Soybean Variety Improves Canopy Architecture and Light Distribution to Promote Yield Formation in Maize–Soybean Strip Intercropping

In maize-soybean strip intercropping (MSI) and maize-soybean relay strip intercropping (MSR) systems, shading by maize reduces soybean leaf photosynthetic capacity and dry matter accumulation, threatening soybean yield. Selecting appropriate soybean varieties is effective for minimising the yield loss caused by shading and achieving stable soybean production. Field experiments were conducted during 2021-2022 to evaluate the effects of sole soybean (SS), MSI and MSR planting patterns on the growth and yield of four soybean cultivars (GQ8, GX7, ND25 and NN996). Soybeans in the MSI and MSR systems exhibited compensatory growth after maize harvest, driven by recovery of the light environment. Compared to SS, MSR sustained the yield of later-maturing and highly branching cultivars (ND25 and GQ8), whereas yields of all varieties in the MSI system decreased by 24.2%-37.1%. Although the MSR system reduced whole-canopy photosynthetically active radiation (PAR) transmittance and biomass accumulation during the vegetative stage, maize harvest at the flowering stage significantly improved light conditions. The PAR transmittance of MSR in the middle and lower soybean canopies increased to > 70%, enhancing photosynthetic rates by 5.1%-19.0% and 3.9%-55.6%, respectively, compared to SS. Compared to MSI, MSR promoted soybean branching and branch leaf formation, increasing leaf number by 17.4%-63.1%, resulting in a comparable leaf area index to SS during the grain filling stage. MSR increased biomass in medium- and late-maturing cultivars by 5.6%-21.3%, but reduced that of the early-maturing NN996 by 8.8%-14.7%. Therefore, medium- and late-maturing soybean cultivars with strong branching capacities are suitable for relay intercropping systems. Their extended growth duration and abundant branching help optimise canopy structure, enhance light interception efficiency and duration, and compensate for early biomass loss, contributing to yield stability or improvement. Our results offer valuable guidance for optimising cultivar selection and planting strategies in maize-soybean intercropping systems.