Objective Nitrogen (N) and phosphorus (P) are essential nutrients for plant growth and play vital roles in modern agricultural production. however, imbalanced of nitrogen (N) and phosphorus (P) nutrients is common in farmland across China. The application of N or P fertilizer alone (the N or P treatment) has limited effects on crop yield, whereas combined N and P application(NP) can markedly increase yield. The magnitude of this interaction depends on fertilization practices, soil properties, and climatic conditions. Previous studies have largely been carried out in a specific region or condition, lacking a comprehensive quantitative assessment of N-P interaction effects at the national scale. Therefore, quantifying the interaction effects of the NP application on crop yield at the national scale, and clarifying how these effects vary with climate, soil, and fertilizer inputs, can provide both a theoretical foundation and practical guidance for the efficient use of N and P fertilizers in China.

Methods A total of 225 group paired observations were compiled from 158 published studies, including four treatments: no fertilization (CK), N alone(N), P alone(P), and combined N and P application(NP). The main and interaction effects of N and P on crop yield under NP treatment were quantified by meta-analysis and random forest model.

Result Compared with the N or P alone treatment, the NP application treatment significantly increased crop yield by 17.0% and 39.7%, respectively. At regional scale, the yield-increasing effect of the NP treatment was highest in North China, and relatively lower in Northeast and Northwest China. With increasing N fertilizer input, the N-P interaction effect decreased significantly, with synergistic effects gradually shifting toward additive or even antagonistic effects. Synergistic effects were more likely to occur in soils lacking N and P nutrients, and were primarily regulated by soil pH and available phosphorus. The N-P interaction effect was highest in regions with annual average temperature of 12–16℃ and annual precipitation of 600–1200 mm, with mean annual temperature (MAT) identified as the main driving factor. In addition, crop type significantly affected the interaction between nitrogen and phosphorus, and the interaction effect was the highest in rice and the lowest in maize, but the synergistic effect of wheat was the highest among the three interaction types.

Conclusion In general, NP application significantly improves crop yield in China, with pronounced regional variability. The N-P interaction is predominantly synergistic, especially under low nitrogen input conditions (N≤150 kg/hm²). Crop-specific responses indicate that rice benefits most from N–P interactions, whereas maize shows relatively weaker responses. Random forest model analysis further clarified that annual average temperature is the core climatic driving factor, while soil pH and available phosphorus are the main soil regulatory factors. These interactions are comprehensively regulated by soil properties and climatic conditions and are more likely to show significant synergistic effects in temperate continental climate zones with moderate temperature and precipitation and in nutrient-deficient soils.