Optimizing Water and Nitrogen Management for Green Pepper (Capsicum annuum L.) under Drip Irrigation in Sub-Tropical Monsoon Climate Regions

Green pepper (Capsicum annuum L.) is one of the major vegetables cultivated in sub-tropical monsoon climate regions. However, with the unreasonable use of water and nitrogen (N) fertilizer, efficient water and N fertilizer management systems need to be identified. The goal of this project was to investigate the coupling effects of different amounts of water and N on green pepper yield, water use efficiency (WUE), as well as N use efficiency (NUE) in sub-tropical monsoon climate regions. The optimum combination of water and N inputs was determined for multi-objective optimization through the multiple regression analysis and the combinations of likelihood functions. The pot experiment was conducted during the green pepper growing seasons (May-September) of 2019 and 2020 in a greenhouse at Nanchang, Jiangxi of China that included three water deficit levels, i.e., mild deficit (W1: 95 similar to 80%theta FC, %theta field capacity simplified as %theta FC), moderate deficit (W2: 80 similar to 65%theta FC), and severe deficit (W3: 65 similar to 50%theta FC), and four levels of nitrogen application (N-app) rate, i.e., 6.0 (N1), 3.0 (N2), 1.5 (N3), and 0.0 g plant(-1) (N4), for a total of twelve treatments (i.e., 3 x 4) with six replications. Results show that water levels have an extremely significant effect (p < 0.01) on green pepper yield and WUE, but no effect on NUE (p > 0.05). N treatments have significant effects on green pepper yield, WUE, and NUE. Meanwhile, the effects of water levels and N treatment interaction on WUE and NUE were extremely significant (p < 0.01), but varied on yield between the two years. The maximum yields (576.26 and 619.00 g plant(-1)) occurred when the water level and N-app rate were 80 similar to 65%theta FC and 6.0 g plant(-1). While the water level and N-app rate were 80 similar to 65%theta FC and 3.0 g plant(-1), the WUEs and NUEs reached the maximum, which were 20.14 and 17.71 g L-1, 76.54, and 77.73 g(-1) in 2019 and 2020. The dualistic and quadric regression equations of irrigation amount and N-app rate indicated that the yield, WUE and NUE cannot reach the maximum at the same time. By establishing a multiobjective optimization model using combinations of likelihood functions, it was concluded that the water level shall be controlled in 80 similar to 65%theta FC and the N-app rate is 3.78 g plant(-1), which can be used as the suitable strategy of water and N management for the maximum comprehensive benefits of yield, WUE, and NUE for green pepper. The obtained optimum combination of water and N inputs can provide a scientific basis for irrigation and fertilization optimization and management in sub-tropical monsoon climate regions.