNC) model and topdressing formula were used to calculate the actual nitrogen application ratio. The fertilizer of the actual nitrogen application ratio was applied in the adjusted N application treatments at the fruit expansion and color-changed period. A comparison was made to clarify the effects of control and adjusted nitrogen application on the yield, fruit quality, and nitrogen use efficiency. The results show that the LFNC model performed the higher accuracy using the back propagation neural network, where the R2 were 0.78 (fruit expansion period) and 0.74 (fruit color-changed period). The Nr1 and Nr3 treatments increased the yield by 5.49, and 4.43 kg/ plant with the rate of increments of 48% and 40%, respectively, compared with the N1 and N3. Compared with the N1, the single fruit weight and soluble solid content of the citrus increased significantly by the adjusted N treatment Nr1. The single fruit weight of adjusted N treatment Nr1 increased by 54.05 g, where the soluble solid content increased by 0.77%, compared with the N1. However, there was no change in the transverse and longitudinal diameter of the citrus fruits and fruit shape index between the control and adjusted N treatments. The partial factor productivity of applied (PFP-N) of adjusted N application treatments with the Nr1 was 10% lower than that of the control with the N1. There was only a little change in the fruit shape index and soluble solids of Nr3. Specifically, the single fruit weight increased by 24.45 g, compared with the N3. The PFP-N increased by 123% in the Nr3, compared with the N3. The agronomic efficiency of the Nr2 and Nr3 increased by 290% and 364%, compared with the N1 and N3, respectively. There was no significant difference in the yield, quality, and nitrogen use efficiency between the Nr2 and N2. In conclusion, the adjusted nitrogen application using the non-destructive monitoring model of the citrus leaf functional nitrogen concentration can be expected to reduce the effects of insufficient or excessive nitrogen application on the citrus yield and quality, in order to improve the nitrogen partial productivity and agronomic efficiency. The finding can provide the theoretical basis and technical support to realize the non-destructive monitoring of functional nitrogen concentration in the citrus leaves and adjusted nitrogen application.