Rational Maize–Soybean Strip Intercropping Planting System Improves Interspecific Relationships and Increases Crop Yield and Income in the China Hexi Oasis Irrigation Area

Abundant light and heat in the Hexi Oasis Irrigation Area in China provide superior natural conditions for agricultural development. To study the maize–soybean planting system of intercropping and determine superior group yield and economic benefits in the Hexi Oasis Irrigation Area, eight treatments were set up in 2022–2023: maize–soybean intercropping with a bandwidth of 1.8 m and a row ratio of 2:3 (M1S3), a bandwidth of 1.8 m and a row ratio of 2:4 (M1S4), a bandwidth of 2.0 m and a row ratio of 2:3 (M2S3), a bandwidth of 2.0 m and a row ratio of 2:4 (M2S4), a bandwidth of 2.2 m and a row ratio of 2:3 (M3S3), a bandwidth of 2.2 m and a row ratio of 2:4 (M3S4), monocropping maize (M), and monocropping soybean (S). We analyzed the effects of changes in bandwidth–row ratios on photosynthetic characteristics, yield, and interspecific relationships in these treatments during two crop reproductive periods. Our results showed the following: (1) Under the intercropping system, the photosynthetic capacity of maize was highest when the row ratio was 2∶3 and bandwidth was 1.8 m. The net photosynthetic rate (Pn) increased by 1.72% to 48.90%, the transpiration rate (Tr) increased by 5.53% to 118.10%, and stomatal conductance (Gs) increased by 2.82% to 86.49% compared with other planting systems. Increasing the bandwidth from 1.8 m to 2.2 m improved the photosynthetic characteristics of soybean, increasing Pn, Tr, and Gs by 3.44% to 74.21%, 3.92% to 53.69%, and 2.41% to 55.22%, respectively. (2) The yield of maize and soybean under monocropping was significantly higher than that under intercropping. In the intercropping treatments, the average yield of crops in the M3S3 system was 16,519.4 kg ha−1, an increase of 6.48% compared with the M3S4 system, indicating that the reduction of one row of soybean in the same bandwidth system increases crop yield; The average economic benefit of the M3S3 system over two years was 35,171.73 CNY ha−1, which increased by 13.3 and 80% compared with the average economic benefit of maize and soybean monocropping, indicating that the intercropping system leads to better economic results for farmers than monocropping. (3) In the two-year experiment, the land equivalent ratio (LER) was highest in the M3S3 model, averaging 1.25 over the two years, showing better land productivity compared with other intercropping systems. (4) When bandwidth was 1.8, 2.0, or 2.2 m, the LER decreased by 8.3, 5.9, and 5.6% when planting an additional row of soybeans, the relative crowding coefficient of soybeans in the respective bandwidths increased by 4.59, 4.72, and 0.75%, the competition ratio of maize (CRM) increased by 22.94, 16.97, and 12.74%, the competition ratio of soybean (CRS) decreased by 20.47, 17.61, and 16.78%, and the competitive power of maize was greater than that of soybean, indicating that the increase in soybean rows in the same bandwidth system would weaken the competitive advantage of soybean, resulting in crop yield and economic benefit reduction. When the row ratio was 2:3 or 2:4, bandwidth increased from 1.8 m to 2.2 m, LER decreased by 3.31 and 0.86%, intercropping maize aggressiveness (AM) decreased by 7.55 and 12.50%, CRM decreased by 18.04 and 24.84%, and CRS increased by 17.32 and 22.77%, respectively, which indicated that the increase in bandwidth under different row ratio systems could improve the competitive advantage of intercropping soybean, thereby improving crop yield and economic benefits. (5) The AHP method, entropy weight method, and TOPSIS analysis showed that M3S3 ranked first, with the highest comprehensive evaluation (0.6017). In conclusion, the M3S3 planting system can better coordinate crop interspecies relationships, with higher land yield and economic benefit, and can be used as a suitable maize–soybean intercropping system in the Hexi oasis irrigation area.