Effects of Water and Nitrogen Coupling on Yield, Quality, and Water Use Efficiency of Drip-Irrigated Watermelon Under Organic Fertilizer Application

A two-factor experiment was conducted using the cultivar ‘Xin you No. 2’ (Citrullus lanatus) to identify an efficient and green production model for drip-irrigated watermelon under plastic mulch in Southern Xinjiang. A basal organic fertilizer was applied at 2250 kg·ha−1. The experimental design comprised three irrigation levels, maintaining soil moisture at 60–70% (W1), 70–80% (W2), and 80–90% (W3) of field capacity, and three nitrogen application rates: 180 (N1), 240 (N2), and 300 (N3) kg·ha−1. This study systematically investigated the effects of water–nitrogen coupling on watermelon yield, quality, water use efficiency, and nitrogen partial factor productivity. The W2N2 treatment achieved the highest yield of 64,617.59 kg·ha−1. Vine length, stem diameter, and dry matter accumulation increased with increasing nitrogen application under the W1 and W2 irrigation levels, but exhibited an initial increase followed by a decrease under the W3 condition. Water restriction combined with increased nitrogen application significantly enhanced the central sugar content, with the W1N3 treatment increasing it by 15.69% compared to CK. Conversely, the W1N1 treatment was most conducive to vitamin C accumulation, showing a 49.88% increase over CK. The total water consumption across the different treatments ranged from 362.12 to 493.92 mm. Both water use efficiency and irrigation water use efficiency reached their maximum values under the W1N3 treatment, at 21.94 kg·m−3 and 35.05 kg·m−3, respectively. In contrast, the highest partial factor productivity of nitrogen (NPFP) was observed under W3N1, reaching 239.33 kg·kg−1. A comprehensive multi-index evaluation using the Technique for Order Preference by Similarity to an Ideal Solution (TOPSIS) method indicated that the W1N3 treatment achieved the highest relative closeness (0.669), identifying it as the optimal water–nitrogen combination.