Relay-intercropping soybean with maize maintains soil fertility and increases nitrogen recovery efficiency by reducing nitrogen input

Optimized nitrogen (N) management can increase N-use efficiency in intercropping systems. Legume-nonlegume intercropping systems can reduce N input by exploiting biological N fixation by legumes. Measurement of N utilization can help in dissecting the mechanisms underlying N uptake and utilization in legume-nonlegume intercropping systems. An experiment was performed with three planting patterns: monoculture maize (MM), monoculture soybean (SS), and maize-soybean relay intercropping (IMS), and three N application levels: zero N (NN), reduced N (RN), and conventional N (CN) to investigate crop N uptake and utilization characteristics. N recovery efficiency and ¹⁵N recovery rate of crops were higher under RN than under CN, and those under RN were higher under intercropping than under the corresponding monocultures. Compared with MM, IMS showed a lower soil N-dependent rate (SNDR) in 2012. However, the SNDR of MM rapidly declined from 86.8% in 2012 to 49.4% in 2014, whereas that of IMS declined slowly from 75.4% in 2012 to 69.4% in 2014. The interspecific N competition rate (NCR_ms) was higher under RN than under CN, and increased yearly. Soybean nodule dry weight and nitrogenase activities were respectively 34.2% and 12.5% higher under intercropping than in monoculture at the beginning seed stage. The amount (Ndfa) and ratio (%Ndfa) of soybean N₂ fixation were significantly greater under IS than under SS. In conclusion, N fertilizer was more efficiently used under RN than under CN; in particular, the relay intercropping system promoted N fertilizer utilization in comparison with the corresponding monocultures. An intercropping system helps to maintain soil fertility because interspecific N competition promotes biological N fixation by soybean by reducing N input. Thus, a maize-soybean relay intercropping system with reduced N application is sustainable and environmentally friendly.