Optimizing maize‑soybean strip intercropping: effects of row management and maize density on photosynthetic enzyme activity, grain quality, and yield in subtropical China

In subtropical agroecosystem of Guangxi, China, where land availability is constrained optimizing the maize-soybean strip intercropping system (IS) is critical for sustainable intensification of smallholder farming. A two-year field experiment (2022–2023) investigated the effects of two distinct row configurations: four maize rows intercropped with four soybean rows (F) and two maize rows intercropped with two soybean rows (T), under three maize planting densities (D1: 45,000; D2: 52,500; D3: 60,000 plants ha ^–1 ). Maize and soybean monocultures were included as controls. Over two growing seasons, key yield components, grain nutritional profiles, photosynthetic enzyme activities, and land equivalent ratio (LER) were measured. Intercropping resulted in a significant increase in maize yield compared to monoculture, with increases ranging from 23.24%–45.34%. The T configuration consistently outperformed the F configuration in terms of system-level productivity. LER for both IS ranged from 0.93 to 1.21, with the TD2 treatment maintaining an LER > 1 in both years, demonstrating enhanced land use efficiency. Additionally, optimized intercropping significantly increased maize crude protein content, with an average increase of 1.79% to 26.79% across years. In contrast, soybean grain quality improved significantly under intercropping, with soluble sugar, starch, and crude fat content increasing by 10.24% and 13.57%, 14.89% and 19.18%, and 7.39% and 7.97%, respectively. The superior performance of the TD2 was associated with enhanced photosynthetic enzyme activity, particularly the 44.36% increase in RuBisCO activity, and greater post-anthesis dry matter accumulation. Correlation analysis revealed strong positive relationship between dry matter accumulation, photosynthetic efficiency, and yield formation. Among the maize-soybean intercropping combinations tested, TD2 provided the optimal balance between yield, grain quality, and land-use efficiency, offering a sustainable and economically viable solution for enhancing productivity and resource use efficiency for smallholder farmers in subtropical regions.