Experimental design and field management

Experiment 1: Experiment 1 was conducted in 2014 under field conditions at the Teaching and Experimental Farm of Sichuan Agricultural University in Ya’an (29°59’N, 103°00’E), which is located at the western border of the Sichuan Basin. The soil of the experimental field is a purple clay loam (pH 7.5). On June 19th, the seeds of 20 soybean varieties (names refer to Supplementary Table 1) were sown, each varieties were sown in one plot with a row spacing of 0.5 m and a row length of 2.5 m, and three rows of each variety were planted for each condition. The shade and full sun light treatments were started immediately after sowing. The shade treatment was achieved by installing green shading nets above the experimental field at a height of 2 m, ∼40% transmittance, PAR was around 500 μmol m^–2 s^–1, average daily temperature was 30.7 °C, average daily humidity was 71.9%. The average daily temperature and humidity of full sun light treatment was 32.9 °C and 64.5% (Yang et al., 2014; Wu et al., 2017b; Figures 1A,B). After the first trifoliolate leaves expanded, soybean seedlings were thinned to 0.1 m between plants in each row.

Photographes of the experiment. (a) Shade treatment in 2014; (b) full sun light treatment in 2014; (c) maize-soybean relay strip intercropping in 2015; (d) soybean monoculture in 2015.

Experiment 2: In order to further verify the shade response of soybean and variety performance of 2014 in a real intercropping shade environment, we conducted Experiment 2 in 2015 at the same field of 2014, so the soil of the experimental field is a purple clay loam (pH 7.5). It was conducted by a split-plot experiment design with three replications. The planting pattern was set as main plot with two levels: maize-soybean relay strip intercropping (shade) and soybean monoculture (full sun light) (Figures 1C,D). 14 soybean varieties (in order to reduce the field work, we only selected 14 varieties with large difference in growth in shade) were used in sub plot based on the results of 2014 (Supplementary Table 1). The field arrangement of relay intercropping was carried out as follows. Briefly, For the full sun light treatment in sole cropping, soybeans were planted as solid rows with a 0.5-m row spacing. For the shade treatment in intercropping, soybean and maize were planted as alternating strips, and every soybean strip was relay intercropped between the maize strips. Each plot contained two maize and two soybean strips, and each soybean strip and maize strip consisted of two soybean and two maize rows. The strip spacing (distance between maize and soybean rows), soybean row spacing and maize row spacing were all 0.5 m (Wu et al., 2017b). Each plot was 6 m long. The maize cultivar, Zhenghong 505, was sown on 9 April and harvested on 9 August, the soybeans were sown on 20 June and harvested on 23 October. When soybean was planted, maize was 2.5 meters high and reduced the light interception rate of soybean by 66%, the average daily temperature and humidity of intercropping treatment was 30.1 °C and 68.6%. The weather condition (rainfall and temperature) of two years are shown in Supplementary Figure 1.