However, most studies focus on relay intercropping systems in which the legumes play a secondary role, i.e., the legumes are used as a cover crop, and the legume yield is not considered. Previous studies show that facilitation and competition coexist in intercropping systems, particularly in legume/non-legume intercropping systems . Xia et al. reported that legumes facilitate the root system and grain yield of maize considerably , whereas Fan et al. found that the grain yield of fava bean decreased in a wheat/fava bean intercropping system in contrast with a monoculture . Generally, the architecture of a plant influences the relative competitive ability. Soybean seedlings often grow under the shade of the maize canopy and then are transferred to full sunlight after the harvest of maize . Moreover, the distribution of the root system plays a key role in the acquisition of belowground nutrients. The roots of cereals occupy soils both near the surface and in deeper layers, whereas the roots of legumes are distributed in the upper soil layers . The competitive ability of cereals for soil N is stronger than that of legumes in an intercropping system . Furthermore, the separation of the root ecological niche between component crops is well known to affect the total grain yield of intercropping systems. In a maize/fava bean system compared with that of a wheat/fava bean system, the total grain yield was significantly higher . Nitrogen recovery efficiency and uptake efficiency can increase significantly through the interaction of roots between crops. Furthermore, the bi-directional N transfer and positive N competition between crops are advantageous to improve to NUE in the wheat/maize/soybean relay intercropping system .