Effect of cereal-legume spacing in intercropping system on alleviating “N inhibition” in pea plants

Optimization of spatial structure in intercropping system is important for increasing crop yield and improving resource utilization efficiency. However, the theoretical basis for improving nitrogen fixation functions of legume and nitrogen utilization efficiency by optimizing the spatial structure of intercropping system has not been well investigated. Thus the synergistic effect of legume/cereal intercropping system on nitrogen nutrient utilization and regulation potential of legume plants was investigated in a field experiment on pea/maize intercropping system in the Hexi Corridor. The experiment consisted of monoculture pea, monoculture maize and pea/maize intercropping systems. In intercropping system, three intercropping spacings between pea and maize were set, which were 15 cm, 30 cm and 45 cm, with unchanged pea row spacing and changed maize row spacing; two nitrogen application rates were also set for intercropped maize, 0 kghm^-2 and 260 kghm^-2. The aim of the experiment was to determine the effect of spatial structure of pea/maize intercropping system on alleviation of “nitrogen inhibition” (N inhibition) on pea nodulation caused by N fertilization. The results of the study also provided a much better understanding of the optimization of the spatial structure of legume/cereal intercropping system, and the improvement of N use efficiency. The results for 2013–2014 showed that pea/maize intercropping significantly increased nodule number and weight of pea, compared with single-cropping pea system. Increase in nodule number varied from 0 to 500%, with the largest nodule number and weight appearing under intercropped spacing of 30 cm. N inhibitory effects (Ca) calculated by using nodule number and weight were all positive. Under N application, the effect on alleviating “N inhibition” under 30 cm spacing was significantly higher than that under 15 cm and 45 cm intercropping spacings. With N application, Ca calculated from nodule number reached 78.70% and 161.21% in both 2013 and 2014 growing seasons, and Ca calculated from nodule weight reached 154.87% and 406.52% without N fertilization. The ratio of nutrient competition of pea plants (CRpm) was greater than 1, which suggested that the capacity of interspecific competition of pea was higher than maize. Intercropping system significantly improved N use efficiency and N use efficiency under 30 cm spacing was significantly higher than that under 15 cm and 45 cm intercropping spacings. N use efficiency under spacing of 30 cm was average 21.90% and 21.88% higher than that of 15 cm and 45 cm respectively in 2013 and 2014 growing seasons. The optimization of spatial structure in intercropping increased nodule number and weight of pea plants, alleviated “N inhibitory effect”, regulated N uptake effect and enhanced N use efficiency of pea/maize intercropping system. The results of this study were critical for developing N-saving intercropping systems, enriching the theory of N use efficiency and improving the contribution of N fixation of legume to legume/cereal intercropping systems.