Optimized phosphorus partitioning enhances phosphorus use efficiency in strip-intercropped soybean

Intercropping systems are widely adopted to enhance nutrient use efficiency and overall system productivity. However, shading from tall-stalked crops often lowers phosphorus use efficiency (PUE) and yield in understory crops compared to monocropping. The mechanisms by which soybean maintains PUE and productivity under low light remain poorly understood. We conducted a two-year field screening of 120 soybean genotypes in a maize soybean strip intercropping system, using P use efficiency for seed production (PUES) as the primary evaluation index. Two genotypes with comparable P uptake but contrasting PUES—QH34 (P-efficient) and SK90 (P-inefficient)—were selected for further analysis. QH34 achieved an average PUES of 180.98 g yield g⁻¹ P, compared to 119.44 g yield g⁻¹ P for SK90. The P-efficient genotype exhibited optimized P allocation, with greater partitioning to leaves and seeds during the reproductive stage, while minimizing P investment in stems. It also sustained a higher photosynthetic P use efficiency (PPUE) in the intercropping system by enhancing the allocation of metabolite P to the leaves (163 % higher than the P-inefficient soybean) while reducing the allocation of lipid P (45 % lower than the P-inefficient soybean). Additionally, QH34 showed smaller reductions in leaf mass per area and higher net photosynthetic rates, reflecting structural stability and better adaptability to shade. The enhanced PUE of P-efficient soybean in intercropping is thus driven by strategic P allocation and stable leaf structure, ensuring sustained photosynthetic performance. These findings provide theoretical and practical insights for breeding soybean cultivars with high resource-use efficiency suitable for intercropping systems. • Soybean yield in intercropping depends on P use, not P uptake efficiency. • High-PUE soybean in intercropping assigns more P to leaves and seeds, less to stems. • Increased metabolic P and reduced lipid P improve photosynthetic P-use efficiency. • Higher leaf mass per area boosts metabolic P storage and photosynthesis. • Functional optimization and structural stability enhance PUE in intercropped soybean.