Effects of different irrigation lower limits and combined application of reduced nitrogen with organic fertilizer on spring-season tomatoes (Solanum lycopersicum)

Greenhouse tomatoes (Solanum lycopersicum) are a high-value cash crop globally; however, inefficient water and nitrogen management in greenhouse systems often leads to resource waste, environmental risks, and reduced yield/quality, thereby posing challenges for sustainable cultivation. Herein, we aimed to optimize water and fertilizer management for spring greenhouse tomatoes by investigating how different irrigation lower limits, combined with reduced nitrogen and organic fertilizer, affect photosynthetic performance, yield, fruit quality, and water–nitrogen use efficiency. A three-factor experiment was conducted in 2024. Treatments included two irrigation levels (L) [75–90% field capacity (FC; L1) and 85–90% FC (L2), where FC refers to the soil field capacity]; two basal organic fertilizer application rates (O) [high (O1: 400 kg decomposed sheep manure) and low (O2: 280 kg decomposed sheep manure)]; and three topdressing nitrate-nitrogen levels (N) [conventional (N1: 352.14 kg·hm ⁻ ²), 20% reduced (N2: 281.72 kg·hm ⁻ ²), and 40% reduced (N3: 211.29 kg·hm ⁻ ²)]. Data were analyzed using three-way ANOVA, principal component analysis (for photosynthetic and quality traits), and entropy-weighted TOPSIS (comprehensive evaluation). The first-level indicators in the system included photosynthetic indicators (F1), yield indicators (F2), quality indicators (F3), water use indicators (F4), partial factor productivity of nitrogen (F5), and nitrogen–phosphorus–potassium accumulation (F6). The results revealed that the treatments L2, N3 and O2 can increase the average yield of tomatoes, and the combination of L2 (85%–90% FC), O2 (low organic fertilizer), and N2 (20% reduced nitrogen) not only displayed the highest levels of vitamin C (21.46 mg/100g) and soluble solids content (5.62%) in fruits but also exhibited the highest total nitrogen content in fruits, stems, and leaves across all treatments, surpassing the lowest-performing treatment by 62.83%. Consequently, this treatment yielded the highest comprehensive performance.