Effect of maize/soybean relay strip intercropping system on soybean morphology, chlorophyll fluorescence, and yield in Sichuan area

Maize/soybean relay strip intercropping can make efficient use of light and land resources to improve yield per unit of land area. The research was conducted to explore the influence of row spacing of maize on the soybean canopy light environment and morphological traits. Two maize varieties, semi-compact (Chuandan 418) and compact (Rongyu 1210), were used to study the efficient utilization of land resources to enhance yield and yield components. Three arrangements of 20 cm, 40 cm, and 60 cm of maize narrow row spacing were set with same width of strip intercropping (SI) belt and soybean row spacing. Solely soybean with a fixed strip width of 200 cm was planted as a control. The light transmittance, morphology, photosynthetic pigment, fluorescence parameters, biomass of soybean and grain yield of both maize and soybean were analyzed. The results showed that the light transmittance and red light/far-red light (R/FR) ratio of the soybean canopy decreased with an increasing narrow row spacing of maize. The same trend was visible in the stem diameter, node number, and dry matter of stem and leaves. The maximum values were measured in the maize narrow row spacing width of 20 cm. Compared with the control, the stem diameter, node number, dry matter of stem and leaves of soybean were significantly reduced under intercropping with two maize varieties, while the second internode length and main stem length of soybean were significantly increased. Photosynthetic pigments contents in intercropped soybean leaves gradually decreased with the increase of narrow row spacing of maize, and were lower than monocultured soybean. The fluorescence parameters of soybean leaves of F_v/F_m, NPQ, F_q'/F_m', and F_q'/F_v' first increased and then decreased, while a reverse trend was visible for F_o with the increase of narrow row spacing of maize. The soybean rapidly recovered after exposure to a full light environment by harvesting the maize crop. The difference in soybean morphological and physiological indexes between intercropping and monoculture was reduced. However, owing to the shading effect of maize in the early growth stage, a significant difference in the grain yield was observed between different intercropping treatments. A benefit analysis of the maize/soybean intercropping system demonstrated that the comprehensive yield was the highest for a narrow row spacing arrangement of maize (40 cm). The yield of maize and soybean were on average 8 559.52 kg·hm^-2 and 1 717.60 kg·hm^-2, respectively. The average land equivalent ratio was 1.57. There was no significant effect on soybean morphological and physiological indexes under intercropping conditions between both types of maize in this experiment. It can be concluded that selecting compact or semi-compact maize varieties and moderately reducing the narrow row spacing of maize can significantly improve the growth of soybean in a maize/soybean strip intercropping system, and result in higher biomass and yield.