Optimizing trade-offs between light transmittance and intraspecific competition under varying crop layouts in a maize–soybean strip relay cropping system

Light is one of the most important environmental factors for plant growth and development. In relay cropping systems, crop layouts influence light distribution, affecting light use efficiency (LUE). However, the response of light interception, light conversion, and LUE for relay maize and relay soybean to different crop layouts remains unclear. We aimed to quantify the effect of crop layout on intraspecific and interspecific competition, light interception, light conversion, LUE, and land productivity between relay maize and relay soybean. We conducted a field experiment for four consecutive years from 2017 to 2020 in Sichuan province, China, comparing different crop layouts (bandwidth 2.0 m, row ratio 2:2; bandwidth 2.4 m, row ratio 2:3; bandwidth 2.8 m, row ratio 2:4), with sole maize and sole soybean as controls. The results showed that relay maize in the 2.0 m bandwidth layout had the largest leaf area index and plant biomass, the lowest intraspecific competitive intensity and the highest aggressiveness. Compared to a bandwidth of 2.0 m, a bandwidth of 2.8 m significantly decreased relay maize leaf area index by 11% and plant biomass by 24%, while a 2.4 m bandwidth caused roughly half these reductions. The 2.0 m bandwidth layout also significantly improved crop light interception and LUE compared to sole maize. The light interception, light interception rate, light conversion rate and LUE in relay maize all decreased significantly with increasing bandwidth, but they increased in relay soybean. The increased light transmittance to the lower and middle canopy with increasing bandwidth did not compensate for the loss of relay maize yield caused by increased intraspecific competition. However, it enhanced the yield of relay soybeans. Increasing the bandwidth by 80 cm increased the relay maize intraspecific competition by 580%, and reduced maize yield by 33%, light interception by 12%, and LUE by 18%. In contrast, the relay soybean intraspecific competition was reduced by 64%, and the soybean yield was increased by 26%, light interception by 32% and LUE by 46%. Relay cropping systems with a 2.0 m bandwidth optimize the trade-off between light transmittance and intraspecific competition of relay crops. These systems achieve the highest LUE, group yield and economic benefits, making them a recommended crop layout for the southwest regions of China. Our study offers valuable insights for developing strip relay cropping systems that maximize light utilization and contributes to the theoretical understanding of efficient sunlight use in relay cropping practices.