Competition ratio and system productivity index are used to assess interspecific competition and intercropping advantages, respectively. As an indicator, competition ratio is used to measure the degree of competition between crops in an intercropping system  and is calculated with the following formula: CRSM=YIS/(YMS×AS)YIM/(YMM×AM)(3) where CRSM is the competitive ratio of maize relative to soybean, YIS and YMS are the yield or nitrogen acquisition per unit area of maize under intercropping and monoculture, respectively, YIM and YMM are the yield or nitrogen acquisition per unit area of soybean under intercropping and monoculture, respectively, and AS and AM are the ratios of the area occupied by maize and soybean under the intercropping system relative to that of the corresponding monoculture, respectively. In this study, AS and AM were the same. A competition ratio greater than 1 indicated the competitive ability of soybean was greater than that of maize in the maize-soybean relay intercropping system. A ratio less than 1 indicated the competitive ability of soybean was less than that of maize in the maize-soybean relay intercropping system. System productivity index, SPI, is another indicator used to assess intercropping that standardizes the yield of the secondary crop in terms of the primary crop  and is calculated as follows: SPI=(SSSM)×YM+YS(4) where SS and SM are the average yields of soybean and maize under monoculture, respectively, and YS and YM are the average yields of soybean and maize under intercropping, respectively.