Optimal interspecific distance maintains soybean yield by promoting canopy–root synergy in a maize–soybean relay strip cropping system

Maize–soybean relay cropping increases land-equivalent ratio, but shading often limits soybean productivity. Optimizing strip relay configurations improves the light environment for soybean, enhancing its photosynthetic capacity and yield. In a four-year trial, we tested maize–soybean relay strip cropping at interspecific distances of 30, 45, 60 and 75 cm, and monocropping soybean. We measured photosynthetic characteristics, photosynthate allocation, root traits, nitrogen (N) uptake and yield to elucidate the canopy-root synergy driving spacing-induced yield gains and identify the optimal interspecific distance. Increasing interspecific distance significantly improved canopy transmittance and photosynthetically active radiation (PAR). The 60 cm treatment (MS60) increased transmittance and upregulated leaf antioxidant enzyme activity, thereby enhancing leaf area index, SPAD and net photosynthetic rate. Compared with other relay cropping treatments, MS60 increased ¹³C content and sucrose accumulation by 17.8%–69.7% and 7.1%–34.9%, respectively, and increased N uptake by 20.1% on average. The dual boost in carbon and nitrogen accumulation led to an 11.6%–29.3% yield increase under MS60, with a soybean yield of 1.9 t ha⁻¹ that was close to the monocropped soybean yield of 2.1 t ha⁻¹. This yield advantage was attributed to increased canopy radiation and carbon (C) accumulation that increased root development and N uptake. MS60 optimizes the balance between interspecific compensation and intraspecific competition in the relay strip cropping system, increasing maize yield while maintaining soybean yield at monoculture levels.