Nitrogen Application Under Aerated Irrigation Mitigated Drought Stress by Improving Leaf Carbon and Nitrogen Reserves in Tomato

Nitrogen (N) application can improve drought tolerance and water use efficiency (WUE) in crops. Previous studies have shown that aerated irrigation improves crop nitrogen absorption and utilization. However, the mechanisms behind the interaction of water and nitrogen under aerated drip irrigation and its impact on crop WUE remain unclear. This study conducted a 2‐years greenhouse experiment with spring‐summer and autumn‐winter tomato to investigate the effects of water and nitrogen coupling on leaf carbon (C) and nitrogen content, photosynthetic characteristics, plant dry matter accumulation, yield, and WUE. The experiment included three irrigation levels (W1, 50% ETc; W2, 75% ETc; W3, 100% ETc) and three nitrogen application rates (N1, 0 kg ha−1; N2, 150 kg ha−1; N3, 250 kg ha−1). The results showed that increased nitrogen application and irrigation levels significantly increased leaf carbon and nitrogen content, net photosynthetic rate (Pn), and stomatal conductance (Gs) (p < 0.05). Under deficit irrigation, nitrogen application increased leaf carbon content by 2.17% and nitrogen content by 9.34%, improved leaf photosynthetic capacity, and increased Pn by 15.57% and Gs by 19.32%. The W2 treatment demonstrated more significant improvement compared to W1. The W3N3 treatment produced the highest plant dry matter accumulation for both tomato types, with no significant difference from W2N3 (p > 0.05). The W2N3 treatment produced the highest yield, 8.67%–9.13% higher than W3N3. The highest WUE occurred in W2N3 for spring‐summer tomato and W1N3 for autumn‐winter tomato. Although W1N3 had 1.02% higher WUE than W2N3, it had a 15.25% lower yield. Thus, W2N3 is recommended as the optimal water‐nitrogen management strategy for greenhouse tomato production. Correlation analysis revealed that leaf carbon and nitrogen contents positively correlated with Pn, plant dry matter accumulation, and yield, whereas the leaf ratio of carbon and nitrogen (C/N) negatively correlated with WUE, suggesting that leaf carbon and nitrogen contents regulate tomato WUE. Nitrogen application under deficit irrigation enhanced leaf carbon and nitrogen contents, photosynthetic capacity (Pn, Gs), plant dry matter accumulation, yield, and WUE. Regression models suggest that the optimal water and nitrogen application rates for greenhouse tomatoes are 192.30–225.67 mm and 205.93–243.43 kg ha−1 for spring‐summer tomato, and 162.00–181.18 mm and 194.98–237.73 kg ha−1 for autumn‐winter tomato. These findings provide a theoretical basis for water‐efficient agricultural practices and sustainable greenhouse tomato production.