Optimizing Irrigation and Fertilization for Greenhouse Pepper Under Slightly Saline Water in Arid Regions

Water scarcity and soil salinization are major challenges for sustainable agriculture in arid regions, affecting crop growth, yield, and quality. In greenhouse systems, optimizing irrigation and nutrient management under brackish (slightly to moderately saline) irrigation water (1–5 g L−1) is essential for maintaining productivity and resource efficiency. This study investigated the effects of irrigation water salinity, irrigation volume, and nitrogen, phosphorus, and potassium application on growth, yield, fruit quality, and water–nutrient use efficiency of greenhouse-grown pepper (Capsicum annuum L., cv. ‘Qilin 99’) in southern Xinjiang. A five-factor, five-level half-fractional quadratic orthogonal rotatable design was employed. Pepper yield showed a unimodal response to increasing salinity, peaking at 3 g L−1 with 4800 m3 ha−1 irrigation and N, P2O5, K2O rates of 225, 160, and 500 kg ha−1, respectively. Water use efficiency and fertilizer partial factor productivity decreased significantly with increasing salinity and irrigation amount, reaching maximum under moderate irrigation water mineralization combined with low and medium irrigation levels, respectively. Fruit quality traits, including vitamin C, capsaicin, and free amino acids, were enhanced under moderate to relatively high salinity levels, whereas fruit size and single fruit weight were highest at lower salinity combined with higher irrigation. Irrigation water salinity was identified as the main limiting factor for yield and quality. Overall, greenhouse pepper exhibited a nonlinear dual-threshold response to combined water, nutrient, and salinity management, with an optimal threshold around 3 g L−1. These findings provide practical guidance for improving water and nutrient use efficiency in greenhouse agriculture under slightly saline irrigation.