Maize–soybean relay cropping increases soybean yield synergistically by extending the post-anthesis leaf stay-green period and accelerating grain filling

Relay cropping of Poaceae and Fabaceae promotes high yield and land-use efficiency by allowing a double harvest. However, it is difficult to increase yield synergistically because of the reduced photosynthetic abilities of legume leaves under the shade of graminoids. Leaf photosynthetic capacity in relay cropping systems is associated with ecological niche differentiation and photosynthetic compensation after restoration of normal light. We conducted a field experiment in southwest China in 2020–2021 to evaluate the effects of three cropping patterns: maize–soybean relay cropping (IMS), monoculture maize (MM), and monoculture soybean (SS), and N application levels: no N application (NN:0 kg N ha⁻¹), reduced N (RN: 180 kg N ha⁻¹), and conventional N (CN: 240 kg N ha⁻¹). Compared to monocropping, relay cropping increased the stay-green traits of maize and soybean by 13% and 89%, respectively. Relay cropping prolonged the leaf stay-green duration in the maize and soybean lag phase by almost 4 and 8 days, respectively. Relay cropping maize (IM) increased the leaf area index (LAI) by 79.4% to 88.5% under NN and 55.5% to 148% under RN. Relay cropping soybean (IS) increased the LAI from 115% to 437% at days 40 to 50 after anthesis. IM increased yield by 65.6%. IS increased yield by 9.7%. HI and system yield were at their highest values under RN. In the relay cropping system, reduced N application extended green leaf duration, increased photosynthesis inside the canopy at multiple levels, ultimately increases soybean yield synergistically.