Spatiotemporal changes in light conditions enhance the nutritional quality of soybean seeds in the soybean-maize strip relay intercropping system

• The nutritional quality of soybean seeds is enhanced in the soybean-maize strip relay intercropping (RI) system as compared to the soybean sole cropping system. • Temporal light recovery following maize harvest in the RI system contributes to the improvement of nutritional quality in soybean seeds. • The optimized spatial light distribution resulting from the strip planting pattern also plays a significant role in enhancing the nutritional quality of soybean seeds. Soybean-maize strip relay intercropping (RI) facilitates an additional harvest of soybean while stabilizing maize yields through optimized resource allocation, representing a crucial approach for expanding soybean cultivation and improving soybean productivity in China. However, the impact of RI on the nutritional quality of soybean seeds and the associated regulatory mechanisms remain unclear. In this study, differences in the metabolomic profiles and nutritional quality of soybean seeds between the RI system and an equivalent-density soybean sole cropping system (SC) were investigated based on three years of field experiments. Results showed that the nutritional quality of soybean seeds from the RI system is significantly enhanced compared to those from the SC system, with increases of 6.97, 21.89, 22.80, 19.18 and 10.94% in the levels of oil, soluble sugars, isoflavones, carotenoids and total carbon, respectively. Conversely, protein content slightly decreased by 2.15%. Metabolomic analysis revealed that RI promotes metabolic pathways associated with carbon fixation and the formation of nutritional quality in soybean seeds. Further artificial shading treatments confirmed that spatiotemporal changes in light conditions within the RI soybean canopy are critical drivers of these improvements. Temporally, shading from maize during the vegetative stage of soybeans decreased light availability in the canopy; however, light recovery following maize harvest increased this availability during the reproductive stage, resulting in a 4.82% increase in carbon fixation in RI seeds compared to SC seeds. Spatially, maintaining a spacing of ≥60 cm between soybean and maize strips effectively mitigated the shading impacts of maize, thereby reducing the marginal disadvantage for soybeans. After maize harvest, the wide-narrow-row configuration improved light conditions in the middle and lower parts of the soybean canopy, alleviating intraspecific shading and enhancing marginal advantages. This effect, akin to light enrichment, led to a 3.51% increase in carbon fixation in RI seeds compared to SC seeds. In conclusion, this study demonstrates that RI alters carbon-nitrogen dynamics and secondary metabolism through the synergistic regulation of temporal light recovery and spatial light enrichment, ultimately enhancing the nutritional quality of soybean seeds. These findings provide a theoretical basis for optimizing strip intercropping systems and promoting the sustainable production of high-quality soybean seeds.