Research on Marginal Effects of Dry Matter Accumulation and Extinction Coefficient in Maize–Soybean Intercropping Composite Population

Maize–soybean intercropping can improve land use efficiency and increase crop yields, thereby contributing to sustainable agricultural development. This study aimed to investigate the marginal effects of intercropping on dry matter accumulation and the extinction coefficient (K) of maize and soybean canopies, thereby providing theoretical guidance for efficient resource utilization and the realization of high-yield and high-efficiency maize–soybean intercropping systems. This experiment employed maize variety Jincheng 316 and soybean variety Heinong 531. Two intercropping treatments (S4M4: four rows soybean alternating with four rows maize; S6M4: six rows soybean alternating with four rows maize) and two sole cropping controls (M: sole maize; S: sole soybean) tested the marginal effects of different intercropping patterns on crop dry matter accumulation, SPAD, and K. Combined with analyses of yield, land equivalent ratio (LER), and intercropping advantage (IA), we determined the optimal row ratio configuration for intercropping. The results showed that, compared with monocropped maize, the leaf SPAD at the R1 stage and grain dry matter accumulation at the R6 stage under S4M4 and S6M4 treatments increased by 17.28%/20.12% and 8.07%/5.35%, respectively, while K values decreased by 21.80% and 14.37%. For soybean, relative to monocropping, the R4 leaf SPAD and R8 grain dry matter accumulation under S4M4 and S6M4 were elevated by 23.87%/19.40% and 26.66%/13.56%, respectively, with corresponding K values reductions of 21.94% and 21.82%. Moreover, S4M4 exhibited a 51.67% higher IA and a 9.48% higher LER than S6M4. In summary, maize–soybean intercropping significantly boosts dry matter accumulation and resource use efficiency, with the S4M4 configuration exhibiting the most distinct advantages.