The level of N application affected the competition ratio of soybean relative to maize (CRSM) in the maize-soybean relay intercropping system (Fig 3). The CRSM increased with the increase in N input, and the ratio in all treatments was greater than 1 in 2012, suggesting interspecific facilitation. Although the CRSM decreased in 2013, the CRSM of all treatments remained greater than 1. However, in 2014, the CRSM of the NN treatment was less than 1, whereas that of RN and CN treatments was greater than 1. The trends for CRSM indicated a positive interspecific interaction between component crops in the maize-soybean relay intercropping system, and N application was advantageous and improved the CRSM. With the increase in years of planting, the interspecific facilitation converted to interspecific competition in the NN treatment, although interspecific facilitation was retained under RN and CN, which suggested that N input was required to achieve interspecific facilitation in long-term maize-soybean relay intercropping.